Journal of Structural Engineering

This investigation concerns the structural stability of two medieval buildings, “Torre Sineo” and “Torre Astesiano,” rising in the center of Alba, a characteristic town in Piedmont, Italy. The geometrical and structural aspects of these masonry towers were analyzed and tests were performed to assess the evolution of damage phenomena. The slant of the towers was determined with the customary topographic survey methods. Nondestructive testing methods were used, instead, to determine the extent of damage and cracking and to assess the evolution of these phenomena over time. The damage processes underway in some portions of the masonry were monitored using the acoustic emission (AE) technique. The latter method makes it possible to estimate the amount of energy released during the fracture process and to obtain information on the criticality of ongoing processes. Finally, an ad hoc theory based on fractal concepts for assessing the stability of masonry structures from the data obtained with the AE technique is proposed.

In the domain of civil engineering it is often necessary to monitor constructions such as buildings or bridges to maintain safety conditions. Nondestructive test methods outmatch other methods, because they do not influence the constructions performance. One approach is based on the experimental determination of modal parameters (eigenvalues, damping, mode shapes). From the changes in modal data damage can be detected. By updating a reference finite-element model such that its numerical modal data correspond well with the experimental modal data of the damaged structure, the damage pattern can be identified. When applying this technique to prestressed concrete, the problem arises that, because of the prestressing, cracks close again. Therefore a research program was started to investigate the change of modal parameters of a gradually damaged prestressed concrete beam. Static loads with increasing amplitude were applied. In between each load step a dynamic measurement was performed. The results show that the damage detection remains difficult for prestressed concrete in the early damage state. However, a damage pattern could be identified, which corresponds to the locations of the final failure.

Displacement coefficients and profiles are presented as promising kernel condition and damage indices along with real-life examples. It is shown that dynamic tests, which do not require stationary reference measurement locations, can also be used to generate data for the computation of modal flexibility. Modal flexibility can then be employed to obtain the displacement profiles. It is also shown that the modal flexibility can be obtained from the frequency response function measurements of the structures. Problems such as environmental effects on measured data and limitations such as incomplete dynamic measurements, spatial and temporal truncation effects are commonly faced in damage detection and condition assessment of real structures. Possible approaches to mitigate these obstacles are discussed. The level of variation and the uncertainty that may be expected when displacement coefficients are extracted from real civil infrastructure systems are also presented. The methods are demonstrated on two real-life bridges and the findings are validated by independent test results.

A method for shape selection for lattice structures is reported. The method uses static joint equilibrium equations to determine a minimum set of adjustments to lattice geometry needed to achieve equilibrium reference states. Symbolic relations among joint coordinates, member forces, and external loads are obtained. Symbolic relations allow designers to select one equilibrium shape among a range of available shapes. The method is suitable for structures with members that carry axial force only, have zero-moment connections (pin joints), and accept external loads at joints only. Examples for two-dimensional and three-dimensional cable structures are presented.

A new pushover analysis procedure derived through adaptive modal combinations (AMC) is proposed for evaluating the seismic performance of building structures. The methodology offers a direct multimode technique to estimate seismic demands and attempts to integrate concepts built into the capacity spectrum method recommended in ATC-40 (1996), the adaptive method originally proposed by Gupta and Kunnath (2000) and the modal pushover analysis advocated by Chopra and Goel (2002). The AMC procedure accounts for higher mode effects by combining the response of individual modal pushover analyses and incorporates the effects of varying dynamic characteristics during the inelastic response via its adaptive feature. The applied lateral forces used in the progressive pushover analysis are based on instantaneous inertia force distributions across the height of the building for each mode. A novel feature of the procedure is that the target displacement is estimated and updated dynamically during the analysis by incorporating energy-based modal capacity curves in conjunction with constant-ductility capacity spectra. Hence it eliminates the need to approximate the target displacement prior to commencing the pushover analysis. The methodology is applied to two existing steel moment-frame buildings and it is demonstrated that the AMC procedure can reasonably estimate critical demand parameters such as roof displacement and interstory drift for both far-fault and near-fault records, and consequently provides a reliable tool for performance assessment of building structures.

