Journal of Structural Engineering

This is the first part of a two-part investigation on the method of reliability-based seismic design. In reliability analysis and reliability-based design under stochastic loads such as seismic excitations, repeated time-history solutions of multi–degree-of-freedom (MDOF) inelastic structures are often required, which can become computationally expensive. To alleviate this difficulty, an approximate method is developed by replacing the MDOF system with a simple equivalent nonlinear system (ENS) that retains the dynamic characteristics of the first two modes and the global yielding behavior of the system. The equivalence is achieved by the use of two empirical response modification factors based on regression analyses of responses to a large number of actual ground accelerations recorded in past earthquakes. Numerical examples on reliability analysis of the special moment resistant space frame (SMRSF) under seismic excitations are given. The computational advantage and accuracy of the method are demonstrated.

This is the second part of a two-part investigation on the method of a reliability-based seismic design. To account for large uncertainties in structural design under seismic load, a method of calibration of the design parameters in seismic codes is proposed based on the consideration of satisfactory performance under future earthquakes in terms of reliability. The calibration is carried out by a minimization of the difference between target and actual reliabilities against serviceability and ultimate limit states over a given period of time. The response surface method (RSM) in conjunction with a central composite design is used to expedite the minimization solution process. To reduce the computational effort required for reliability evaluation under seismic loads, the equivalent nonlinear system (ENS) developed in part I is used for dynamic-response analysis of multi–degree-of-freedom (MDOF) inelastic structures. Numerical examples on the calibration of the earthquake load factor and drift limit for selected target limit-state probabilities are given. Parametric studies are also carried out to examine the dependence of these parameters on the target reliability levels against both serviceability and ultimate limit states.

The need for reassessment of the fatigue life of existing structures is increasing as the world's infrastructure ages. A fatigue life reassessment typically begins with an assessment of the current condition of the structure. The condition assessment techniques range from visual inspection to X-ray inspection or detection of acoustic emissions. The fatigue reliability of the structure can be estimated from probabilistic fatigue life or fracture mechanics models. The data obtained from the condition assessment can be combined with these models to estimate the remaining service life of a structure using Bayes' theorem. Simulation techniques are often used to facilitate these calculations. If the remaining service life is inadequate, it may be desirable to repair the structure; however, repairs must be performed carefully to provide the desired benefit. On the other hand, economic factors may dictate a course of action other than repair, such as replacing the structure or changing the operation of the structure.

The problem of assessing the damage to and remaining life of structures subjected to fatigue is explored, with particular emphasis on railroad bridges, highway bridges, and offshore structures. In railroad structures, the fatigue reliability estimates are typically calculated based on fatigue life predictions. The resulting predictions are used for budgeting purposes rather than for scheduling repairs. Examples are given of economic decision based on fatigue life calculations. Fatigue reliability estimates for highway bridges are also typically based on fatigue life calculations. Target reliability values have been suggested and are used in both the design of new bridges and the evaluation of existing bridges. Given the economics of offshore structures, it is feasible to develop more elaborate fracture mechanics models. These models have the advantage of explicitly allowing updated estimates of fatigue reliability based on inspection results. The economics of inspection of offshore structures also dictates that inspection scheduling be based on fatigue realiability estimates.

Corrosion of reinforcement is a major problem affecting a large number of reinforced concrete structures. At present, most reliability-based design studies of reinforced concrete structures do not consider the effects of corrosion. In this paper, we present a reliability-based approach to the design of reinforced concrete bridge girders that are under corrosion attack. Both reserve and residual reliability constraints are satisfied. The approach is based on the American Association of State Highway and Transportation Officials (AASHTO) standard specifications for highway bridges and on data of chloride corrosion of steel in concrete. First, the effects of corrosion on both moment and shear reliabilities are investigated. Second, a reliability-based design approach based on minimization of total material cost including corosion effects is proposed. This approach is demonstrated on several design examples. Finally, we suggest and illustrate a reliability-based design approach based on minimization of expected lifetime cost including corrosion effects and cost of failure consequences.

This paper presents a decision model for minimizing the life-cycle cost of prefabricated concrete elements and structures, taking into account the building design-manufacturing-construction-service life process as a whole. The decision model utilizes principles of engineering economic analysis under uncertainty in considering costs and benefits of construction, maintenance, repair, and consequences of failure. The concepts are illustrated for plant precast hollow-core concrete floor slabs. Decision variables can include parameters related to concrete quality, manufacturing process technology, and maintenance. The optimal solution identifies those values of the decision variables that result in minimum expected total cost. The performance constraints, in the form of limits on the probability of flexural failure, shear failure, cracking, and excessive deflection are formulated using principles of structural reliability theory.

One of the most important aspects of the construction of concrete bridges using the balanced cantilever method is the limit state of the loss of stability by overturning. This problem is especially relevant because of the lack of guidance in most structural codes about the actions involved and the safety factors to be used. First, the main assumptions, actions, and the methodology involved in the construction of such bridges are presented. The result of the reliability-based calibration process is the definition of a set of partial safety factors to apply in the design of the elements used for ensuring stability during the construction of bridges using this method. The bridges covered in this study are those with main spans between 80 and 140 m. This range includes most of the concrete bridges constructed with the cantilever method. The derived partial safety factors are also valid for a wide range of estimated live loads and wind forces on the deck during consruction. Finally, an example is presented comparing the design values of the stabilizing elements for a bridge with a main span of 120 m, obtained using the proposed safety factors and those computed with a global safety factor of 1.5.

