Advanced Technology in Structural Engineering

Research efforts related to computational control of active structures are contributing to the development of intelligent structures. Intelligent structures are a type of active structure that reacts to changes in their environment (loads, movements, etc.) through performing specific tasks. These tasks are i) measurement of behavior, ii) evaluation using knowledge bases, iii) use of computational control to modify structural characteristics and iv) use of past events to improve performance. A computational framework based on intelligent control methodology is presented that combines reasoning from explicit knowledge, search, and learning to illustrate capabilities of intelligently controlled structures. Using this framework, a computational system and its application to tensegrity structures are under development. Such computational control systems are stimulating the design and construction of innovative structures, thereby extending design possibilities for structural engineers.

The past few years have seen advancements in electrical engineering fields, such as microelectromechanical systems (MEMS) and microsensors. Together with more mature technologies like wireless data communication, these advancements have begun to make embedded microdevices for use in civil infrastructure a serious and cost-effective reality. This paper begins with a brief discussion of deterioration, damage, and current detection methods for concrete infrastructure. Following this background discussion is a presentation of the microsensor and electronic system technologies available for addressing deterioration and detection with distributed, embedded microdevices. Where possible, existing microsensors are proposed to augment or replace current nondestructive evaluation techniques. System technologies for power and communication are similarly proposed. In the area of local strength testing, where neither current NDE methods nor existing microsensors have proven effective, novel MEMS testing devices are proposed. Based on these needs and available technologies, the discussion concludes with a description of a concept being developed for a "Smart Aggregate" microdevice for distributed, embedded concrete infrastructure monitoring.

This paper summarize a research effort towards the development of an integrated system for monitoring the condition of a civil structure, utilizing advanced sensing, microprocessing, wireless communication and damage diagnosis methods. Specifically, the discussions focus on the following issues: (1) the development of modular wireless vibration sensing, data acquisition and processing units; (2) the development of advanced structural damage assessment procedures; and (3) environmental effects on experimentally obtained modal parameters.

Design of civil engineering structures is done using models. These models are mathematical abstractions of real structures. During the modeling process we simplify structural behavior and scale the structure down to fathom it. In doing so, we quickly loose the sense of dimensions, displacements, and forces. For example, it becomes very hard to imagine a 2% drift of a six-story building. Furthermore, it is virtually impossible to sense the resistance of a structure, especially when some of its elements yield. Experienced engineers simply develop a feel for the structure...but how do they do this? It is as if they can touch the structure they are designing. The sense of touch is the only active sense we have. It allows us to learn about the world around us by active probing. The rest of our senses are simply passive. Out of the four passive senses, we use only vision to design and analyze structures. Why? The models we use simply defy any other sense. Even with the development of computers, we limited ourselves to developing only the visual means or representing the results of structural analyses. The term "scientific visualization" tells all. The limit on representing the structure using only vision is artificial. We need to break through this self-imposed barrier: we need a model we can touch. This can be done by making a virtual environment for modeling and analysis of structures which has a haptic as well as a visual user interface.

Electrorheological (ER) suspensions represent a broad class of materials which exhibit reversible property changes when subjected to high electric fields. Specifically, material properties change from those of a Newtonian liquid to a those of a viscoelastic-plastic solid, within milliseconds, and with very little required external power. The adaptable behavior, and simplicity, of these materials are attractive features in implementing semi-active (variable-damping or variable-stiffness) vibration control. Because the ultimate behavior in ER materials is yielding, the stresses in these materials are limited, and are typically small compared to the stresses in structural materials. Therefore, in designing high force ER devices, the material must be designed to flow over large surface areas. One embodiment of this design consists of multiple concentric annular flow ducts. This paper presents preliminary experimental results from one such design.

In this paper, a semi-active electromagnetic friction damper (SAEMFD) is proposed for control of peak dynamic response of seismic-excited structures. In the proposed SAEMFD device, the friction force between two sliding plates is regulated by controlling the normal force using an electromagnetic field across the damper. The friction force is proportional to the square of the current passing through solenoids attached to two friction plates and it can be operated using stand-by batteries. A control algorithm is proposed for the semi-active control of the friction damper using a simple static output. The effectiveness of the SAEMFD device and the control strategy in reducing the peak dynamic response of structures is investigated through an application to a 5-story base-isolated building subject to near-field earthquakes. Numerical results demonstrate that the proposed SAEMFD device and the control strategy are effective in reducing the peak drift of rubber-bearings of the base-isolated building subject to near-field earthquakes.

This paper discusses the results of a series of simulations conducted to compare the effectiveness of various control systems for earthquake hazard mitigation. A systematic comparison of the performance of different control systems will be useful for the designer in selecting the most effective control system for a structure. Ideal passive, active and semi-active control systems are employed in this study. For determining the control action, an H₂/LQG control algorithm was selected for the active system and a clipped optimal control algorithm was selected for the semi-active system. In both cases the control algorithm was based on acceleration measurements for determination of the control action. To evaluate and compare the controllers, the structural responses of various three-story building are examined. An El Centro earthquake is used as the ground excitation. The mass is held constant while the stiffness of the structure is varied to examine structures with a range of natural frequencies.

The design of passive energy dissipation systems, referred to as passive dampers, for complex civil infrastructures involves the determination of optimal locations and capacities. In this paper, optimal design methodologies for passive dampers are presented based on two different performances; namely, H_∞ and H₂. Emphasis is placed on the application of the Linear Matrix Inequality (LMI) to the effective design of passive dampers. The advantages of the optimal design obtained based on H_∞ or H₂ performance are demonstrated through simulation results.

In this paper the efficiency of various dissipation mechanisms to protect structures from pulse-type and near-source ground motions is examined. Physically realizable pulses are introduced, their resemblance to recorded near-source ground motions is illustrated, and their shaking potential is examined. It is found that the response of structures with low to moderate isolation periods is substantially affected by the high frequency fluctuations that override the long duration pulse, therefore the concept of seismic isolation is beneficial even for motions that contain a long duration pulse. Furthermore, although pulse motions can capture many characteristics of the displacement and velocity time histories recorded near the earthquake source, they are not dependable motions for structural analysis. It is recommended that near source records should be used instead of their pulse approximation. Dissipation forces of friction type are efficient in reducing displacement demands, although occasionally they are responsible for substantial permanent displacements. The study concludes that a combination of relatively low friction and viscous dissipation forces is attractive since base displacements are substantially reduced without increasing appreciably base shears and superstructure accelerations.

It is generally recognized that structures having high rigidity can be characterized as having relatively small displacements when a seismic transient or wind loading is applied. These small deflections are such that the implementation of added damping components has proven to be difficult. Research was performed combining fluid dampers with a simple mechanical toggle brace assembly that magnifies the small displacement of the structure, while simultaneously producing the required damping force. Test results are provided with the toggle braces and fluid dampers installed on a 14,500 kg structure, subjected to seismic ground motion transients on a large seismic shake table.

This paper summarizes the results of a recent investigation on the dynamic response of asymmetric-plan buildings with supplemental viscous damping to harmonic ground motion using modal analysis techniques. It is shown that most modal parameters, except modal damping ratios and dynamic amplification factors, are affected very little by the plan-wise distribution of supplemental damping in the practical range of system parameters. The first modal damping ratio increases while the second decreases as CSD moves from right to left of the system plan, and their values increase with larger plan-wise spread of the supplemental damping. Trends for the dynamic amplification factors are reversed, as they are inversely influenced by the damping ratio, i.e., higher the damping lower the dynamic amplification factor. The largest reduction in the flexible edge deformation occurs when damping in the first mode is maximized.

This paper outlines large-scale tests of tension/compression yielding braces (also called "unbonded braces") in support of their first applications in the United States. The core steel in these braces provides stable energy dissipation by yielding under reversed axial loading, while the surrounding concrete-filled steel tube resists compression buckling. The paper summarizes a series of tests on large-scale unbonded braces, having yield forces of 1200, 1600, and 2100 kN (270, 360, and 470 kips). Each brace was subjected to a cyclic loading pattern consistent with that used widely for testing steel beam-column connections. Additional tests explored the behavior of the braces under a near-field loading history, a displacement time history derived from a seismic analysis of an idealized 5-story building, and a low-cycle fatigue test.

This paper describes the application of GA (Genetic Algorithm) to the shim adjustment in the erection accuracy control for the Tsuneyoshi Bridge ,which is a cable-stayed bridge and constructed in Osaka Bay, Japan. Some methods have such a problem that weghting for various factors is difficult in a job site. Therefore, an attempt is made to present pareto-optimal solutions and apply GA as an optimum shim adjustment method. It is noted that the proposed method can provide us with discrete combination values of shim plates instead of continuous values. Furthermore, it is expected that even inexperienced engineers can easily select an appropriate solution suitable for individual situations without parametric examination of various weighting factors, because the method can give a set of solutions (called parato-optimal solution) with different characteristics

In recent years important progress has been made in the assessment of the lifetime behaviour of concrete bridges. Due to the large uncertainties related to the loading and the deterioration of such bridges, an assessment based on stochastic modelling of the significant parameters seems to be the only relevant modelling. However, a great number of difficulties are involved in the modelling. The main purpose of this paper is to give an overview of areas where more research is needed and in some cases propose solutions. There seems to be a general agreement that optimal design of concrete bridges should include assessment of not only the initial costs but also expected costs related to preventive and essential maintenance, and expected failure costs. The total lifetime costs of a system of bridges or a single bridge will therefore not only depend on load and strength parameters but also on inspection and repair costs and also on design criteria (limit states). Further, it is of interest to study the reliability of a single bridge from a systems point of view but also to study whether bridges can be considered from on an individual basis or whether a system (group) approach should be used. The conclusion of this study is that reliability based optimal design of an infrastructure system of concrete bridges can now be performed based on recent research results obtained worldwide. However, much more research in this area is needed before a general procedure can be established. A number of important factors are only partly investigated or not addressed at all. It is believed that optimal design in this context will reduce drastically the lifetime costs of operating this type of infrastructure systems.

Two methods are proposed and evaluated for discrete variable optimization of structures subjected earthquake loads. The first method, called the two-phase method, uses a combination of continuous and discrete optimization algorithms and the second one, called an adaptive discrete assignment method (ADAM), uses only a continuous optimization algorithm. The proposed methods are applicable to any discrete variable problem that can be formulated as a continuous problem. A lattice tower is solved to demonstrate the methods. In addition, the problem is solved with a genetic algorithm (GA). Efficiency and practicality of the proposed methods are compared to those of the GA. From the results, it is concluded that GA is a very straightforward method to use for discrete problems; however, it requires a significant amount of CPU time. On the other hand, the two-phase method and ADAM do not require as much computational effort but the discrete designs found with them are more expensive. Also they require more effort for their computer implementation.

The beneficial effects of redundancy on the safety of structural systems have been recognized. However, as indicated in Bertero and Bertero, 1999, "... few studies have examined the effects of redundancy in a quantitative way. In fact, even the definition of redundancy .... remains unclear." This paper uses the concept of system function, borrowed from Leemis, 1995, to define system redundancy and component importance factors for deterministic structures. Extension of these definitions to nondeterministic structures under random loads are also briefly presented. Based on the results, recommendations for optimal system design are stated.

Response of sliding isolated bridges with smart dampers in near fault earthquake ground motions is evaluated in this study. The smart damper used is a Magnetorheological (MR) damper. A 1:20 scale single span sliding isolated bridge model with four sliding bearings and a MR damper is studied. New sliding mode controller is developed and implemented in real time. Several near fault ground motions recorded in the Northridge earthquake are used in the testing. Shake table test results of the scaled sliding isolated bridge model with MR damper are presented to demonstrate the effectiveness of the dampers. It is shown that the smart MR dampers can reduce displacements and forces in the piers further than the passive dampers. While these displacement reductions can be achieved by increasing the passive damping further, it can only be done at the expense of greater forces in the piers.

This paper investigates the effectiveness of several base isolation strategies for historical earthquakes of various magnitudes and characteristics. The strategies studied herein include isolation systems with lead-rubber bearings and with "smart" (semi-active) dampers. To demonstrate the advantages and disadvantages of the various approaches, historical earthquakes scaled to different magnitudes are used to excite an isolated building structure. Responses examined include: peak base and structural (relative) displacements, peak base and structural (absolute) accelerations, and peak applied forces. The adaptable nature of the smart damper system is shown to protect a structure from extreme earthquakes, without sacrificing performance during the more frequent, moderate seismic events.

Recently, a resetting or switching semi-active stiffness damper (RSASD or SSASD) has been shown to be effective in reducing the structural response due to dynamic loads. In this paper, a semi-active hybrid isolation system, consisting of rubber-bearing isolators and resetting or switching semi-active stiffness dampers is proposed. The performance of such a semi-active hybrid isolation system is studied extensively for the response control of buildings subject to near-field earthquakes. Numerical results clearly demonstrate that the resetting semi-active stiffness damper is capable of protecting the integrity of base-isolated structures during near-field earthquakes.

Seismic dampers represent an advanced technology that is increasingly being utilized to dissipate energy from an earthquake. The seismic dampers can be positioned either within the frame of a building or within a seismic isolation system. When used as a component of a seismic isolation system, seismic dampers are used to limit the isolation system displacement. However, for certain types of earthquake ground motions, seismic dampers that are used within an isolation system have been shown to increase the interstory drifts of the superstructure. The research presented herein investigates the ability of adaptive seismic isolation systems to resist disparate earthquake ground motions. The isolation system consists of sliding isolation bearings in combination with a controllable fluid damper. The results of the research show that the adaptive sliding base isolation system is capable of simultaneously limiting the response of the isolation system and the superstructure for disparate earthquake ground motions.

This paper summarizes some of the previous work on optimal bridge maintenance planning and presents new developments. The optimization process is reliability- and cost-oriented. The purpose is to optimize lifetime maintenance expenditures associated with bridge stocks. The degradation of the reliability of individual bridges and the present value of future maintenance costs are both considered in this process. Numerical examples illustrate different aspects of the optimal bridge maintenance planning approach.

Many important suspension and cable stayed bridges have been built recently having very long span lengths. For these types of bridges wind loadings, related to wind speed, are the most important issue and aeroelastic considerations are the crucial issue in the design. In this paper an general formulation of wind forces in bridge deck is introduced and the aeroelastic analysis of bridges is presented. The non-linear mathematical problem of obtaining the critical velocity giving way to flutter instability is developed. A sensitivity analysis of the flutter speed with regards to the mechanical parameters of the bridge deck is presented and a simple example to explain the methodology is included.

Discrete design variables are encountered in most practical design applications. An overview of the optimization methods that can treat such design variables in their solution process is presented. Many times selection of an appropriate method depends on type of discrete variables and problem functions. Therefore, discrete design variable problems are classified into six different types. Methods suitable for each problem type are identified. Some recent applications of discrete variable optimization methods are described. Methods for structural optimization with available sections are discussed.

This paper presents a constrained multiobjective optimization method in the form of a robust, practical, problem-independent formulation based on genetic algorithm combined with game theory, and investigates the effect of multiobjective optimiztion on structural design. The study includes objectives, constraints, and design variables as well as their effect on structural design and behavior. The algorithm comprises — multiobjective fitness function, niche method, and Pareto set filter. A 10-story setback building is used to illustrate the evaluation of three choices: steel frame, reinforced concrete frame, composite steel and reinforced concrete frame. Evaluation is based on economics as well as performance. Comparison of optimum design results is obtained with consideration of weight, structural cost, and seismic energy for all three frames subjected to the same seismic input. Constraints are based on Uniform Building Code specifications including stress, displacement, drift, and ratio of story stiffness. The new approach provides a powerful tool to locate a global solution.