The reinforced concrete building considered in this study has 34 typical floors above a $2.7\mathrm{m}$ thick transfer plate and a three-level podium. The transfer plate is a reinforced concrete thick plate that transfers the loads from the walls at the typical floors to widely spaced columns in the three-level podium. A microconcrete model representing the high-rise building was constructed in 1:20 scale. Shaking table tests were conducted and the model was subjected to earthquake actions representing minor, moderate, major, and supermajor earthquakes for a region of moderate seismicity, with basic seismic intensity at the VIIth degree pursuant to GB50011-2001. Seismic performance was qualitatively assessed, and it is predicted that the prototype building will not collapse when subjected to major earthquakes. The majority of the damage and failure occurred at the story above the transfer plate. To minimize the damage, it is desirable to strengthen the walls between the 4th and 15th floors as well as reducing any change in stiffness within the transfer plate zone. Data obtained from the shaking table tests were analyzed. Spectral frequencies of the acceleration spectra estimated by the fast Fourier transform method and deformation measurements are good indicators on structural damage. Plots of acceleration against displacement would be useful in assessing the locations of structural damage. The use of the ratio of lateral stiffness to check the existence of a soft story may not be appropriate for high-rise buildings. However, story drift relates well with the degree of structural damage.

The potential effects of seasonal freezing on seismic response of integrated bridge column-foundation systems were investigated using three outdoor large-scale test units installed in glacial till soil. Two of these test units had identical dimensions and reinforcement details and consisted of $0.61\mathrm{m}$ diameter columns and foundation shafts. These units, which were tested during summer and winter at ambient temperatures of $23°\mathrm{C}$ and $-10°\mathrm{C}$ , respectively, revealed the impact of the effects of seasonal freezing. When compared to the test at $23°\mathrm{C}$ , the identical unit tested at $-10°\mathrm{C}$ showed 170% increase in effective elastic stiffness, 44% increase in average lateral force resistance, a $0.84\mathrm{m}$ upward shift to the maximum moment location, and a 64% reduction in the length of the plastic region in the foundation shaft. The third test unit had a $0.61\mathrm{m}$ diameter column with a $0.91\mathrm{m}$ diameter foundation shaft. The use of the oversized foundation minimized the effects of frozen condition on the response of the column although the construction of this column-shaft system was relatively complex. A summary of this experimental investigation is presented in this paper.

One of the greatest sources of uncertainty in the calculation of wind loads occurs in the selection of the wind exposure. This paper compares the traditional power-law exposure–coefficient curves used in North American codes and standards such as ASCE 7-02, 2002, with curves derived from modern models of the planetary boundary. It is concluded that the traditional exposure coefficients are reasonably consistent with modern boundary layer theory for heights below about $300\mathrm{m}$ . Above that height the traditional exposure coefficients increasingly depart from modern theory. The paper also examines the relationship between the dimensions and density of ground roughness obstacles and the exposure coefficient. This relationship enables a more objective assessment to be made than by the typical method of judging exposure “by eye,” at least where the ground roughness is fairly uniform. However, many sites are affected by upwind changes in ground roughness. Simple expressions, suitable for use in a building code or standard, are proposed for calculating the effect of an upwind change in roughness on the exposure coefficient.

A strut–tie model approach has proven to be useful for the detailing of disturbed regions of structural concrete. This approach promotes a better understanding of force transfer mechanisms and improves the designer’s ability to handle unusual circumstances. Despite the advantages of the strut–tie model approach, portions of them lack adequate definition and have not been extensively verified for use in structural analyses and designs. One of the specific areas in which further research is required is nodal zones. Before the strut–tie model approach can be practically implemented into various regions of structural concrete, a proper way of checking the strength limits of nodal zones to allow safe transfer of strut and tie forces through nodal zones must be established. In this study, a literature review of the approaches for evaluating the limits of nodal zone strength was conducted, and a consistent and general approach applicable to any type of strength verification situation of two-dimensional nodal zones was proposed. The validity of the proposed approach was examined by evaluating the ultimate strength of several nodal zones that were tested to failure.