In current engineering practice, to evaluate the stability of columns in frames with rigid and semirigid connections, one must estimate the effective length factor (K-factor) of columns considering the effects of the nonlinear moment-rotation characteristics of beam-to-column connections. Herein, using the alignment chart approach and based on a recent study on the stability of semirigid frames and the concept of Shanley's inelastic column buckling theory, the governing equations on the K-factor of columns for various boundary conditions with rigid and semirigid beam-to-column connections are derived for unbraced frames. With a proper evaluation of the tangent connection stiffness for semirigid beam-to-column connections at buckling state and with the introduction of the modified relative stiffness factors, the alignment chart in the present American Institute of Steel Construction-load and resistance factor design specification can be applied to find the corresponding column K-factor in semirigid frames. This is described in the present paper. Furthermore, using a semirigid and unbraced portal frame example, the effects of the nonlinear moment-rotation characteristics of connections on the K-factor are numerically studied and the K-factors obtained by two simplified methods are also compared with the present method.

The pull-through/local dimpling failure strength of screwed connections is very important in the design of profiled steel cladding systems to help them resist storms and hurricanes. The current American and European provisions recommend four different test methods for the screwed connections in tension, but the accuracy of these methods in determining the connection strength is not known. It is unlikely that the four test methods are equivalent in all cases and thus it is necessary to reduce the number of methods recommended. This paper presents a review of these test methods based on some laboratory tests on crestand valley-fixed claddings and then recommends alternative tests methods that reproduce the real behavior of the connections, including the bending and membrane deformations of the cladding around the screw fasteners and the tension load in the fastener.

A consistent design approach for pegged timber framed connections can be developed by modifying the current National Design Specification for Wood Construction to account for the lower bearing and bending strength of the wood connectors. This paper addresses the bearing strength of white oak dowels in both red oak and Douglas fir Specimens. The effects of peg strain orientation, peg diameter, peghole clearance, specimen grain orientation, and specimen wood species were studied. A modified bearing strength test was developed to eliminate localized crushing of the white oak dowel by the test machine. Bearing strength tests were performed on 183 wood specimens. The bearing strength of pegged connections was found to be relative insensitive to the factors investigated and lower than bearing strengths using similar steel fasteners for all test configurations.

The approach based on using the finite-element method for the three-dimensional dynamic bridge analysis is used to explore how different factors influence bridge behavior. The example used is a continuous nonprismatic four-span bridge that was previously monitored in the field by the writers. The influence of the dynamic behavior is shown with plots of the dynamic deflections at different locations. The factors that have an effect on the dynamic behavior of the bridge are the dimensions; the material properties; the section properties; the damping characteristics; the surface roughness; and the vehicular weights, speeds, and locations. Comparisons are made for each of these factors.

An optimal design formulation is presented that incorporates the use of standard prefabricated sections for the design of steel transmission poles. This standardization is achieved by discretizing the design variables for the pole structure that correspond to the prefabricated sections available in a catalog. The cross-sectional shape and the steel grade are also treated as design variables for some solution cases. The cost function expression is altered for a case to include the cost of welding the pole joint. Optimal designs are obtained using a genetic algorithm, an enumeration method, and the simulated annealing method. From the results, it is concluded that standardization of the design of transmission poles can be implemented into the optimization process for practical applications.

Design optimization of trusses consists of arriving at optimum topology, configuration, and sizes of cross-sectional parameters, when weight of the truss is minimum, satisfying a set of specified constraints. Genetic algorithms-based methods provide ideal techniques for modeling many practical considerations, which make the optimal solution constructible. Two improved methods for size and configuration optimization and topology optimization are presented in this paper. The first one is a two-phase method, which in addition to arriving at better solutions compared to those obtained using earlier methods, also improves the computational efficiency. The second method, based on a variable string length genetic algorithm (VGA), addresses the topology optimization problem, taking into account a number of practical issues. Classical problems from literature are solved and the results are compared. Features of the proposed method, which help in modeling and application to optimal design of large truss structures, are demonstrated by solving a problem of a microwave antenna tower.

The conventional definition of redundancy is applicable to skeletal structural systems only, whereas the concept of redundancy has never been discussed in the context of a continuum. Generally, structures in civil engineering constitute a combination of both skeletal and continuum segments. Hence, this paper presents a generalized definition of redundancy that has been defined in terms of structural response sensitivity, which is applicable to both continuum and discrete structures. In contrast to the conventional definition of redundancy, which is assumed to be fixed for a given structure and is believed to be independent of loading and material properties, the new definition would depend on strength and response of the structure at a given stage of its service life. The redundancy measure proposed in this paper is linked to the structural response sensitivities. Thus, the structure can have different degrees of redundancy during its lifetime, depending on the response sensitivity under consideration. It is believed that this new redundancy measure would be more relevant in structural evaluation, damage assessment, and reliability analysis of structures at large.

The results of a fire load survey, carried out in Kanpur, India, are presented. Eight office buildings with a total floor area of about 11,720 m² were surveyed. The inventory method was used in the present survey. Details of survey methodology and processing are given. Analyses have been made to determine the influence of room use, room floor area, and floor level on fire loads. It is found that room use and room floor area are major parameters affecting the fire loads in a room. An attempt is made to calculate the composition of fire loads in the buildings surveyed. In office buildings wood and paper contribute to a substantial portion of the total fire load. The movable contents contribute to about 88% of the total fire load. The fire loads obtained from the survey have been compared with those of similar surveys in the United States and the United Kingdom.