The 2000 foot (609 meter) tall 7 South Dearborn tower proposed for Chicago, will be, at time of completion, the tallest building in the world and also one of the most slender with an overall aspect ratio of approximately 8.5 to 1. The 110-story building is a multi-use project with retail comprising the ground floor, offices in the lower section, residential comprising the upper sections, and telecommunications (including digital television broadcasting) occupying the highest floors. The innovative lateral load resisting system developed for the tower consists of a reinforced concrete core wall, with structural steel outriggers at the mechanical floors, connecting the core to structural steel perimeter columns in the lower half of the building. An overall structural economy of the lateral system for the tower is a requirement for the feasibility of the project. Based on structural analysis and wind tunnel testing, the building responses to wind loadings (deflections and accelerations) were determined. The sizing of the members of the lateral system for high-rise buildings is often controlled primarily by serviceability requirements rather than strength. In order to achieve an economical building, it is prudent that these elements are appropriately sized and the structural materials are efficiently distributed among the various components to result in a least cost solution. This paper summarizes the structural system for the 7 South Dearborn tower and presents the optimization techniques utilized to determine a least cost solution for a given deflection or period incorporating member constraints.

This paper presents a computer-based optimisation technique for the lateral stiffness design of tall buildings of mixed steel and concrete construction. Given the topology of a tall buildup structure and a set of prescribed wind loading conditions, the optimal distribution of structural materials will be automatically sought by a systematic goal-oriented design technique which satisfies a set of wind drift design performance constraints. The optimisation technique described herein has been used at the early design stage of the North East Tower, which, when complete, will be the tallest building in Hong Kong.

The paper presents the conceptual details of a new mathematical model for nonlinear static push-over analysis for use in performance-based design of frameworks under earthquake loading. Studies have shown that the behavior of semi-rigid frameworks can be modeled using a `fixity-factor' that measures the degree of connection fixity. Assuming a potential plastic hinge section of a beam-column member is a kind of connection, a similar approach is adopted by this study to monitor the rigidity degradation (plastification) of members of frameworks under push-over loads. Through the use of `rigidity-factors' that measure the degree of plastic hinge formation, the conventional elastic stiffness matrices of frame elements (beams, columns, etc.) are progressively modified to account for nonlinear elastic-plastic behavior under incrementally increasing loads. The concepts are illustrated for first-order analysis of planar frameworks. They are readily extended to second-order analysis and space frameworks.

Structural optimization and performance based engineering concepts are simultaneously emerging as useful tools in the design of building structures. While these concepts have generally evolved on different tracks, successful implementation in the professional design environment requires an integrated approach. In this paper, ideas for developing an integrated approach are offered.

This paper presents a distributed object-oriented design for a nonlinear finite element analysis using the message-passing paradigm and a single-program-multiple-data scheme. In recognition of the costly communication startup time-penalty of distributed processing, the design uses data structures to minimize communications frequency. The proposed object design was implemented and tested on the nonlinear static push-over analysis of a 20-story moment-resisting frame.

A sequential object-oriented framework is extended for parallel processing using an Actor programming model. The paper presents results showing the performance improvements that can be obtained on a network of workstations.

This paper outlines a software framework that will serve as the core for collaborative structural analysis software development over the Internet. The collaborative framework would allow researchers and engineers to easily access the analysis platform and to incorporate new element technologies and solution strategies for structural analysis. The objective is to provide a "plug-and-play" environment where users can define and specify their own model, element types, and preferred solution techniques, and possibly the strategy most appropriate for solving their specific problem.

In the present work a parallel algorithm for the solution of general structural dynamic problems has been developed. This algorithm is referred to as the Iterative Group Implicit (IGI) algorithm. The IGI algorithm is an extension of the Group Implicit (GI) algorithm. In both methods the structure being analyzed is partitioned into subdomains and each subdomain is integrated independently and in parallel using any existing time-stepping solution algorithm. Also, in both methods the compatibility condition at the interface degrees of freedom is restored using a mass averaging rule. However, equilibrium at the interface degrees of freedom is not satisfied in the GI algorithm, while in the IGI an iterative procedure has been devised to recover equilibrium in these degrees of freedoms. The IGI method has the same algorithmic characteristics as the underlying solution method used to solve each subdomain. A numerical example has been developed to study the performance of the IGI algorithm. It is found that the present method is superior to the GI algorithm both in terms of accuracy and efficiency.

We describe the parallel implementation and performance of a general-purpose toolkit, ZPLCLAW, for solving wave propagation problems in 2 and 3 dimensions. The methods, developed originally in the Fortran CLAWPACK package, are based on solving Riemann problems, problems with piecewise constant initial data, at individual grid cells and propagating the resulting waves in multi-dimensional manner. The effectiveness and high-resolution features of these methods have been demonstrated in the CLAWPACK package. The toolkit is implemented in ZPL, a data parallel language that provides high-level support for operations on parallel arrays independent of the underlying communication libraries used. We demonstrate the performance of the toolkit on an elastic wave propagation problem arising in pavement engineering, and illustrate its scalable performance on the CRAY T3E and on a cluster of workstations. The toolkit ZPLCLAW is available on the web.

Nonlinear static and nonlinear dynamic procedures in the Guidelines for the Seismic Rehabilitation of Buildings are used to analyze an existing four-story steel moment frame building previously damaged in the Northridge earthquake. Results from both types of analyses are compared with the objective of exercising and evaluating the FEMA-273 procedures for low-rise steel moment frames.

Among the more challenging aspects of performance-based seismic design is the development of techniques to reliably establish the so-called "collapse prevention" performance objective. Possible approaches include one from FEMA 273 Guidelines for the Seismic Rehabilitation of Buildings that employs linear or nonlinear seismic analyses together with acceptance criteria in the form of element-based limits on forces and deformations. Another method, proposed in the recently published SAC Seismic Design Criteria for New Moment-Resisting Steel Frame Construction, defines acceptance criteria in terms of probabilistically determined interstory drift limits. While both of these methods are major steps towards achieving comprehensive performance-based design, neither fully accounts for aspects of inelastic system stability as influenced by nonlinear strength and stiffness degradation under cyclic loads. An new approach is proposed herein that utilizes cumulative damage indices and a second-order analyses to evaluate the remaining stability of structures that have been subjected to strong ground motions. Application of the technique is demonstrated through a series of incremented time-history analyses of a composite steel-concrete moment frame.

In the past few years, a number of performance-based design guidelines have been published for use in the structural engineering community. These documents are in response to a need for design guidelines that have the ability to better predict performance during earthquakes. A principal component of these guidelines is the use of nonlinear analysis procedures. Until recently, nonlinear analysis was not deemed to be computationally economical nor sufficiently well understood by the profession to be used in structural engineering practice. The development of nonlinear analysis as an available tool opens many new options to engineers. The anticipated nonlinear behavior of a structure during severe earthquake shaking can be examined more accurately. For existing building evaluations, this tool can allow engineers to justify the safety of buildings that would otherwise be deemed unsafe by building codes and other procedures that are solely based on linear approaches. Nonlinear analysis can also lead to a cost-effective design by taking full advantage of the capacity design methodology though such a procedure is iterative and time consuming. With the common availability of nonlinear analysis in the structural engineering profession comes a number of advantages that can be seized and built upon, as well as numerous disadvantages that need to be avoided and addressed. The paper will use examples from actual projects to illustrate these from the perspective of practicing engineers.

The introduction of a performance-based design philosophy in FEMA-273, 1997 for seismic rehabilitation of buildings marks a significant departure from traditional seismic design. Two types of analysis methods for estimating seismic demands in building structures, which can be broadly classified as linear and nonlinear procedures, are outlined in the document. This implies that an engineer can make reliable estimates of deformation demands using either one of these analysis methods. This paper is an attempt to confirm or demystify the validity of linear and nonlinear static procedures in arriving at reasonable estimates of deformation demands in relatively regular building structures. Two structures: an existing 6-story steel building and a 7-story reinforced concrete building are used as case studies to evaluate the different analytical procedures. It is concluded that commonly-utilized static procedures, including those recommended in FEMA-273, are generally inadequate and substantially enhanced procedures are needed if static methods are to be successfully used in performance-based seismic evaluation of structures.

With the inclusion of the Non — Linear Static Procedure (NSP) or pushover analysis into the Federal Emergency Management Agency Document 273 (Fema 273), the need for non — linear pushover analysis tools for structural design in seismic zones is apparent. Focusing on reinforced concrete structures, this entails the ability to perform the NSP on lateral force resisting systems consisting of moment frames, structural walls, or any combination of these. The authors are currently developing a pushover analysis tool, using an existing finite element program, which can accomplish these tasks. This paper details the steps in performing the NSP, compares preliminary results of the NSP with a full non — linear dynamic analysis, and discusses a simplified procedure to model shear deformations in reinforced concrete members.

A draft version of the Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges has been compiled through National Cooperative Highway Research Program 12-46. The manual extends the Load and Resistance Factor approach to the evaluation of highway bridges in the United States. The objectives of bridge rating are different from the objectives of new bridge design resulting in the use of different combinations of limit states and reliability indices in evaluation. The draft manual proposes a system for bridge load rating and permit evaluation based on a uniform reliability index that uses site specific information to reduce the uncertainty in some of the evaluation variables. The primary goal of this project is to produce a practical evaluation manual that is consistent with the Load and Resistance Factor philosophy, is easy to understand and use, and will receive broad support for adoption and distribution by the AASHTO Subcommittee on Bridges and Structures.

A draft version of the Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges has been compiled through National Cooperative Highway Research Program 12-46. The manual extends the Load and Resistance Factor approach to the evaluation of highway bridges in the United States. The objectives of bridge rating are different from the objectives of new bridge design and these differences result in different limit states and reliability requirements. The draft manual proposes a system for bridge load rating and permit evaluation based on a uniform reliability index (where possible) that uses site specific information to narrow the uncertainty in some of the evaluation variables. The results of this system are compared to the current load factor ratings based on trial evaluations.

A procedure for evaluating the remaining fatigue life of in-service bridges is included in the Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges, which was developed in NCHRP Project 12-46. This procedure and other parts of the Manual have been under review and testing by transportation departments in volunteer states since early 1999. The goal of the paper presented here is to provide an introduction to this new procedure. The fatigue life evaluation procedure contained in the Manual follows very closely with the design methodology contained in the AASHTO LRFD Bridge Design Specifications. This makes the procedure up-to-date and should make it easy to use as bridge engineers become more familiar with AASHTO LRFD. The procedure is very similar to the method presented in NCHRP Report 299, which was incorporated into AASHTO's Guide Specifications for Fatigue Evaluation of Existing Steel Bridges. As a result, the procedure provides an estimate of remaining life that is based on calculated applied stress ranges of realistic magnitude instead of excessively high stress ranges used for design under the AASHTO Standard Specifications for Highway Bridges.

The first bridge rehabilitation project in Washington, D.C. to use the AASHTO LRFD Specification reveals several lessons to be learned from using the newly implemented design approach. The LRFD Specification has influence on the project in terms of inspection, analysis and rehabilitation design of this 1960's vintage structure. The intent of this project is to rehabilitate the Southbound 11^th Street Bridge to ensure safety and extended service life as well as to save updated operational needs. The structure is a built-up deck plate girder bridge with stringers and floor beams. The project involves winning, redecking, repair and elimination of pin connection details. Construction is to be performed while keeping the bridge in service. An instrumentation program will likely be implemented during the construction in order to monitor the structure and ensure that the actual structural behavior is as predicted by analysis. The use of the LRFD Code has many implications for this project Due to the nature of the existing structure, several aspects of LRFD design are interpreted and applied in situations not fully addressed by either the LRFD Specification or published software. These include but are not limited to curved girders, application of distribution factors for non-traditional framing systems, and application of design factors and limit states to existing members. Resolution of these issues is presented in a format that provides a basis for future LRFD Specification considerations.

This paper describes briefly the resent activities of the research works and vibration control of the wind induced vibrations of inclined cables of cable-stayed bridges, in particular, rain-wind induced vibration, which becomes one of major subjects on the safety of cable-stayed bridges.

As is now quite well known and documented, cable-stayed bridges around the world are commonly exhibiting excessive and unanticipated vibrations of the main stays, often associated with the simultaneous occurrence of wind and rain. This paper presents a characterization of the nature of these high-amplitude vibrations based on a comparative evaluation of data collected from the long-term instrumentation of two cable-stayed bridges in the United States: the Fred Hartman bridge in Houston, Texas; and the Veteran's Memorial Bridge in Port Arthur, Texas. Each of these bridges has exhibited high-amplitude stay oscillations. The contributions of various cable modes in the observed responses are investigated, and the influence of rainfall is discussed.

Rain- and wind-induced vibration in the stay-cables of cable-stayed bridges is a problem that is generating greater attention as more of these bridges are constructed around the world. Large-amplitude stay-cable vibrations have been observed on existing bridges at low wind speeds of less than 30 mph with and without the presence of rain. This problem has challenged the very integrity of these aesthetically pleasing structures and is alarming to the passing motorist. Rain- and wind-induced vibration is an aerodynamic phenomenon that was relatively unknown during the design of many cable-stayed bridges, resulting in the need for vibration countermeasures. In this paper, various aspects of the rain and wind vibration phenomenon are presented based on a series of wind tunnel section model tests that were performed with a newly developed two-degrees-of-freedom elastic suspension system. The effectiveness of a vibration mitigation device was also examined.

In February 1998, stay cable oscillations were observed on the Cochrane Bridge, Mobile, Alabama in winds accompanied by rain. Rowan Williams Davies & Irwin Inc. (RWDI) was retained by A.G Lichtenstein & Associates to assist them in identifying the causes of the cable oscillations, to assess the potential of future wind-induced cable vibrations, and to design a damping system to mitigate the oscillations. RWDI performed on-site oscillation decay measurements for some of the stay cables. Inherent damping values for the cables were calculated from the decay traces. Using these damping values and a criterion based on Scruton number (mass damping parameter), the potential for rain/wind type oscillations for the stay cables was assessed. In addition, the cables susceptibility to other forms of wind-induced vibrations was reviewed. The program of work provided an opportunity to check the validity of criteria for wind-induced oscillations against field observations.

Segmental bridges started in the United States in the early 1970s with the precast Corpus Christi Bridge in Texas and the Pine Valley Bridge in California. Since then, the effect of creep and shrinkage effects on segmental precast and cast-in-place bridges has been under constant debate. Many design engineers, however, are still unsure what effect creep and shrinkage has on the final stresses of the structure. Shrinkage is the property of concrete to change in volume independently of the loads it sustained. Both creep and shrinkage are time-dependent deformations and may occur concurrently. In this paper, the writers present a parametric study by comparing results of structural analyses with and without consideration of creep and shrinkage effects. All structural analyses have been carried out using TANGO, a commercially available software specially designed to handle this type of structures.

The Richmond-San Rafael Toll Bridge, spanning the northern end of San Francisco Bay, is undergoing extensive earthquake upgrading and retrofitting. The West Trestle spans of the bridge represent 2.0 km of the total 6.75-km total bridge length. The low-lying overwater trestle connects the main bridge structure to the west abutments at Point San Quentin. This paper discusses the technical features of the West Trestle reconstruction that uses innovative design and construction methods to replace the trestle on the same alignment as the existing structure, while traffic is maintained in all lanes during commute hours. Special methods used include staged foundation construction and full-width, span-by-span deck erection using prestressed concrete double-tee girders.

In recent years many highway agencies have experimented with the use of innovative contracting techniques in their quest to deliver better transportation projects for their users. The highway industry has traditionally depended on low bid contracting in building infrastructure projects, however, in recent history this practice has started to be questioned. Cost and quality of the project and the time it takes to build these projects has come under closer scrutiny and as a result many have concluded that we may be better served by shifting more of the development of a project from the design side to the side of the builder. The New Jersey Department of Transportation has experimented with this process, having advertised a score of projects to date utilizing design-build contracting. New Jersey utilizes a modified design-build process where the design-build team bids on designs which are 30% to 40% complete.

Condition ratings are important to automated decisions on actions for maintenance, repair and replacement of bridges. Bridge management systems use condition ratings to identify bridges that need repair now, or are expected to need repair within a selected planning period. Condition states, the definitions of ratings, determine what is reported and what kinds of decisions can be supported. The relation of condition states to decisions and planning activities has been increasingly recognized, prompting new definitions of condition states as a part of the development of bridge management systems. Damage, reported as poor condition ratings, is one factor that triggers repair or replacement actions for bridges. There are other factors such as functional inadequacies and vulnerability to sudden failure. Actions are needed for deficient bridges; that is for bridges where damage, or function, or vulnerability is unacceptable. Each factor, damage, function, and vulnerability, can be expressed as condition, and can be reported as condition ratings. Bridge condition is, simply, the need for action. By mapping condition states explicitly to actions, a consistent basis for creation of condition states is obtained. For each action, the sufficient conditions in damage, function or vulnerability are established. These sufficient conditions become definitions of condition states. The number of feasible actions determines the number of condition states. Inspections and assessments of bridges are selected to detect the conditions specified in the state definitions. Beginning from actions, a system for definition of condition states, for selection of inspection and testing operations, and for coordinated examination of deficiencies is achieved. This paper proposes a framework for definition of condition states for damage, function, and vulnerability according to the actions that are needed.

The paper describes some of the experiences of the Wroclaw University of Technology (WUT) in applications of the neural network technology in the Railway and Road Bridge Management Systems in Poland. The conception of the expert functions based on the multi-level neural networks as well as the expert function creation process is presented.

The Ministry of Transportation of Ontario (MTO) has engaged Stantec Consulting, Ltd. to develop a new state-of-the-art Bridge Management System to support the management of the Province's 3000 bridges. Developed in Visual Basic, the software will exploit object technology to produce a practical decision support system that recognizes the complexities of bridge management. A bridge management system (BMS) supports policy and programming decisions by predicting the engineering and economic outcomes that may result from those decisions. To do this, a BMS incorporates deterioration models, cost models, business rules for treatment selection and costing, and an analytical framework for calculating and presenting information relevant to the decision at hand. In Ontario, nearly all project-level decision-making in bridge management is performed by structural engineers, based in MTO's five regions, which conduct biennial inspections and plan future work. The new BMS is therefore designed to satisfy the decision support needs of these engineers, by providing life-cycle cost and tradeoff information in the field while the engineer visits each bridge for inspections. This paper describes the engineering models used in project-level analysis.

Ongoing efforts to develop a comprehensive road structures management system in Switzerland have resulted in a project called KUBA-MS. The system is based on ideas underlying the AASHTOWare^TM Pontis System but distinguishes itself in several features, reflecting differences in Swiss and US maintenance practices. The paper describes the methodology adopted in the software from user perspective. The general structure of the system and some distinctive features are outlined.

Monitoring structures such as bridges, dams, and buildings has received significant attention in the last decade. Several owners and researchers are actively pursuing development of monitoring systems, with reliable sensor technologies and remote monitoring systems for decision making. Most of the system identification process thus far is concentrated in three phases, namely data acquisition, structural identification, and damage detection, and lacks decision-making phase. This paper presents a comprehensive view on the engineering of structural health process integrating the decision making process.

Reliability of remote bridge monitoring systems (RBMS) for long-term health monitoring, and variability of modal parameters to operating conditions have not yet been addressed in detail in the literature, but are of major concern to many bridge owners. In this regard, New York State has installed RBMS on three bridges in the past, to study the reliability of then state-of-the-art monitoring equipment and sensors for long-term bridge monitoring. These collected data have been used to extract modal information from recorded transient signals for their use in long-term health monitoring of bridge structures. This paper addresses the signal processing methods used in this study and discusses the results.

The monitoring system SMS 2001 was developed in co-operation with the Infokom GmbH Neubrandenburg as an academic-industrial partnership especially for the needs in long-term observations of infrastructure buildings, mainly with respect to realize a supervised life time extension. On some bridges and industrial buildings such systems have been installed, when preceded advanced loading test has been finished successful. The modular constructed intelligent system allows to be used in two different modes, long-term and event dependent measurements. A larger number of different kinds of sensors can be included. Interactive communications between system and the central office can be made for all important reasons. After discussion of main demands on such systems the structure of SMS 2001 and two-year practice experiences are dealt with. Finally an overview on main conclusions for preparing the next generation of this system is given.

Following the Mianus River Bridge collapse in 1983, the State of Connecticut began a concerted effort to develop and use remotely accessible monitoring systems placed on bridges to continuously evaluate their movements. This is being done to answer questions on the performance of existing bridges, to refine techniques and equipment which could be used in the evaluation of different bridge components and to develop approaches that could be used to provide a continuous indication of a bridge's structural integrity. This work has led to a federally funded research project to implement remote monitoring systems on several bridges in the State. The first of these systems has been installed on a continuous three-span, post-tensioned curved concrete box-girder bridge. The remote monitoring of this structure is accomplished with sensors that measure strain, tilt, temperature and acceleration. This system is used to describe the behavior of this structure and to provide structural performance information. Considerations include short-term monitoring to explore the live load and temperature affects and long-term monitoring over the course of several years to examine variations in the structural behavior.

Genetic Algorithms (GA) are powerful tools for solving large and complicated optimization problems. Objective functions used in parameter estimation (PE) are commonly nonlinear due to available measurements at a limited number of degrees of freedom for a structure. Sparse measurements create a fairly complicated objective function surface that requires a robust algorithm to find its global minimum without converging to a local minimum or diverging. This paper examines the potential of genetic algorithms to find the global minimum associated with modal stiffness and flexibility based objective functions used in PE. A finite element model of a six-story two-parameter shear building is used for this study. Using three-dimensional plots of the two objective functions, few cases with complex surface and several local minima are selected. FlexGA^TM genetic algorithm software is then used to estimate parameters of the model. The overall performance of GA to find the global minimum for these cases is compared with gradient-based optimization methods (commonly referred to as hill climbing, HC). It is concluded that GA's performance in locating the basin of the global minimum is superior to HC. It is further recommended that when GA converges, switching to HC yields more accurate parameter estimates.

Identification of damage location based on modal measurements is an important problem in structural health monitoring. The damage index method, which attempts to evaluate the changes in the modal strain energy distribution, has been found to be effective under certain circumstances. In this paper a new damage index method based upon strain and kinetic energy is proposed and compared with the former. The objective is to evaluate the feasibility of the damage index method to localize the damage and estimate the severity using FEM simulations of a bridge. Finally this method was used to predict the damaged location in an actual structure, using the results of a modal survey from a large concrete bridge.

A multiplexed Bragg Grating optical fiber monitoring system was designed and retrofitted to an in-service interstate highway bridge. The structure is a two span continuous, composite steel girder bridge with a reinforced concrete deck. A network of 64 sensors was used to measure the strain at several locations in the bridge, with sensors bonded to the top, bottom flange and web of the girders. The installed system was used for a dual purpose: structural evaluation as well as traffic monitoring. Vehicle speed, vehicle count, and traffic statistics are extracted from the data. A fatigue analysis of the bridge is performed.

A distributed fiber optic sensor is developed for embedment in concrete structures. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The study reported here is aimed at demonstrating the applicability of this sensor in monitoring cracking of concrete structures.

Optical sensor technology is a potentially important element in the development of a bridge monitoring system. The present research has utilized the inherent characteristics of fiber-optic cable to develop a variable gage-length interferometric sensor. The major objective of the research was to develop a sensor for detecting changes in bridge structure dynamic response to be used in conjunction with pattern recognition techniques. Because sensor location is critical when using point sensors, long gage-length sensors are expected to be more effective in detecting changes in global dynamic response, effectively integrating the response along predetermined lengths of the structure.

We explore current advancements in radiographic and computer technologies that greatly improve performance characteristics for industrial computed tomography (CT). Non-film radiography methods, necessary for CT, have improved significantly and personal computers and software are advancing at an unparalleled rate. By combining these advancing technologies, we construct low cost, fully functional CT systems with high data acquisition rates. Normally, CT scans are made on a line by line basis, requiring constant axial repositioning of the specimen or x-ray equipment. However CCD cameras and scintillator screens allow for image plane views of the specimen—a collection of image plane views is equivalent to an entire series of line scans—to be made and thus have greatly reduced or eliminated the need to axially reposition. A consequence of collecting image plane views of a specimen, as opposed to line scans, is that the total data acquisition time can be reduced by up to a factor of one hundred. The next generation of technology is based on flat panel x-ray detectors that have a number of important advantages over the scintillator-optics-CCD approach. Additionally, flat panel detectors are robust under several source types, which makes this CT system practical for many applications. The rapid enhancements of personal computers (PC) and PC hardware and software have also contributed to advancements in CT technology. Hardware improvements result in efficient data acquisition, fast data transfer, precision staging and real-time high-resolution displays. Software improvements result in better hardware control, efficient data processing and functional graphical user interfaces. In this paper will further discuss the advancements in emerging CT technology and offer comparisons to past systems. We will also present results from various specimen tests and conclude with examples from recent tests.

The design and construction method for the proposed concrete alternate for the US Grant replacement bridge is described. The proposed design is a precast segmental box girder structure with a single plane of stay cables along the centerline of the structure. The proposed bridge is a unique unsymmetrical bridge with spans of 350'/875/452', the unequal side span being a result of geometric constraints. The design focuses on providing a robust and serviceable bridge, while maintaining an optimized design that is cost competitive with a steel alternate cable stayed bridge prepared for this site.

The Port Authority of New York & New Jersey is building an elevated light rail system at John F. Kennedy Airport that is supported by 13.5 kilometers (8.7 miles) of precast segmental superstructure. This is the first application of precast segmental construction for a project in the New York City area. The light rail system loops around the terminals in the Central Terminal Area (CTA) of JFK Airport and connects with spurs to Howard Beach and the NYC Subway and along the median of the Van-Wyck Expressway to Jamaica Station where it meets the Long Island Railroad. Eventually, it will connect with the LaGuardia Airport and ultimately with Manhattan. The current project for elevated structures, stations, trackwork and trains is worth about $1 billion of which approximately $100 million is for the precast elevated structure.

The Woodrow Wilson Memorial Bridge is the only Potomac River crossing in the southern half of the Washington Metropolitan area. It carries the Capital Beltway (I-495), which is a part of I-95, the main north-south interstate route on the East Coast. The current bridge is a six-lane structure with a bascule span that accommodates shipping to Alexandria, Virginia and the District of Columbia. Built in the early 1950's for a daily volume of 75,000 vehicles, it is rapidly deteriorating under the present daily traffic volume of 175,000 vehicles. Replacement of the structure is estimated to be necessary by 2004, or weight restrictions may have to be imposed. The new structure will be a part of the monumental core of the Washington Metropolitan area. Approximately 6,000 feet long, the new bridge will be substantially wider than the existing bridge to accommodate the traffic growth. This project has raised many concerns on the part of community groups and agencies regarding visual and urban design effects on the historical, architectural and community resources.

The seismically vulnerable East Span of the San Francisco-Oakland Bay Bridge will be replaced with a dual east bound and west bound 3.6 km long parallel structure. The cost of the replacement bridge is estimated at $1.5 billion and the bridge will be constructed by the year 2004. The Bay Bridge lies between the Hayward and the San Andreas faults which can generate magnitude 7.5 M and 8.1 M earthquakes, respectively. Performance criteria require that the bridge must be operational immediately following a 1500-year return period earthquake from either of these two faults. Four distinct structures will make up the bridge crossing: a low rise post-tensioned concrete box girder near the Oakland shore; a 2.4 km long segmental concrete box girder; a self-anchored suspension signature span; and a post-tensioned concrete box girder that connects to the east portal of the Yerba Buena Island tunnel. This paper focuses on the design parameters and constraints, the design evolution, the bridge type selection process, the selected design's major structural and architectural characteristics, and the structural details.

New York State installed its first Fiber-reinforced-polymer (FRP) composite bridge in October 1998. Since FRPs are relatively new to such civil engineering applications, a comprehensive test program, comprising of load tests and visual inspections every six months, and detailed finite-element analysis were planned to monitor the structure and also to evaluate its long-term durability. This paper will describe the bridge design details, load testing procedures, and results from the first two load tests. The data presented in this paper can be used to refine the design of future FRP bridges.

Continuous, long-term monitoring systems have been designed for two polymer composite bridges recently built in Delaware. The Magazine Ditch bridge has been continuously monitored for more than a year. The monitoring system provides data to investigate the effects of sustained load, environmental factors and live load on the bridge. Early results show that daily and seasonal temperature changes can induce strains in the bridge that are equal in magnitude to the maximum live load strains. A similar system has been designed for the first State owned composite bridge in Delaware. The monitoring system for bridge 1-351 is expected to be on-line by the summer 2000. Presented in the paper is a brief overview of the systems and sample results from the data collected for the Magazine Ditch bridge.

This paper describes application of Fiber Reinforced Polymer (FRP) composite laminates to strengthen a seventy-year old reinforced concrete T-beam bridge in Rensselaer County, New York. Leakage at the joints of this single-span bridge lead to substantial moisture and salt infiltration in the superstructure — manifested in efflorescence, freeze-thaw cracking, and delamination at some locations. Integrity of the steel reinforcing and overall safety of the bridge was suspected. This concern was heightened by the absence of any documents pertaining to the bridge design, such as bar size, steel type, concrete strength, and design loads. The bridge was strengthened using bonded FRP laminates. Load tests were conducted before and after the FRP laminates were installed to assess effectiveness of the strengthening system. Test results are also compared with those obtained analytically using classical approaches.

The Constructed Facilities Center at West Virginia University developed a glass fiber reinforce polymer composite hexagonal deck system through a pultrusion process. This developmental work was carried-out in cooperation with the US Dept. of Transportation's Federal Highway Administration and West Virginia Department of Transportation-Division of Highways. Laurel Lick Bridge and Wickwire Run Bridge built in West Virginia in 1997 have been field tested to establish deflection response and load distribution factors. In addition, simple theoretical computations were carried-out using orthotropic plate theory, resulting in excellent correlation with field data.

Cities have evolved throughout history because they are solutions. They cluster areas of work and housing for ease of communication and trade. They provide essential cultural, educational, and entertainment opportunities. The tall building is a key player in the city. In fact, in the largest cities, they are the major driving force with regard to the urban infrastructure. This paper provides examples of both the "good" and the "bad" with respect to the changing scene of the city during the Second Millennium and with particular reference to the high-rise. One finds that proper attention to infrastructure (primarily transportation), to environmental factors, and to the integration of housing within the urban core makes all the difference. The debilitating effect of uncontrolled fringe development stands out as a major warning to planners of cities of the Third Millennium. What makes a city work? That's the question.

The Arabian Tower is a 321m high, 7 Star hotel located on the shore of the Arabian Gulf, 15km south of the city of Dubai, in the United Arab Emirates. It forms the centrepiece of the world class Chicago Beach Resort Development, which also includes the 5 Star, 600 bedroom, Jumeirah Beach Hotel, and the themed, 15 acre Wild Wadi aquapark. The Tower is, both internally and externally, an extraordinary building. The structural framing includes dramatic, bold forms, which are both part of the architecture and demonstrate structural design of a high standard. This synergy of innovative yet efficient structural design with dramatic architecture, has produced a striking building that is currently the tallest hotel in the world. Internally, the 202 super luxury suites, each with a minimum floor area of 169m², are arranged in a V formation, either side of a 180m high atrium, one of the world's largest. The Arabian Tower is designed to reflect the seafaring heritage of Dubai, resembling the billowing sail of a traditional Arabic Dhow. This has been achieved both by utilising the profile of the building and by integrating the architecture and the structure so that each complements and takes advantage of the other. The rigging of the sail is expressed as a clad framework, external to the building façade. This exoskeleton, however, is not merely decorative, it forms an integral part of the horizontal stability system of the structure. The desire to give the front elevation of the building the appearance of a sail led to the innovative use of a structural Teflon coated fibreglass fabric as a tensioned membrane façade. The siting of the Tower on a man made island 300m offshore not only reinforced the sail concept but also had the practical benefits of freeing up more space on shore for the other parts of the development and of reducing the impact of the Tower's shadow on the Resort's beach. WS Atkins & Partners Overseas were appointed by the Client to provide a complete multi-disciplinary design service, including architecture, civil, structural and building services engineering, interior design and landscape architecture. WSAPO were also awarded the commission for the Construction Management of the entire development.

Design-build has been around for a number of years in one form or another. The most recent trends take on many different forms. All of us as structural engineers will be required to adapt our practices and participate or be left out. This paper includes the perspective of a principal and a firm that has been extremely active in design-build in the last several years. The experiences that are described will help consulting engineers avoid some of the pitfalls which can trap the uninitiated. The projects described include those for several public agencies as well as for private industry.

In the spring of 2000 the American Institute of Steel construction will publish a fully revised code of standard practice for steel buildings and bridges. The committee of 23 individuals has fully reviewed and made substantial revisions. Of interest to structural engineers is that, for the first time, the structural engineering community was represented on this committee. As a result a number of important revisions are significant for the structural engineering community. Sections have been added to the code to address in more detail the responsibilities of the various parties including the fabricator. Responsibilities for work in preparation for the erection of Steel in existing structures have also been addressed. More detail has been added with regard to requirements for design drawings and specifications. Responsibility for connection design has also been addressed along with more detail regarding shop drawing procedures. This paper will address the above subjects from the perspective of the structural engineer. While there are numerous other changes, these are of particular importance to our profession.

For more than 100 years, licensing of construction industry design professionals has served well to protect the public. In combination with building codes, licensing laws have reduced the number of structural failures to a very low level. In recent years, work done by structural engineers across state borders has increased. Because licensing laws are currently enacted on a state-by-state basis, practice across state lines is becoming impeded unnecessarily. This is because adjacent states often enact laws that have small unimportant differences thereby impeded cross-border licensing and restraining trade. This paper provides a model that would further increase the protection of the public while eliminating current restraints of trade. Through use of Federal legislation regulating trade, it is proposed that a structural practice act be established. Those who qualify under this act would be able to practice structural engineering in all jurisdictions.

The current demand for structural engineers trained to practice in a global economy coupled with the need for higher levels of expertise in various civil engineering specialties is forcing civil engineering faculties and technical societies to reexamine the objectives and role of their programs for addressing these needs. Issues related to accreditation, minimum entry level education for practice, ability to utilize effectively advanced communication technologies, and the attraction and retention of undergraduate students to civil engineering will be discussed.

Codes and standards for structural design are prescriptive, quantitative, and detailed, and in most instances they have served the profession reasonably well. During the past decade, however, a number of advances and significant changes have occurred in building design and construction. The performance concept provides a new design paradigm for responding to new building technologies and increased public expectations. When performance goals are expressed for different occupancy classifications in probabilistic terms, performance-based design becomes a vehicle for implementing risk analysis tools directly in structural design of new buildings and evaluation of existing facilities.

A structural deterioration life-cycle reliability model is used herein to calculate probabilities of cracking and spalling for typical RC structures. The influence of mix composition (water-cement ratio, concrete compressive strength) and cover on corrosion rates are included in the analysis. Life-cycle costs are estimated using time-dependent probabilities of failure calculated for annual increments over the lifetime of the structure (100 years). It was found that reducing the cover from that currently specified for design results in a significant increase in life-cycle costs.

Field testing of old concrete slab bridges designed for smaller truck loads shows that these bridges possess much higher strength than that indicated by current rating procedures. Extensive experimental and analytical investigations were carried out to evaluate the behavior of concrete slab bridges. Information gained from conducting the experimental and numerical analysis is used in this paper to develop a state of the art method to rate concrete slab bridges using a probabilistic approach. Monte Carlo simulation technique is used to develop probability density functions (PDF) for both load effect and resistance. The PDFs are then used to develop the probability of failure of the bridge. Results of this investigation indicate that concrete slab bridges, including those originally designed for H15 loads, have large reserve capacities and most of these bridges could carry modern traffic loads.

Evaluation of concrete bridge decks is performed due to natural aging, increasing load spectra, deterioration caused by fatigue, freeze/thaw cycles, shrinkage and temperature gradient. Based on field observations and tests, the analysis is provided for different damage scenarios. The FE model is built to show the behavior of whole superstructure and to check the influence of particular parameters affecting deck degradation. Model is calibrated using strains and deflections measured on a real bridge.

In this paper, the long-term performance of eccentrically-loaded high-strength concrete (HSC) columns under sustained loads is evaluated. The column performance is seen from the column reliability viewpoint and a probabilistic analysis is used to assess column performance. The reliability of reinforced concrete short columns representing normal-high-, and very high-strength is investigated. Also, the effects of the concrete compressive strength, amount of longitudinal steel, load eccentricity, and sustained load ratio (ratio dead load to total load) on the column reliability are evaluated. It was found that all these parameters have a considerable impact on the resulting column reliability. Regarding the influence of the concrete compressive strength it was found that the higher the concrete compressive strength the smaller the column reliability. Also, very low reliability levels were found for the combination of higher strengths, higher sustained load ratios, higher load eccentricities, and minimum amount of longitudinal steel.

Wind-induced loads on a building are invariably spatially non-uniform and some twisting about its vertical axis results. The magnitude of this wind-induced moment or torque depends on the shape of the building, the influence of surrounding buildings and the direction of the wind. Even for symmetrical plan forms such as a squares or a rectangular in homogeneous surroundings, torsional forces occur when the direction of the wind is not along a line of symmetry. The wind-induced torque is accentuated in the presence of eccentricities between the effective centres of rigidity, mass and aerodynamic force and for buildings with relatively long torsional periods of vibration. Torsional effects can substantially increase wind-induced loads on the main wind force resisting system; accentuate the racking of exterior walls and therefore influence the detailing of the curtain wall system; and add to the wind-induced motions, which if excessive can adversely affect occupant discomforts. Despite its importance, wind-induced torsion is overlooked or at best dealt with in a nominal fashion by most building codes. The National Building Code of Canada and more recently ASCE Standard 7 require designers to consider partial as well as full wind loading. In the partial load case, the wind loading on any portion of the building is reduced by 25% for buildings over 60 feet. This results in a wind- induced torque which corresponds to a base shear eccentricity of just under 4% of the building width. This is far short of actual eccentricities for some buildings, predicted from wind tunnel model tests and empirical data bases. While the effects of torque on the structural frame are seldom dominant, they can substantially alter the distribution of the wind-induced forces. If not allowed for in design, this has the effect of diminishing the effective safety index of some parts of the structural frame. Torsional load effects are not the domain of tall buildings alone and significant eccentricities of wind-induced forces also occur for low and intermediate height buildings. The effects of wind-induced torque are discussed for buildings of tall, intermediate and low height.

The wind-induced torsional response is important for the design of tall buildings in terms of both serviceability and survivability. Nevertheless, current codes and standards provide very little guidance in this regard. This paper aims to provide a framework for the formulation of torsional load effects for possible inclusion in codes and standards. The formulation is based on the high frequency force balance (HFFB) technique that has generally been recognized as an effective wind tunnel technique. The major shortcoming of this technique is that it provides mode-generalized torsional loads associated with a uniform mode shape. This paper first discusses the effects of a non-uniform mode shape on the results. An example from a database derived from the HFFB measurements of a host of prismatic and a group of unconventional building configurations is presented. Based on the aerodynamic force information, a "gust loading factor" (GLF) formulation is developed for estimating the torsional response. It is envisaged that the proposed formulation will be useful for inclusion in codes and standards along the format of the along-wind response. An example is presented to illustrate the effectiveness of the proposed procedure and the significance of torsional load effects for design application.

Torsional wind loading on buildings is not as well understood as lateral or overturning loads, and is not as amenable to analytical treatment. Because of this, many designers—and indeed most codes of practice—ignore this aspect of the load, and simply apply the lateral load at the geometric or elastic center of the structure. Wind-tunnel tests on model buildings have revealed that torsional loads usually exist, and span a great range of significance. Several causes of the torsion can be identified, and this categorization aids the designer in providing methods to either reduce or manage the loading. This paper identifies some common sources of torsional loading in terms of building shape, interfering effects of nearby buildings, and dynamic characteristics of the structural frame. In addition, it is shown that torsional loading is routinely larger than that provided for in most standards.

Key factors which govern the wind-induced torsional response, especially the torsional loads, are discussed in this paper. These factors include building geometry of the floor plan and of the elevation, building surroundings, and structural dynamic properties. Due to the complication and variation of these factors from case to case, the presented discussion is focussed on the floor plan geometry for torsional static loads, and natural frequency and dynamic coupling for the torsional dynamic loads. With definition of equivalent eccentricity, e, and load reduction factor, r, the magnitudes of torsional loads for several tall buildings are investigated with respect to the corresponding horizontal wind loads. The complicated nature of torsional response also affects the precision of wind tunnel tests, although wind tunnel tests are still considered to be the most reliable approach. Normally, the torsional response cannot be measured with the same degree of completeness as for sway response using the wind tunnel force balance model. Selecting an appropriate wind tunnel method for torsional response is therefore also considered to be a key factor. With discussion on the recent developments in wind tunnel technique, guidance for selecting the wind tunnel method is suggested.

Many local U.S. Building Code requirements for wind loads do not address the issue of unbalanced wind loads. Typically code-defined pressures are assumed to be uniform and applied at the center of the projected building façade. Unbalance loading, while required for most other types of loads, is not required. With the unusual geometric shapes given to modern buildings, this can lead to unconservative design approaches for mid-rise and high-rise structures. ASCE 7-95 and the National Building Code of Canada do address this issue. Their requirements along with the resulting load unbalances for a specific project are discussed. The results of wind tunnel studies along with current design recommendations from one laboratory is also presented and compared to the current national standards. Strength vs. serviceability requirements is also discussed with respect to unbalance load requirements.

Building codes typically specify the pressure coefficients or shape factors for determining the wind forces on buildings and structures, as well as, the peak pressures and suctions which act on the components of their exterior envelope. Even though these are selected to envelope wind action in most situations, the requirements of many tall buildings fall outside code provisions. Most building codes recognize this difficulty and permit and, in some circumstances, even require special wind tunnel tests or other studies in unusual situations. Tall buildings, which have been wind tunnel tested over the past 30 years or so, number in the thousands. The motivations for such studied invariably include concerns for integrity, performance and economy. Some areas, where wind tunnel based studies can improve design, are discussed below:

Background to the generation of perceptible accelerations in tall buildings is developed to demonstrate that these tend to be dominated by the cross-wind response to wake, vortex, excitation. A sensitivity analysis demonstrates the dependence on wind speed, damping, building massiveness and the cross-wind force spectrum. The remainder of the paper gives guidelines to the way in which the shape or configuration of a building may be directed to achieve reductions in the cross-wind force spectrum and hence reductions in acceleration response.

There is a myriad of constraints to be addressed in the design of the tall building curtain wall, and the response to each contributes to its appearance and performance. Wind, its forces and flows, defined through wind tunnel testing is one such constraint. This paper outlines the impact wind has on the curtain wall design process as it relates to resisting wind on the one hand and harnessing it to benefit on the other. As to other constraints, seismic design procedures deal with displacements on the order of 1/50 of the floor-to-floor height on occasion resulting in 75mm or greater vertical joints near the building corners. Energy conservation limits on heat gain/loss translate to average vision areas of approximately 50% of curtain wall area, the remainder being opaque glass, metal, stone or some combination thereof. Building maintenance equipment explains the presence of vertical rails or the pattern of tie back buttons. Concern for the formation of ice that might join the wind flow and possibly cause injury to people and property below is among the reasons for the oft-seen flush curtain walls. And control of vertical flame spread often translates into rated spandrel construction of approximately 1 meter in height.

Several recent projects for high-rise building designs by the Chicago office of Thornton-Tomasetti Engineers (T-T) involve significant collaboration between the structural engineer and the wind engineering consultants. One of these projects is "Project 2000", a mixed use development to be proposed for Chicago, Illinois. This paper is intended to illustrate the importance of wind tunnel consulting in the design of tall buildings, particularly the value in the initial design phases of a project.

The building of many large buildings in close proximity in large cities raises some issues to do with wind that have received little public discussion. When a new building is inserted into the existing conglomeration of buildings it changes the wind flow patterns around those that are nearby. These effects tend to diminish with distance from the new building but within the nearest few blocks the changes in flow patterns can be expected to affect the following: wind speeds at ground level; wind loads on the exterior cladding of nearby buildings; wind-induced loads on the main structural systems of nearby buildings; wind-induced motions of nearby buildings; dispersion of air pollution in the area surrounding the new building; and the accumulation of snow in drifts, and therefore the snow loads, on nearby roofs.

The design of tall buildings is predominantly governed by the need to provide adequate strength and stiffness against lateral dynamic forces induced by wind, and to satisfy stability and serviceability conditions. While serviceability requirements are fulfilled by limiting lateral displacements (drifts) to maintain the integrity of architectural elements such as cladding and partitions; the minimization of perceptible accelerations assures occupant comfort. These conditions are directly affected by the dynamic and aerodynamic characteristics of the building and the severity of the local wind climate. The design process involves the selection of an appropriate structural system, and the evaluation of that system under design wind environments utilizing code, analytical methods, and wind tunnel testing. This may be an iterative process for exceptionally tall, slender, or unusual building forms. Interestingly enough, while tall building structures serve as one of our most vital constructed facilities, the design process is based almost solely on the information provided by analytical and scaled models. Understandably, while full scale models are not feasible, considering their sheer size and cost, monitoring the performance of actual structures is paramount and must be undertaken following construction as a means of verification and improvement of current design practices and analytical models.

Post-modern architecture has moved significantly from directly expressing the structural systems of tall buildings, which symbolized their strength and stability. Tall buildings have become more slender, require large open views along their perimeter, include multiple setbacks, and have been proposed for regions with potentially large wind forces and seismic events. These factors have influenced structural system selection significantly. In an effort to reduce the structural cost, and to resist overturning forces, several structural systems have been proposed that are nearly determinate and thus have relatively little redundancy. These structures are highly efficient, from a material expenditure standpoint. The issue of redundancy as it relates to the underlying robustness of the structural system has traditionally been left to the discretion of the individual engineer. Unlike highly redundant framed tube structures such as the Sears Tower, Chicago, and the World Trade Center Towers, New York, unpredictable events and design forces, including acts of terrorism, could have a large impact on these nearly determinate systems. Research is needed to study the approach and philosophy that engineers should adopt to evaluate the redundancy and robustness of these systems and to establish design criteria.

One of the consequences of Hurricane Andrew in southern Florida during 1992 is that many condominium buildings in the fifteen to thirty story range are now being designed with balcony- edge storm stutters. These shutters are intended to reduce the devastating damage that strong wind and rain can have when the building envelope is breached. In fact, recent experience in the United States suggests that residential damage claims are 40% larger when the building envelope is breached. The increase in internal pressure after cladding failure can create other cladding breaches on that floor or in that apartment. These events are frequently the cause of massive insurance. With the shutter option the prudent cladding designer is now interested in two physical geometries: (i) the open balcony condition with no storm shutters present and (ii) the building with slab-edge storm shutters installed. The latter effectively changes the shape of the building from a rough-surfaced collection of protruding balconies to a much cleaner, often rectilinear, structure. The purpose of this paper is to demonstrate the significant differences in cladding pressure for the two building configurations.

In mid November of 1972 Mr. Albert Prouty, Vice President in charge of the construction of the 795-foot John Hancock Tower in Boston contacted me. Windows were still being installed near the top of the building. He asked me to look at an area where some windows had apparently been blown out by the wind. Figure 1 is an Isometric Schematic of the building. The area of blown out windows on the Stuart Street façade is indicated. Figure 2 is a map of the local area and includes a roof plan of the building. The heights of surrounding buildings in stories are also indicated. Most windows in the area shown in Figure 1 had been replaced with plywood, because the windows had broken in strong SW winds even after being replaced. Also, windows in other areas had cracked. A wind tunnel test had been run to determine the design loads for the windows, but the windows had been designed to Boston Code. I obtained a copy of the wind tunnel report.

On a cold and windy night in Boston almost 27 years ago, the curtain rose on one of the most extraordinary glass facade failures the building industry has known. The investigations that ensued followed varied and diverse paths but ultimately led to verifiable and well-founded conclusions. Unfortunately, that truth was lost in legal protectionism. But even such impediments run their course, and the time has come to lay the facts on the table. From the beginning, the John Hancock Tower was controversial. The very concept of placing a 790 foot tall tower next to one of the most precious architectural works in Boston, the Trinity Church designed by H.H. Richardson, constituted, to some, a monumental failure in city planning. But the chief designer, Henry Cobb of I.M.Pei, defended the appropriateness of this project, engaged the Boston architectural community in the debate, endured his critics and brought his vision for the architecture of Boston's Back Bay to fruition. The tower survives as a beautiful modern spire, still deferring to its stately neighbors by reflecting their grand images.

The behavior of structures under the effects of impact and explosions is one of the last frontiers in structural dynamics. Many serious difficulties exist in material modeling, load definition, and selecting effective approaches for addressing the behavior of complicated geometrical domains in a structure. Significant success can be achieved by including careful modelling of structural resistance mechanisms and insuring a physics-based approach in the numerical simulations.

Terrorist-bombing events throughout the world have demonstrated the vulnerability of conventional reinforced-concrete buildings to blast effects. Typical columns and floor slab systems are not designed to resist the complex blast loading, such as uplift or reverse loading of floor slabs and combined lateral and tensile loading of columns. Two-story, quarter-scale reinforced-concrete models were used to investigate the blast response of a typical flat-plate structural system in support of the Technical Support Working Group (TSWG), Blast Mitigation of Structures research program. Experiments were conducted on five models, allowing a variation in explosives standoff and cladding configuration. The High-Performance Computing facilities available at the US Army Engineer Research and Development Center (ERDC) Major Shared Resource Center were used to perform analyses to support the TSWG quarter-scale experiments. Analyses were performed to predict the response of the exterior column for different ranges and exterior wall conditions.

Procedures for developing retrofit designs for reinforced concrete (RC) columns and slabs to improve their survivability to blast loads are described. These procedures allow conventional RC columns, even ones designed with little lateral resistance, such as in seismic zone 1, to survive large explosives loads at very close standoffs. The design procedures for blast loads were adapted from those already used to develop seismic retrofits for RC columns but differ because they consider both shear and bending failures and include effects peculiar to blast loads. The effectiveness of the resulting retrofit designs is demonstrated through high-fidelity physics based calculations using the PRONTO3D finite element code and through comparisons to experimental data.

Retrofit methods are being developed to make conventional structures more resistant to the effects of explosive attacks by terrorists. Past efforts have concentrated on retrofitting columns and exterior walls. However, the structural roof system must also be evaluated and possibly upgraded to be consistent with the protection level provided by the retrofitted walls and columns. An experimental/analytical program has been performed to determine the response of a lightweight steel roof system to blast loads. The experimental results have validated the analysis methodology. A method of designing retrofits has been developed and evaluated against experimental data; this method will be extended to other roof systems.

This paper will examine relevant issues in the development of probability-based design guidelines for conventional wood-frame roof systems in high wind regions. Relevant limit states, performance criteria, load combinations, and target reliabilities will be discussed. The objective of this paper is to identify some of the key issues in developing risk-consistent design criteria for conventional wood-frame roof systems, thereby laying the groundwork for future study.

The CUREe-Caltech Woodframe Project is currently underway in California as a combined research and implementation project to improve the seismic performance of woodframe buildings, a need which was brought to light by the January 14, 1994 Northridge, California Earthquake in the Los Angeles metropolitan region. The project, funded by FEMA, has five main elements, which together with a management element are designed to make the engineering of woodframe buildings more scientific and their construction technology more efficient for targeted seismic performance. The main research components of the project are included in the Testing and Analysis Element managed at the University of California, San Diego. The objective of this paper is to briefly describe the projects currently underway in the Element 1 - Testing and Analysis of the CUREe-Caltech Woodframe Project. The other elements of the CUREe-Caltech Woodframe Project are Element 2 —Field Investigations, Element 3 — Codes and Standards, Element 4 —Economic Applications, and Element 5 — Education and Outreach.

Simplifying assumptions in the analysis and design of light-frame wood buildings can result in either uneconomical or unsafe structures. This paper describes the development of improved tools for performance analysis of light-frame buildings under wind and earthquake loads that can be used to improve current design procedures. We present herein the details and preliminary test results of a full-scale one-story house tested under static cyclic loading. The irregularly shaped house is being used to experimentally validate a three-dimensional Finite Element (FE) model of the house. Once validated, the whole building FE model will be used to develop improved design procedures that take into account systems behavior. Sample analysis results are presented.

This paper presents an evaluation of conventional residential construction with respect to the actual performance of single-family detached dwellings subjected to extreme earthquakes and hurricanes. Design and construction implications are also addressed in a scientifically rigorous approach based on statistical sampling of the affected housing populations, engineering analysis, and fragility modeling. Such an approach is believed to be an effective means to identify real performance issues, evaluate the strength of cause-and-effect relationships, and properly focus attention on cost-effective solutions to performance problems that will improve future design methods and construction practices.

Inelastic dynamic time-history analyses of nine steel frames were performed to understand effect of ground motion characteristic on structural response. Particular ground motion characteristics studied were the effect of near-fault ground shaking, soft-soil shaking and vertical shaking. Record frequency content and magnitude effects were also investigated. It was found that near-fault strike-normal records caused demands as high as 6.7%, residual displacements were approximately 52% of the peak inelastic displacements, and the SRSS method for predicting the maximum increase in column axial force for simultaneous vertical and horizontal shaking was non-conservative due to frame inelasticity. Soft-soil records increased displacements of a 3 story frame by up to 56% above those from design level motions. Drifts did not increase linearly with increasing earthquake ground acceleration and showed significant variation depending on the earthquake record. Changing the record frequency content while maintaining the spectral response at the fundamental period of the structure affected the story in which peak drift demand occurred.

This paper presents a summary of two year long comprehensive parametric study of the impact of various deterioration characteristics on seismic response of moment resisting steel structures. More than 14,000 nonlinear analyses were performed utilizing more than 500 earthquake time histories. The results indicate that various hysteretic deterioration types affect seismic demand characteristics differently. Three types of hysteretic deterioration were considered: stiffness, slip or pinching, and strength degradations.

This paper summarizes the results of current research, sponsored by the SAC Steel Project, on the reliability of nonlinear static methods for predicting the seismic performance of steel moment frame buildings. As part of previous SAC studies, three steel moment frame buildings (3, 9, and 20-story), located in Los Angeles, were designed using Pre-Northridge earthquake connection details. Two DRAIN-2D models (M1 and M2) were created for each building. Model M1 is a centerline-to-centerline model, while model M2 explicitly accounts for the strength and stiffness of the panel zone and represents the more accurate model. Nonlinear dynamic time history analyses were performed for each building model using a total of 60 earthquake ground motions with seismic hazard levels having a 2%, 10%, and 50% probability of exceedance in 50 years. In the current study, nonlinear static "pushover" analyses of the buildings were performed with the same models and ground motions used in the nonlinear time history analyses. The Coefficient Method, Capacity Spectrum Method, Equivalent System Method were used to calculate building performance response quantities. The maximum roof displacement and maximum interstory drift response for models M1 and M2 were compared with the model M2 results from the nonlinear dynamic time history analyses.

A reliability-based performance based method for performance prediction for steel frame buildings was developed for the SAC Phase 2 research program. A load and resistance format was developed where the variations due to randomness and uncertainties in the loads and resistances are accounted for explicitly. The confidences in achieving the performance objectives are also calculated.

Given the uncertainty in the timing, location and magnitude of future earthquakes, for most engineering purposes it is most meaningful to use a probabilistic approach to characterizing the ground motion that a given site will experience in the future. A probabilistic seismic hazard analysis takes into account the ground motions from the full range of earthquake magnitudes that can occur on each fault or source zone that can affect the site. The probabilistic approach is very compatible with current trends in earthquake engineering and the development of building codes, which have embraced the concept of performance based design. In contrast to the traditional building code approach, performance-based design requires an explicit prediction of the structure's performance at each of several ground motion levels corresponding to a set of performance objectives. Each performance objective is associated with an annual probability of occurrence, with increasingly undesirable performance characteristics caused by increasing levels of strong ground motion having decreasing annual probability of occurrence. Performance based design requires a more comprehensive representation of ground motions than has conventionally been used. For example, ground motions estimates are needed at multiple annual probability levels, and may need to be specified not only by response spectra but also by suites of strong motion time histories for input into time-domain non-linear analyses of structures, because response spectrum analysis is based on a linear elastic model and therefore does not address the non-linear response that is the essence of building damage and failure.

Near fault earthquake shaking may cause structures to experience inelastic displacement demands which are significantly greater than that expected from design level earthquakes. This paper describes the inelastic demands for elastic-perfectly plastic single-degree-of-freedom oscillators to actual recorded earthquake motions. Inelastic response spectra from sites at known locations relative to the fault are used to quantify the inelastic demands in terms of a relationship between lateral force reduction factor, period and ductility. For medium and long period oscillators the severity of inelastic demand increased as the distance along the fault from the epicenter in the direction of rupture increased. Short period oscillators were relatively unaffected by the near-fault shaking effects.

A new format for representing inelastic response spectra is described. The format is similar to that used in the Capacity Spectrum method, but is simpler to use in practice. Like the Capacity Spectrum method, this format allows peak displacement response to be estimated. However, instead of relying on assumed relations between hysteretic response and "equivalent" viscous damping, Yield Point Spectra (YPS) directly apply strength reduction (R) factors associated with constant ductility (μ) responses. The R-factors are exact when computed for specific records or are approximate when existing R-μ-T relations are applied. Like any of the Nonlinear Static Procedures (NSPs) of FEMA-273, Yield Point Spectra can be used to estimate peak displacement response. Unlike the other NSPs, Yield Point Spectra may be used to determine admissible combinations of strength and stiffness to satisfy performance-based design objectives.

We simulate long-period near-source ground motions due to hypothetical events on a strike-slip fault (M_W 6.9) and a buried thrust fault (M_W 7.0). We include the dynamics of the rupture process using a model of sliding friction. The directivity of the rupture creates large displacement and velocity pulses in the ground motions in the forward direction. For the strike-slip fault the peak values occur near the tip of the fault, while for the buried thrust fault the peak values occur up-dip from the top of the fault. The acceleration response spectra in the 2.0 sec to 3.0 sec range exceed 1.0 g near the strike-slip fault in the forward direction, and 0.5 g up-dip from the top of the thrust fault. These results quantify the threat posed to long-period structures near faults.

This paper describes the seismic analysis and design procedures used for the seismic upgrade of an existing 3,000,000 gallon (11,355,000 liters) steel elevated water tank supported by a steel framework in Sacramento, California. The initial seismic upgrade design consisted of a base isolation system. The final seismic upgrade design incorporates a passive energy dissipation system in the braces of the framework to create a friction damped braced frame. The seismic upgrade using the friction damped braced frame is compared with the originally proposed base isolation retrofit scheme.

This paper presents application of viscous energy dissipating devices for seismic upgrade of an 8-story concrete building located on the grounds of the UCLA campus in Los Angeles. The lateral load resisting system in the longitudinal direction consists of a series of non-ductile reinforced concrete frames and this is the direction in which the building was fortified by these devices. The lateral system in the transverse direction consists of reinforced concrete shear walls and was not subjected to upgrade at this stage.

The purpose of this paper is to evaluate the effectiveness of Fluid Viscous Dampers (FVD) when used to control blast loading responses on lateral load resisting frames. In particular, this paper addresses the following issues: 1) Development of a blast loading time history for a 3,000 pound TNT blast, 2) Blast effects and performance comparisons of a conventional special moment resisting frame (SMRF), SMRF with FVD, and a conventional shear wall building. Nonlinear dynamic force history analyses were conducted on three different types of structures: 1) Conventional SMRF, 2) SMRF with FVD, and 3) Conventional concrete shear wall. The lateral load resisting frames of these structures were designed to conform to the 1994 Uniform Building Code, Zone 4 criteria. Nonlinear computer models with and without FVD were subjected to a dynamic blast loading from 3,000 pounds of TNT at 100, 40, and 20-foot standoff distances. Nonlinear analyses indicated that structures with FVD provided a cost effective way to control displacement and plastic hinge rotation of lateral load resisting frames under blast loading.

Energy dissipation systems are being employed in the United States to provide enhanced protection for new and retrofit building and bridge construction. The hardware utilized includes yielding steel devices, friction devices, viscoelastic solid devices and mostly, so far, viscous fluid devices. This hardware has been used in either diagonal or chevron brace configurations. Two new developments in the field of energy dissipation systems utilize unusual configurations which substantially increase the effectiveness of the system. These configurations are presented in the paper and their utility is demonstrated.

Up until now, as an earthquake countermeasure, seismic buildings have been designed over eighty years in Japan. On the other hand, many buildings with structural control systems have been constructed in these fifteen years. These design methods intend to reduce the structural response caused by dynamic inputs including earthquakes. These systems are defined as the systems which control the response motion of buildings, structural elements and accommodations inside the buildings, by installing a certain system or mechanism. According to this definition, the structural control system is a method which controls all kinds of vibration caused by wind, traffic, machinery loads and so on. In this paper, the outline of the building examples with structural control systems in Japan is introduced. The number of examples is about 200. The structural control systems which are discussed, are systems using hysteresis dampers (steel plate with slit, steel bell-type and lead damper), friction dampers, viscous dampers including visco-elastic dampers, tuned mass dampers and active control systems including active mass dampers and hybrid mass dampers. And the details of these systems are introduced roughly.

Modular connectors are being developed for use in seismic-resistant steel moment frames. The connectors are engineered specifically to meet performance requirements corresponding to optimal seismic response. The versatility in design required to accomplish this task is not readily available with traditional rolled shapes. Thus, the designs rely on advancements in materials and casting technology to create connectors specifically configured for seismic performance. To date, three modular connection configurations have been developed: (1) a semi-rigid modular bolted connector for partially-restrained frames; (2) a superelastic post-tensioned connecting system; and, (3) a cast modular node for moment-resisting frames. Trial designs have been developed for each of the three configurations and an analytical program has been initiated. Alpha prototypes for two of the concepts have been designed. One prototype has been created, and experimental work is underway.

The behavior of high strength square CFT beam-columns was experimentally investigated. The CFT specimens were 305 mm square tubes made from conventional (A500 Grade-B) or high strength (A500 Grade-80) steel with nominal width-to-thickness (b/t) ratios of 32 or 48, and filled with high strength (110 MPa) concrete. The effect of the stress-strain characteristics and nominal b/t ratio of the steel tube on the stiffness, strength and ductility of the high strength CFT specimens was investigated. Eight monotonic beam-column tests and four cyclic beam-column tests were conducted. The experimental results indicate that the concrete infill delays the local buckling of the steel tube and that the steel tube offers some confinement to the infill concrete thus increasing its compressive ductility. The monotonic curvature ductility of the CFT beam-column specimens decreases significantly with an increase in the axial load level and the b/t ratio. Cyclic loading does not have a significant influence on the flexural stiffness and moment capacity of the beam-column specimens. However, the post-peak moment resistance of the beam-column specimens decreases more rapidly under cyclic loading. This has a significant influence on the cyclic ductility of the specimens.

The object of this paper is to investigate the response of guyed towers to earthquake loading using two methods of analysis: the response spectrum method and the time domain analysis using a step by step integration technique. A unified approach for linear and nonlinear structural analyses based on energy minimization of the total potential work of the structure is used to predict the responses of guyed towers to simulated earthquake histories. These histories have been generated to match the target design response spectrum through an analytical procedure based on an autoregressive model. The approach is applied to three existing guyed towers. A comparative study has been performed in order to determine the response of these towers to normal loading condition as well as to earthquake loading. For each loading condition, the maximum nodal displacements and bending moments along the tower height are determined. Based on these results, the possible need for detailed seismic analysis of tall-guyed towers is assessed. The results are summarized and discussed.

This study presents an overview of an approach that identifies a design earthquake in terms of a critical response spectrum at a site in order to estimate the time variant reliability of the structure against exceedance of a specified inter-story drift. This is accomplished by coupling a technique similar to Probabilistic Seismic Hazard Analysis (PSHA) with performance-based reliability. Once the critical response spectrum is identified in terms of earthquake magnitude, M, and site-to-source distance, R, Monte Carlo Simulation (MCS) is used to account for spectral scatter, and structural resistance uncertainty present in the mass, stiffness, and damping of the model. An illustrative example is included for clarity. This example combines MCS with response spectrum analysis to estimate the static reliability of the structure. Then, the return period of the design earthquake is determined by definition and the time interval of interest is modeled as Poisson with a mean proportional to the length of the interval, and the time variant reliability of the structure determined.

Adding damping by the use of various damping devices has become an accepted method to reduce wind-induced vibrations in tall buildings. An interesting example of a 39-storey office tower is presented where large projected accelerations are the result of the vortex shedding off an adjacent existing 52-storey building. Viscous dampers and a motion amplification system are utilized to suppress the anticipated accelerations. A description of the damping system and its analytical complexities are discussed. Non-linear analysis of the tower, with time history forcing functions, derived from the wind tunnel, is presented. The dichotomy of the wind and seismic design requirements on the damping system is discussed. Cost data for the damper system is also presented.

The idea of performance-based seismic engineering (PBSE), which has become a catch phrase for the next generation of design and evaluation procedures, is not a new concept. This paper briefly reviews the practice of seismic engineering and the development of performance concepts during the past 40 years. Several analytical methods for design, verification, and evaluation that are currently in use are briefly summarized to compare differences, similarities, accuracy of results, and ease of application. Recommendations are made for future directions for PBSE.

A comprehensive, though relatively simple nonlinear method for the seismic performance evaluation of structures (the N2 method) has been developed at the University of Ljubljana. The method combines the nonlinear static (pushover) analysis and the response spectrum approach. The method yields results of reasonable accuracy if the structure oscillates predominantly in the first mode. In the paper, the most recent version of the method is presented, which is formulated in the acceleration — displacement format. The application of the method is illustrated by two examples.

The expected performance of low and medium-rise steel frames designed according to the common force/strength approach of the UBC or NEHRP is evaluated. Several combinations of soil profile type, lateral-force-resisting system, and frame-member proportioning are included in the designs. Nonlinear frame models of the designed buildings are set up for dynamic time history analysis. Nonlinear material behavior, large displacements, interaction of axial force and bending moment regarding both strength and stiffness, and spread of plasticity within each frame-member are considered in the nonlinear frame models. A total of 18 ground motion time histories are selected from the recorded events and scaled to a spectrum-related "intensity" representative of each site. To evaluate the expected damage, appropriate structural response parameters such as, interstory drift, ductility demand, dissipated hysteretic energy are obtained. A modified form of Park-Ang damage index is also calculated for moment-resisting frames. The significance of each damage measure in predicting the performance and correlation between the different measures is investigated. The performance of the steel frames is also assessed according to the acceptance criteria of the NEHRP Seismic Rehabilitation Guidelines (FEMA 273) which sets forth plastic rotation and deformation limits to define the performance levels for various structural and nonstructural components.

An improved capacity-demand-diagram method that uses the well-known constant-ductility design spectrum for the demand diagram is developed and illustrated by examples. This method estimates the deformation of inelastic SDF systems consistent with the selected inelastic design spectrum, while retaining the attraction of graphical implementation of the ATC-40 Nonlinear Static Procedure.

This study investigates an alternative seismic retrofit approach that can be employed in end-sway frames of deck-truss bridges. The proposed approach provides various modified bracing configurations that include supplemental damping systems. Experimental and analytical investigations on a 1:3 scale model of an existing steel end-sway frame tested on the shaking table at SUNY Buffalo demonstrated that proposed alternatives can efficiently improve the lateral strength and stiffness characteristics. Moreover, they modify the load path within the sway frame in favor of the existing steel bearings and reduce the overall demand.

The Williamsburg Bridge is a 95-year-old suspension structure over the East River in New York City connecting the boroughs of Manhattan and Brooklyn. The 7300-ft. long bridge carries a quarter-million passengers daily on eight traffic lanes and two subway tracks. It is undergoing reconstruction valued at $700 million that will be completed in 2002. The reconstruction includes replacement of the 4250-ft long steel approaches including the elevated steel subway approaches. As part of the reconstruction, an in-depth seismic investigation was undertaken. This paper reports on the various findings and the experience from performing a sophisticated analysis on an old bridge on the East Coast. This experience ranges from the lack of information regarding the structural systems to problems with modeling and analysis as well as the retrofit needs of a very old structure, which constitutes a significant transportation link in a congested urban environment.

Deck-truss bridges designed without consideration of seismic loads may suffer severe damage in the event of a large earthquake. A proposed retrofit solution consists of introducing ductile devices as "structural fuses" to protect both the superstructure and substructure of these bridges. Ductile panels having such fuses are designed to replace the end and lower-end panels of deck-truss bridges.

This paper reviews a number of practical and innovative solutions for the seismic retrofit of the steel towers and the deck hinges of the San Mateo-Hayward Bridge. Emphasis is put on the interrelationship between the concept, design and detailing development steps and the importance of addressing constructibility issues during all these steps. The seismic retrofit strategy for the steel towers of the bridge was to increase their overall ductility and to achieve a desirable distribution of strength and stiffness based on failure mode criteria. One of the important retrofit measures used was the addition of anchor bolts at the tower base connection to increase connection strength, ductility and redundancy. The new anchor bolt connection assemblies included a lower yield steel bar above the column base plate to control the location of yielding, and a steel sleeve around the bar to improve rocking behavior by forcing the bars to resist compression if they have previously experienced tension yielding. They were detailed to fit within the limited clearance between the existing anchor bolts. Another cost-effective measure for improving tower base behavior was the addition of a concrete fill at the base of the columns. The retrofit of the deck hinges presented a special challenge in view of their inherent vulnerabilities, high and uncertain seismic demands and clearance constraints, and the need to maintain movement capacity. To limit excessive hinge openings, a relatively simple high capacity longitudinal rod restrainer system that used conventional marine type fender units was developed. It was found that well defined performance and retrofit criteria along with retrofit concepts and details compatible with the existing bridge configuration, easily constructible, and with ample tolerances can have a significant impact on the cost and the effectiveness of the retrofit.

Three examples are presented to illustrate the seismic retrofit of steel bridges through the related techniques of isolation, re-articulation, and damping of bridges. The examples are the isolation of the south side span and damping of the main cables of the Golden Gate Bridge and the re-articulation of the side spans of the Vincent Thomas Bridge with "side span hinges"

Since the "fast-track" construction process became popular during a not-so-recent period of double-digit interest rates, owners and developers have come to believe that Rome really was built in a day. Projects that sit on the shelf for two years, awaiting various approvals, suddenly must be designed and bid in two or three weeks. Often, this "schedule driven" mentality allows design professionals insufficient time for preparation of complete, project-specific contract documents. The structural contractor's bidding process deteriorates into one of speculation rather than meticulous estimation of the work to be performed.

Section D3.1 of the AISI Specification for the Design of Cold-Formed Steel Structural Members has provisions that predict required brace forces in Z-purlin supported roof systems. The provisions were developed using elastic stiffness models of flat roofs and verified by full-scale and model testing.

This paper is concerned with a study on the behaviour of plate girders curved in plan. Experiments on plate girders curved in plan are described and results obtained using finite element method for girders having different curvature and subjected to a point load applied at the mid span are presented. Deformation and the ultimate strength values predicted by the finite element method are shown to be in reasonably good agreement with those obtained from experiments. The effect of curvature and depth to thickness ratio of web on the ultimate strength behaviour are investigated.

Frederic R. Harris, Inc. performed the computer modeling and design of the marine transport system utilized for the twin swing-span truss replacement of the George P. Coleman Bridge across the York River in Yorktown, Virginia. The contractor for the project. Tidewater Construction Corporation, was under a strict schedule for the span replacement procedure. Tidewater would incur an $8,000 per hour penalty for any time beyond an allotted 24-day construction period that the bridge was out of service. Harris designed a support system and erection that enabled the contractor to float six prefabricated bridge sections 64 km upstream on barges from the fabrication yard and into place. A similar transport system was used to remove and demolish the existing spans. The system worked flawlessly, enabling Tidewater to perform the entire operation in just nine days, earning a $1.4 million early completion bonus.

The development of an innovative method to manage traffic detours for highway construction projects located adjacent to waterways presents an interesting challenge. One proposed method involves detouring traffic over a floating structure. In order to establish the feasibility of constructing a floating structure, design criteria were established and a case study area was identified. Three alternatives were developed as potential systems for the floating detour structure and advanced to conceptual design level. A comparison quantifying the various advantages, disadvantages and factors associated with each alternative was made and recommendations were formulated.

Design guidelines for laboratory structural design are presented. Basic design criteria are outlined including floor loading, finishes, framing materials, collateral loads and vibrations. Framing layout and lateral force resisting systems are discussed. Examples, cost information and mechanical interface issues are presented.

Recent research has been conducted at Utah State University regarding the effects of various states of damage on the dynamic characteristics of reinforced concrete bridge bents with the objective of evaluating forced vibration testing as a non-destructive condition assessment tool. The Utah Department of Transportation is currently involved in a very large and intense highway project, the redesign and reconstruction of the I-15 corridor through Salt Lake City. The primary access to downtown Salt Lake from I-15 was the 600 South viaduct. This viaduct was a 2-lane, elevated roadway constructed primarily of reinforced concrete. During demolition, three reinforced concrete bents were left standing and intact. These bents served as the research specimens.

This study presents a time-domain system identification method which is used to identify nonlinear hysteretic properties of isolation bearings installed in seismically isolated bridge structures from quick release experiments. The method utilizes a computational model of the bridge and an optimization technique to minimize an objective function which ensures the energy balance within the structural system at any given instant of time. It is shown that in the proposed method that the objective function is an algebraic quantity which does not require the solution of nonlinear second-order differential equations of motion. Hence, computational effort is minimal.

This paper deals with studying the dynamic response of a fiber reinforced plastic bridge recently constructed in New York State. The dynamic behavior of the bridge is studied using detailed finite element analysis. These models are then compared with field tests performed on the bridge to validate finite element models. In a complex structural system such as a bridge made of laminated fiber reinforced plastic materials, several sources of structural degradation may occur during the life span of a bridge. For example damage due to delamination of plies, cracks and loss of bond between components, and change in material properties, may often take place in various parts of the structures and can not be detected by visual inspection. Thus, a finite element model validated by field testing can be used to foresee several damage scenarios. The correlation of dynamic testing on the bridge with dynamic analyses of the bridge using finite elements will be used to give an indication of the degree of the damage and possible location within the bridge.

An extensive study was carried out for evaluation of different experimental and analytical techniques for damage identification of civil constructed facilities. A three-span steel-stringer bridge was subjected to a regimen of deliberately induced damage to simulate many of the common deterioration and damage states that may affect these bridges. The bridge was extensively tested between April and December 1996. A primary objective of the study was to obtain reliable results reflecting the dynamic properties and responses of the bridge. In this context, improved testing procedures and parameter estimation methods were established such that the civil infrastructure can reliably be tested and the modal data can be post-processed and confidently interpreted. A number of different indices were employed for a comparative study on damage detection. A damage indicator (Bridge Girder Condition Indicator) based on the deflections computed using modal flexibility yielded very promising results for locating damage and quantifying damaged response for certain loading patterns. This offered Bridge Engineers a conceptual index for understanding damage and its significance. In this paper, the knowledge gained as a result of this study will be summarized.

The dynamic responses of a steel deck tension-tied arch bridge subjected to earthquake excitations were investigated. The Birmingham Bridge with span of 189 m (620 ft) located in Pittsburgh, PA was selected as an analytical model for the study. Two analytical models were considered for representing the bridge deck system: in one the bridge deck was replaced by a series of simple beams connected to the tension-ties at panel points; in the other the deck was modeled by using substructures. To evaluate the seismic resistance of the bridge, a comparative study has been conducted. Three orthogonal records of the El Centre 1940 earthquake were used as support excitation for the seismic response analysis. Based on the study, it was found that the simple beam model would be inadequate for representing the bridge deck system in the seismic response analysis of the bridge. The accuracy of the analytical models may be improved by applying a method of parameter estimation. A schematic diagram of the identification process is presented, which controls on the accuracy of the data processing and parameter estimation checked by an expert system program.

This study aims to promote rational quality control approaches to fabrication inspection and weld acceptance which together ensure that the structure has sufficient fatigue performance. In order to evaluate the effects of weld defects on fatigue performance, fatigue tests of butt-welded joint specimens of 25, 50 and 75mm thick with various types of weld defects. Acceptable size of weld defects is established from these test results and fracture mechanics analysis. The computerized automatic ultrasonic inspection systems have been developing and the applicability of these systems have been examined through the round robin tests.

A collaborative effort of the Federal Highway Administration (FHWA), the United States Navy, and the American Iron and Steel Institute (AISI) has resulted in the development of new steel with great potential for use in highway bridge construction. Enhanced performance steel, also known as high-performance steel (HPS), may be defined as any steel with some properties improved over existing ASTM A709, Grades 345W, 485W or 690W. Stated more simply, HPS has improvements in one or more of the following properties: strength, weldability, ductility, corrosion resistance, fatigue resistance, fracture toughness, fabricability, or formability. The HPS currently available has a yield strength of 485 MPa and is produced through improvements in both chemistry and control of the cooling process.

There are a number of important factors that are combining to prompt further development of steel bridge systems: cost competition from conventional materials; the advent of new materials, especially fiber-reinforced plastics; and public perception and community value. This paper focuses on current opportunities and ideas to enhance the economy and aesthetics of short and intermediate span steel bridges. The advent of a new and improved material specification, high performance steel (HPS), has prompted this change in part. This paper focuses on a two-girder bridge system concept which addresses a number of key issues and owner criteria, including cost, reliability, redundancy, replaceability and durability.

In a previous paper, the authors compared the fracture resistance of High-Strength Low-Alloy Steel Shapes of Structural Quality, Produced by Quenching and Self-Tempering Process (QST), ASTM A 913M Grade 345, to that of High-Strength Low-Alloy Columbium-Vanadium Structural Steel, ASTM A 572M Grade 345. The former was at that time a relatively new high performance steel while the latter is still today the most commonly used steel for painted bridges. Since then the authors have tested additional fracture toughness specimens made of different types of steels. Of particular interest here are the tests of specimens made of High-Strength Low-Alloy Structural Steel with 345 MPa Minimum Yield Point to 100 mm Thick, ASTM A 588M, and Carbon and High-Strength Low-Alloy Structural Steel Shapes, Plates, and Bars and Quenched-and-Tempered Alloy Structural Steel Plates for Bridges, ASTM A 709M Grade HPS-485W. The former is the steel of choice for unpainted bridges. The latter is a more modern steel developed in a joint effort by the Federal Highway Administration, U.S. Navy, and American Iron and Steel Institute. A 709M Grade HPS-485W, a weathering-type steel, has corrosion resistance similar to that of A 588M steel. As a follow-up to the previous paper, the fracture resistance of the A 709M Grade HPS-485W steel is hereafter compared to those of the A 572M Grade 345, A 588M and A 913M Grade 345 steels. It should be noted that when the supplementary requirements of A 709M are specified, that is, when a steel is used in bridge construction, the A 572M Grade 345 and A 588M steels are accepted under A 709M as Grade 345 and Grade 345W respectively. For ease of reference, the four subject steels will be referred to herein as A572, A588, A913 and HPS.

The propagation and stability of very long fatigue cracks in a redundant stiffened-panel structure is considered. Cracks have propagated in ships to a length greater than 8.0 m without sudden fracture or instability. To investigate this phenomenon, large box beams were fatigue tested in four-point bending. The crack was started in a wide stiffened panel that comprised the tension flange. The stiffened panels, fabricated from relatively thin A572 Grade 50 plate (less than 26 mm thick), were expected to have a ductile fracture mode. These panels did not fracture at service stress levels despite cracks greater than one meter in length.

Calculation of two dimensional (2D) and three dimensional (3D) linear elastic fracture mechanics (LEFM) demands as well as inelastic fracture toughness demands (e.g., CTOD) are reviewed for a typical steel moment connection. The results demonstrate that both material yielding and 3D behavior must be incorporated in the analysis to accurately predict the demand. In absence of flaws, or in cases where the largest flaw does not drive the observed fracture, an alternative characterization of the fracture demands is sought. Classical engineering models of fracture, and modern micro-mechanical models of fracture initiation, predict that stress triaxiality plays a direct role in the fracture demand. 3D inelastic analysis, conducted without assumed flaws, shows the location of high stress triaxiality and maximum principal stress demands in notched round bars, T-stub tensile tests, and a moment connection. Based on the analysis of these tests a curve is suggested for prediction of fracture in steel structures as a function of the stress triaxiality and maximum principal stress.

Fracture toughness for steels in civil structures is typically measured using the Charpy V-Notch (CVN) test. This test is specified in the overwhelming majority of cases where fracture is a concern, in spite of the fact that test conditions are generally not representative of service conditions. The high strain rate during this test is not representative of strain rates applied to structures in service and test temperatures do not match service temperatures. Additional complications occur when different codes, standards, and specifications require significantly different test temperatures and minimum toughnesses. As an extreme example, bridge design specifications require that CVN tests be conducted at temperatures above the LAST, while guidelines for design of offshore structures require that CVN tests be conducted at temperatures below the LAST. This paper provides a survey of several of the major codes, standards, and specifications and discusses difficulties than can result when requirements differ.

This paper presents an overview of current Standards, Codes, and Manuals that provide guidance in determination of structural loads for transmission line towers. The advantages and disadvantages of a separate loading standard for transmission line structures and alternative options will be presented. The issue of an appropriate Standard is important, as the electric power industry moves into a deregulated competitive market where "responsible" innovation will be the key to success. There is currently no industry consensus design "standard" that specifically addresses transmission line structure loads.

Most utility companies in the U.S. use the National Electrical Safety Code (NESC) provisions to determine loads on transmission pole structures. Although several other standards for the calculation of wind loads on non-utility poles do exist, a major national standard that forms the basis of most other standards and is adopted by the building codes is ASCE 7-95 Minimum Design Loads for Buildings and Other Structures. A comparative review is made in this paper of the wind pressure calculated using both NESC and ASCE standards with the main objective of determining differences between those standards and the significant parameters affecting wind load calculations. The adequacy of the current NESC provisions for extreme wind loads will be discussed. The results should provide added insight into the differences between the two standards and may thus be of interest to the committee that is currently revising the ASCE 74 Guidelines for Electrical Transmission Line Structural Loading (under development).

It is most appropriate that the 21^st Century has begun with a new and exciting innovation in transmission line structure technology. It is also appropriate that this latest structural innovation relies on time proven, indisputably reliable technologies. By combining to best advantage, the structural and material characteristics of both tubular steel and spun concrete poles, a new sectional composite pole has been developed and is being effectively used to save money, and to increase reliability of new and existing transmission lines.

The use of direct embedded prestressed concrete poles for the support of substation equipment and electrical bus conductors is potentially a very cost-attractive alternative to steel substation structures supported on cast-in-place foundations. The analysis of substation structures requires consideration of the structural response to electromagnetic forces due to short circuits (shock loads) especially for the case of rigid bus support structures. In this study, the dynamic response of prestressed concrete pole substation structures is compared to the response of more traditional steel substation structures. A dynamic factor is suggested to consider the impact effect on these structures. A proposed testing method to verify the theoretical results by applying a shock load to the substation structure is also presented.

Three full-scale concrete pole substation frame structures were tested under specified loading conditions to compare the actual test performance to the theoretical computational predictions based on conventional structural analysis. The objective was to develop refined analytical procedures, conceivably based on improved modeling of the structures, which provide better estimation of the induced forces due to structural loads. Various loading cases were applied to each structural configuration, and all structures were loaded to destruction. All testing was conducted in a vertical configuration, with the poles embedded in crushed granular material to simulate actual field conditions.

For several decades, fiber reinforced composites (FRC) have been used in increasing measures to replace other construction materials in a wide range of structural and aesthetic applications. FRC utility poles have been manufactured for over 30 years. Yet, the National Electrical Safety Code (NESC) has not specifically addressed safety issues for FRC poles in previous editions. Change proposals have been conditionally accepted to introduce specific strength and load factors for FRC poles. This paper will review the change proposals submitted to the NESC and the issues being considered before final acceptance. Applications for FRC utility poles will also be discussed.

Limit states are, according to EUROCODE 2, states beyond which the structure no longer satisfies the design performance requirements. Ultimate limit states (ULS) are those associated with collapse or with other forms of structural failure which may endanger the safety of people. Examples are loss of equilibrium of the structure or failure due to excessive deformation, rupture or loss of stability. Serviceability limit states (SLS) correspond to states beyond which specified service requirements are no longer met. Examples are deformations or deflections, vibrations, cracking of concrete, or damaging of concrete. In an earlier paper by Thoft-Christensen the service life for a reinforced concrete structure is defined as the initiation time for corrosion T_corr of the reinforcement. This is a rational definition viewed from a life-cycle cost angle since repair of corroded elements is a major contributor to the life-cycle cost. Using the diffusion modelling of corrosion it is shown on the basis of simulation data that applying this definition the service life can be modelled by a Weibull distribution.

The service limit state control of permanent deflection for steel sections in flexure has serious implications for both design and evaluation of steel bridges. These provisions are found in Article 6.10.3 of the American Association State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specification. Load combination Service II of LRFD Table 3.4.1 is the specified combination for this limit state. This uncalibrated limit state load combination governs the design of compact steel sections in flexure, as demonstrated by several published deign examples. Since the vast majority of modern steel sections in positive flexure qualify as compact sections in the LRFD Specifications, the implications of this governing, uncalibrated limit state are significant. The limit state is also suggested in the proposed AASHTO Manual for Condition Evaluation and Load and Resistance Factor Rating of Highway Bridges the review of permit issuance. These provisions were first introduced with Load Factor Design in the 1970's, based upon listed experimental results from the earlier AASHTO test road bridges. Herein, the basis of these provisions is reviewed and discussed, along with the questions that must be answered to calibrate this important limit state for rational use in both design and evaluation of steel bridges.

A method is presented that accounts in a precise way for the effects of creep, shrinkage and relaxation of materials in the time analysis of prestressed concrete structures under variable loading conditions. The method is applied to a prestressed concrete bridge built as a balanced cantilever. The influence of time related effects on its long term serviceability is discussed.

Design and evaluation of prestressed concrete I-girder bridges are dependent on the service level, live load, tensile stresses induced in the girders and the distribution of these stresses among the supporting girders. This study presents the results of field tests conducted on three prestressed concrete I-girder bridges to obtain service level stresses and girder distribution factors. The three bridges are simple span structures with span lengths of 10.3 m, 23.3 m and 31.1 m. Bridge live load response was measured near the mid-span for the passage of test trucks and normal truck traffic. The study found that measured service level stresses extrapolated to a recurrence interval of 2 weeks to 75 years exceed levels induced by an HS-20 truck in interior girders only. Measured girder distribution factors for one and two-lanes are less than girder distribution factors specified by the American Association of State Highway and Transportation Officials.

This paper addresses serviceability aspects unique to railroad bridge design and evaluation. Emphasis is put on the effects of steel bridge deterioration due to wear, fatigue and corrosion, and timber trestle deterioration on the serviceability criteria for railroad bridges. Serviceability considerations with respect to extreme events such as earthquakes are also discussed.

In 1998, a historic bridge located in Valley Forge National Park, Chester County, Pennsylvania was rehabilitated using advanced composite materials. The bridge, built in 1886, is a wrought-iron through-truss that spans 19 meters and carries one-lane of vehicular traffic on Wilson Road over Valley Creek. Based on the deterioration of stringers that supported a timber deck, a bridge rehabilitation project was initiated. The solution chosen was to remove the stringers, and in their place, install a glass fiber-reinforced polymer (GFRP) slab. The weight of the slab was roughly one-half the weight of the existing stringers, thereby reducing the dead-load effects on the remaining structure. A wood wearing surface was installed on top of the GFRP slab to restore the deck to its initial condition. In 1998, a diagnostic load test was performed on the bridge using a pre-weighed truck. The purpose of the test was to quantify the structural response and establish the load carrying capacity of the restored bridge. Strains and deflections of the GFRP slab and the truss elements were recorded for two different truck weights and for various load passes. This paper will primarily focus on the performance of the GFRP slab as measured during the load test.

Everyone enjoys looking at, or passing through, a covered bridge. Their image conjures up recollections of less stressful times. A cursory observation notes the routine — a roof and siding to protect the main structural components from the detrimental effects of occasional wetting from rain and snow. However, a more careful examination of these older structures often notes details of construction that demonstrate the care of its designers and builders. How have they stood so long and how do we help them continue to serve? There are many covered bridges remaining in the United States that date from the early part of the nineteenth century. They survive in spite of their age and neglect over the years. A renewed interest in their preservation that gathered steam in the 1960s and 70s has led to challenges for their supporters. Built long before the development of standardized codes and specifications, they were sized in most instances based on the experience of previous bridges — if they withstood the loads of the day, they were acceptable. Those that failed provided clues of structural or material weaknesses. There are several technological issues that must be addressed to allow ongoing use of these structures. However, modification and/or repair of them is constrained by historic preservation requirements. This paper addresses these two topics.

Fallingwater is indeed a building that might be called an icon. That is, it is clearly an exceptional structure, one worth saving. In fact the American Institute of Architects recently voted it the most outstanding work of American architecture in the last 125 years. The house is located in Pennsylvania some 72 miles southeast of Pittsburgh and is presently owned by the Western Pennsylvania Land Conservancy. This paper will describe how we went about analyzing the existing historic structure and what the proposed design for the repairs consists of. The major repair campaign is schedule to take place in the winter of 2001–2002. Fallingwater is on the National Register of Historic Places and is one of the most famous houses in America, indeed in all of the world. It must be noted, and without apology, that in addressing serious structural problems such as this one, we have a great deal of difficulty following the strict letter of the Secretary of Interior's Standards for Historic Preservation — we cannot avoid serious interventions and cannot avoid irreversible actions. Our office began working on repairs to Frank Lloyd Wright houses some eight years ago, first with the Darwin D. Martin House in Buffalo, New York and later at Wingspread in Racine, Wisconsin. As a result of a lecture presenting our solution to the difficult problems at Wingspread, the Executive Director of Fallingwater retained our firm to investigate structural problems at the house. We started in 1995 and issued the report of our initial findings to the Board in May 1996. Subsequent evaluation resulted in a full four volume report on the structural conditions of the house. This paper will summarize the results of those findings and the proposed solution to the inherent problems.

The present Dome of the U.S. Capitol Building was designed and built from the 1850s to the 1860s and is a masterpiece of 19^th-century engineering and construction. The majority of the Dome is comprised of cast iron; remaining elements are wrought iron. LZA Technology (LZA), A Division of The Thornton-Tomasetti Group, utilized investigative techniques used in the practice of forensic engineering to perform a comprehensive structural analysis and review of the Dome and concluded the following: (1) the U.S. Capitol Dome was brilliantly designed and well built; (2) the U.S. Capitol Dome is structurally sound, and capable of resisting all reasonably anticipated loadings; and (3) the current undertaking to waterproof the Dome will play an essential role in the longevity of the Dome structure, as it will assist in controlling and minimizing corrosion of the Dome's metal components.

Rebar pullout tests are performed and the relationship between cracking and the components of stress and displacement at the rebar-concrete interface is evaluated. Particular attention is given to the influence of specimen size and geometry. Phase measurement interferometery is used for accurate measurement of surface displacement and cracking. A confinement parameter is proposed to relate the two stress components at peak load. This parameter is geometry dependent but essentially size independent.

Brief history of the idealized truss models for shear design is traced. Modified Compression Field Theory (MCFT) using strut and tie beam is applied to High strength concrete (HSC). Beams. A computer program based on multi-layered elements located across the beam depth and a refined stress-strain curve for HSC are used. Theoretical results obtained are examined in the light of experimental results of beams. Provisions of ACI Code 318 are examined for HSC shear design. Owing to recent development of more accurate fracture mechanics methods, an alternate Fracturing Truss model (FTM) is recommended for inclusion in future design codes.

High performance concrete (HPC), with enhanced durability characteristics and strength parameters, has been promoted recently by the Federal Highway Administration (FHWA) for use in highway structures. Even though the total shrinkage of HPC is lower than that of conventional concrete, it has also been observed that HPC has a higher early-age shrinkage. The higher early-age shrinkage may cause the concrete cracking if the concrete is not properly cured. These shrinkage cracks could also reduce the durability performance of concrete. As shrinkage, especially drying shrinkage, is mainly originated by the moisture loss in concrete, an appropriate curing method can prevent the moisture loss in concrete and thus prevent cracking due to shrinkage. This paper will present the analytical and experimental results on the behavior of deck specimens with different curing methods. Two models are used to predict the shrinkage deformations of the deck specimens. One model is a constitutive model derived from the theory of elasticity and composite theory and the other one is ACI empirical calculation procedure of shrinkage strain. Four curing techniques are used in the tests. A comparison study is conducted on the early-age behavior of specimens under different curing methods. The advantages and disadvantages of each curing method are also discussed based on the behaviors of the specimens and the feasibility in construction. Finally recommendations on proper curing techniques are presented.

In 1996 New York State introduced a "high performance" concrete mixture to improve the performance and durability of bridge decks. Designated Class HP concrete, fly ash and microsilica were substituted for a portion of the cement resulting in lower permeability while maintaining good handling characteristics. A recent study assessed its performance and compared to previously used bridge deck concretes. Results indicate that it has increased cracking resistance of bridge decks and in turn increases service life expectations. This paper summarizes this study and the results.

Advanced fiber reinforced plastic (FRP) composites made of resin impregnated fibers have the potential to become a prominent construction material in lieu of steel bars in the twenty-first century for bridge decks that are susceptible to oxidation when exposed to chlorides. Advances in fabrication, ability for post bending, improvements and consistency in mechanical properties, proper selection and formulation of resins, and strict quality control are some of the issues that make a clear distinction for the new generation of FRP rebars. This paper reviews the design and construction of newly constructed concrete bridge decks reinforced with C-BAR reinforcement. Design and construction techniques are presented with emphasis on the cost effectiveness of using FRP rebars in the concrete bridge deck construction.

External reinforcement of existing structures using FRP has many advantages. High strength, easy application and minimum infringement on the existing clearances are usually important issues when selecting materials for rehabilitating an existing structure. FRP sheet material applied as external reinforcement meets these requirements and, as this case study demonstrates, can be used in ways other than the obvious application of increasing flexural strength of concrete members.

A 90,000 square meter, three-story, precast concrete parking garage in Pittsburgh, PA was strengthened in order to address concerns regarding distress in the double tee beams supporting each elevated floor. Initially, the distress was manifest in inclined cracks forming at the reentrant corner of the dapped ends. During an independent load test of sample tees, additional cracks at approximately 1.5 m from the dapped end formed and became so severe that, at roughly 75% of the design load, the tees were deemed to have failed. This failure condition resulted in an effort to strengthen the ends of the tees in the garage for shear and flexure. In assessing various strengthening alternatives, externally bonded carbon fiber reinforced polymer (FRP) reinforcement was determined to be the most cost effective, the least disruptive to the operation of the parking garage, and virtually unnoticeable once the installation was complete. A load test of several tees after the installation of the FRP reinforcement demonstrated that the retrofitted tees could support loads over 100% of the design load. This paper presents the details of the design, specification, installation, and testing of the FRP strengthening system for this large retrofit project.

This paper reports preliminary results of a field investigation regarding the effect of fiber reinforced polymer (FRP) wraps and concrete sealers on the corrosion of rebar in reinforced concrete. The structure under investigation consisted of twelve columns and associated pier caps of an existing bridge in Minneapolis, MN. Six of the structural elements (i.e. columns and associated pier caps) were wrapped with three types of FRP, three of the structural elements were sealed with conventional sealers, and three of the structural elements served as controls. Half of the wrapped structural elements and all of the sealed structural elements were subjected to electrochemical chloride extraction (ECE) prior to being wrapped or sealed. Preliminary results of the ECE process indicated that it was effective in removing some chloride ions from the concrete structures, but its long term effectiveness on inhibiting corrosion is still under investigation.

The effectiveness of FRP jackets for increasing the shear capacity and the flexural ductility of reinforced concrete (RC) columns was demonstrated in many studies. However, for smaller axial loads, the contribution of FRP jackets to flexural strength is minimal. Using FRP sheets in the direction of a column with end anchorage to improve its flexural capacity at the base is not easily achieved. This paper reports on a research project aimed at upgrading the flexural capacity of RC piers using near-surface mounted (NSM) FRP rods. Flexural strengthening and testing to failure of the piers were carried out on a bridge that was scheduled for demolition during the Spring of 1999. Three of the four piers of the bridge were strengthened with different configurations using FRP rods and jackets. The flexural strengthening was achieved using NSM carbon FRP rods that were anchored into the footings. The piers were tested under static push/pull load cycles. An analytical model was developed to determine the net forces acting on a bridge pier at a given load level based on the measured response. Strengthening techniques, test results, modes of failure, and sample analytical results of tested bridge piers are described and the effectiveness of this technology is demonstrated.

Due to the continued deterioration of our nation's bridges, combined with the increasing cost of bridge maintenance, our bridge inventory continues to experience a backlog of structurally deficient bridges. One potential solution is the implementation of new high-performance materials. Because of their many beneficial characteristics including being lightweight, having high strength- and stiffness-to-weight ratios, and being corrosion resistant, advanced polymer composites represent one such alternative. An effort led by a design group consisting of researchers from the University of Delaware (UD), engineers from the Delaware Department of Transportation (DelDOT) and the Federal Highway Administration (FHWA), and engineers and fabricators at Hardcore Composites and James Julian, Inc. of Delaware resulted in the replacement of the superstructure of a 72-year-old bridge with fiber-reinforced polymer (FRP) material. This project resulted in the installation of one of the first state owned all-composite bridges in the country. Bridge 1-351 is located on Business Rt. 896 over Muddy Run, in Glasgow, Delaware. The original bridge, built in 1926 and widened in 1936, consisted of a reinforced concrete deck on steel beams (with concrete-encased steel facia beams) supported on concrete abutments. Due to deterioration of the beams, the existing bridge was in need of repair. Rehabilitation of the substructure using traditional materials and replacement of the superstructure using advanced composites was proposed. On November 20, 1998, Bridge 1-351 was opened to traffic. This paper will provide a brief overview of the design and construction of the composite bridge, followed by a description of the proposed long-term monitoring of the structure being used to access the in-service performance of this novel bridge.

Unreinforced masonry (URM) walls at a decommissioned building in St. Louis, Missouri were tested to failure. The walls belonging to the present experimental program were subjected to out-of-plane loading. Previous work on URM and reinforced masonry walls strengthened with FRP laminates has shown remarkable increases in capacity and ductility. However, most of this research has been conducted under laboratory conditions, where, many times, it is a difficult task to represent real field conditions. In this context, this experimental program offered a singular opportunity for performing field experimentation on URM walls strengthened with Glass, Aramid and Carbon Fiber Reinforced Polymers (GFRP, AFRP and CFRP, respectively), as well as Glass Near Surface Mounted Rods. Parameters such as the type of composite system, strip width, and FRP installation methods were evaluated. A mechanism of failure caused by a shear-compression effect lead to the fracture of either the upper or lower boundary masonry units. Due to this failure mode, the walls were not able to develop a higher capacity compared to the control specimen.

This study examines the in-situ shear performance and modes of failure of reinforced concrete (RC) T-beams (joists) strengthened with externally bonded carbon fiber reinforced polymer (CFRP) sheets. The experimental program consisted of testing a series of nine structural components at the Malcom Bliss Hospital, in St.Louis, Missouri. The members were tested to validate the different systems and techniques of strengthening. In particular, this paper covers cases of U-Wraps with and without end-anchors. The experimental results clearly indicate the contribution of externally bonded CFRP sheets in increasing the shear capacity of the system. The experimental and theoretical shear strength of the reinforced concrete members (RC) under investigation is presented with and without anchorage.

Many products used by the electric power industry today are either made of, or incorporate, fiber-reinforced composites (FRC). Examples of such products include ladders, grating, construction tools, lift-truck booms, transformer pads, hot sticks, bus bar supports, insulators, crossarms, and pole structures for outdoor lighting and electric power lines. The growing use of FRC materials and products in the utility industry can be attributed to: (1) improvements in the cost and performance of FRC products due to advances in materials, product engineering and process technology, (2) a growing acceptance and use of FRC materials in general infrastructure, construction, industrial and electrical applications, and (3) inherent advantages with respect to weight, non-conductivity, and environmental impact. During the past 50 years, FRC materials have been successfully used across numerous industries in everything from aircraft to recreational products. To meet the requirements of such a broad range of applications, the composites industry has developed a variety of FRC materials and processes. It would be impractical to discuss in one brief writing all of the alternative FRC materials and processes available today, nor would that be of interest here. The purpose of this writing is to provide a general description of the FRC materials and processes commonly used to make composite products for transmission and distribution applications. It is not the intent of this writing to evaluate or qualify the materials or processes described, or disqualify any materials or processes not mentioned.

Applications of fiber-reinforced composites (FRC) are expanding as the experience in using these materials increase. Although reinforced plastic composites have been successfully employed in major structural applications, the use of the material for poles supporting electric distribution lines is relatively new. The purpose of this paper is to present an overview on the general use of FRC poles for electric distribution lines. Information on manufacturing handling, installation and field considerations, testing, and design consideration is covered.

The increased use of high strength concrete (HSC) in buildings has raised concerns regarding the behaviour of such concrete in fire. Spalling at elevated temperatures and the resulting reduction in fire resistance is of particular concern. In this paper, the various issues relating to spalling and its impact on fire resistance are discussed. The spalling phenomenon and its main causes in HSC are presented. This includes the critical parameters that influence spalling in HSC under fire conditions. Design solutions (cures) to minimize spalling in HSC structural members are presented.

A compilation of fire test data which shows distinct behavioral differences between high-strength concrete (HSC) and normal strength concrete (NSC) at elevated temperature is presented. The differences are most pronounced in the temperature range of 20 °C to 400 °C. What is more important is the observed explosive spalling of HSC specimens during fire tests.. A comparison of test results with current code provisions on the effects of elevated temperatures on concrete strength shows that the CEN Eurocodes and the CEB provisions are unconservative. Aspects of analytical modeling for predicting the buildup of internal pressure during heating are discussed. The paper concludes with recommended research needs, identified at a workshop on fire performance of HSC, convened at NIST in February 1997.

Owing to their lightweight and good corrosion resistance, Glass fibre Reinforced Plastic (GRP) composites are increasingly being used as an alternative offshore construction material to steel and concrete. However, their applications in the building industry are limited due to their perceived weakness under fire conditions and a lack of design information. The fire performance of GRP composites is being studied in a number of places world-wide and this paper reviews some of the research studies carried out by the Structures and Fire Research Group at the University of Manchester, UK. This research has so far been only concerned with the experimental and numerical study of the thermal properties of a variety of composite materials and the thermal performance of different offshore composite systems including panels and pipes. This paper describes the completed experimental programme and their results and discusses the numerical heat transfer modelling technique and the selection of appropriate thermal properties of GRP composites for numerical modelling.

The paper reports the salient features of a simple theoretical model (with its predictions backed by mainly large scale test data) which provides an insight into the mechanism of premature plate peeling failure in reinforced concrete beams strengthened in flexure by externally bonded steel or FRP plates glued to their tensile side, and provides a simple means of predicting such largely brittle failures.

This project involved the reconstruction and widening of a four kilometer (2.5-mile) stretch of the Northern State Parkway between the Meadowbrook Parkway and the Wantagh Parkway in Long Island, New York, including the replacement of six stone faced arched bridges, for the New York State Department of Transportation (NYSDOT). One of the primary challenges of this project was to preserve the character of the original stone-faced arches in this stretch of the parkway. This paper presents the development, design and construction of these replacement bridges using stone-faced precast, prestressed concrete box beams with curved soffits.

Use as columns of rectangular or square hollow sections with high-strength concrete in-fill is increasing in high-rise constructions because of their inherent load-carrying qualities. The analysis of such columns is complex and the present codes do not provide sufficient information and guidelines for the designers. The present paper is concerned with an analytical method to predict the elastic and ultimate load behaviour of composite columns consisting of square or rectangular tubes with concrete in-fill. The description of the proposed analytical method is presented in the paper. Analytical results are compared with the corresponding experimental results to establish the accuracy of the proposed method.

The determination of wood diaphragm and shear wall deflections needs to be considered in the structural design of buildings. These deflections calculations are not being made by engineers because many engineers are not aware of how perform these calculations nor the code requirements for diaphragm and shear wall deflections. Many wood diaphragms are actually considered rigid rather than the previous practice of assuming flexible diaphragms.

This paper discusses the role of the design engineer in the quality control process of building construction and the need for Construction Observation to be provided by the Engineer of Record.

Non-structural elements that were once ignored in their effects on the primary structural design and on their large contribution to economic losses after seismic- and wind-induced failures are now playing a more significant role in today's structural engineering world. Lightweight curtain walls with insufficient published information fall under these categories. The intent of this paper is to discuss the concept of curtain walls in this context. The paper also illustrates a few problems associated with curtain walls, especially in rehabilitation projects. Focus is given to the recent requirements for drift limitations in the new codes. Finally, sufficient references are listed to facilitate further study.

Traditional timber bridges designs, while economical and durable, often have difficulty producing adequate stiffness for longer spans. Addition of FRP reinforcement has been shown to improve stiffness to a limited extent, but not nearly as much as it improves strength. To address the stiffness issue, a combined analytical/experimental pilot study is being conducted on a reinforced FRP glulam/concrete composite beam section. The section consists of a glued-laminated beam reinforced on the tension face with E-glass fiber reinforced composite. On the compression face, lag screws were driven into the wood beam before pouring the concrete slab to make the glulam beam and concrete slab composite. The stiffness of the beam was modeled using first order shear deformation theory based on available material properties. The load-deflection ratio was predicted for the limit case of a fully composite concrete slab and the limit case of no concrete slab. A nonlinear model based on moment-curvature analysis was developed and run to help quantify the increased strength and stiffness of the composite beam. An experimental program has been designed to better quantify the beam response and determine whether the strength and stiffness gained through the addition of the concrete in the compression zone sufficient to make up for the additional dead load. The wood used in the glulam was eastern hemlock, a low grade but abundant New England species. A longitudinal E-glass reinforcement mat was bonded to the wood with a phenolic adhesive through the wet lay up method. In order to control shear failure, some of the specimens were wrapped at the ends with additional E-glass shear reinforcement. To date, one beam has been loaded in three point bending within the linearly elastic range, and the results indicate that the stiffness gains are significant. Future tests will examine failure modes and the effects of shear reinforcing, as well quantifying strength gains based on economical amount of FRP reinforcing.

Fiber-reinforced polymeric (FRP) composites are materials that are finding increasing use in civil engineering applications. Despite the excellent mechanical properties and corrosion resistance offered by these organic matrix materials, their susceptibility to the synergistic effects of stress and environmental weathering is a hindrance to their widespread acceptance in civil engineering. The objective of this paper is to characterize the durability of a specific formulation of wood-compatible pultruded E-glass/phenolic composite. This composite is unique in terms of its layered structure so as to make it compatible with standard structural wood adhesives e.g. phenol-resorcinol-formaldehyde (PRF). Due to its unique design, the durability of this wood-compatible FRP reinforcement cannot be directly determined from published work on the durability of E-glass composites.

Fiber Reinforced Polymers (FRP) bonded on the tension side of wood girders, offer opportunities to improve the performance of new timber bridges, to allow wider use of lower grade lamstock, and to repair and upgrade existing timber bridges. This paper compares the cost/performance characteristics of E-glass and carbon tension reinforcements for glued-laminated timber bridge girders. Two performance criteria important in bridge design are evaluated: Girder allowable bending strength (F_b) and girder flexural Modulus of Elasticity (MOE). The reinforced girders are analyzed using a probabilistic, materially nonlinear numerical model that calculates their allowable bending strength and bending Modulus of Elasticity. The paper gives an approximate carbon fiber cost at which PAN carbon fiber is economically competitive with E-glass fiber. The paper also points out important design considerations in using FRP reinforcement for timber bridges.

New forms of engineered timber materials allow engineers to design longer lasting timber bridges with longer spans. Advances in treating processes and construction methods such as stress laminating can lead to longer service live than ever before. One form of engineered timber material being used is parallel strand lumber (PSL). PSL has design stresses up to 35% higher that standard solid sawn or glu-lam material. The unique manufacturing methods associated with PSL allow the engineer to design secondary-laminated structurally efficient shapes like tee and box beams. Post-tensioning these tee and box beam bridges yields even higher strength which can lead to shallower superstructures and longer spans.