This paper presents an analytical study to determine the effects of bearing stress distributions on concrete bearing strength and bearing plate flexural demand, for concrete blocks loaded concentrically through nonanchored flexible steel plates. Finite-element (FE) analyses of concrete specimens tested in previous investigations are employed to determine the bearing strength of the concrete when loaded through flexible plates. An FE parametric study is performed to derive formulas for the calculation of the concrete bearing stress distribution for flexible circular steel bearing plates. The results of this parametric study are used to develop a procedure for the design of concentrically loaded circular steel bearing plates on plain (unreinforced) concrete. The proposed procedure is compared with a standard design procedure.

The use of ultrahigh-performance fiber-reinforced concretes (UHPFRC) improves the structural response and extends the durability of concrete structures. Three basic configurations for structural elements combining UHPFRC and reinforced normal strength concrete are proposed with regard to typical demands on rehabilitation projects. They fulfill a protection function and the resistance can be increased if required. The flexural response of these composite UHPFRC–concrete elements is investigated with an original analytical cross-sectional model in a parametric study: UHPFRC increase stiffness and resistance and delay the formation of localized cracks. For this, a minimum magnitude of strain-hardening of 0.2% is necessary. Additional reinforcement in the UHPFRC layer is the most efficient way to increase the bending resistance. The investigation demonstrates how to use UHPFRC and reinforcing bars best in rehabilitation of existing concrete structures.

The wooden columnar structure of the historical building of Ali Qapu in Isfahan is a notable sample of Iranian traditional construction of 350 years ago. In this study, the structural system and load carrying elements of the structure have been identified. Structural analysis due to applied loads has been performed and structural strength of its members has been examined according to related codes. Based on obtained results, suggestions for reinforcement of the structure have been made. In this paper, the results from study on the wooden columnar structure of the Ali Qapu building are presented.

Formed from a thick layer of polystyrene foam sandwiched between two layers of oriented strand board, structural insulated panels (SIPs) are a viable alternative to traditional wood stud shear walls. Developed in North America, SIPs satisfy building performance criteria while being both sustainable and cost effective. This paper details a comprehensive experimental and parametric study at Napier University examining the structural performance of shear walls constructed of SIPs under the action of both racking loads and combined bending and axial compression. The effects of size and position of openings, for doors and windows, were examined and compared to designs carried out in accordance with BS 5268 and Eurocode 5. The study has demonstrated that SIPs perform as an effective composite material possessing the strength and stiffness necessary to sustain required design loads.

Results from an investigation of snow loads and wind actions on 20 existing buildings in Norway are presented. The objective has been to investigate to what extent existing buildings meet current regulatory requirements relating to safety against collapse owing to snow loads or wind actions. Eighteen buildings have a utilization ratio of more than 1.0 under current regulations. The new design rules have led to most of the buildings investigated having reduced safety against collapse owing to snow loads and greater safety against collapse owing to wind actions than the regulations now demand. The investigation indicates too low reliability for a considerable number of buildings according to current building regulations when evaluating the possible consequences of the conclusions in a national perspective. Scenarios for future climate change indicate both increased winter precipitation and increased temperatures, and thus changing the snow loads on roofs. Wind scenarios for the decades to come indicate an increase in frequencies of strong winds in areas also exposed today. Thus, the future reliability of the buildings in these areas could decrease.

In this paper, the performance of deteriorating bridges is analyzed considering the combination of essential and preventive maintenance actions. Multiobjective optimization under uncertainty is used to find the best combinations of condition, safety, and cost; satisfying all performance constraints of deteriorating bridges during a specified time horizon under multiple maintenance types. The thresholds at which essential maintenance actions are applied and the times of application of preventive maintenance actions are considered as design variables. The evolution in time of nondeterministic performance is modeled as probabilistic condition, safety, and cost profiles, and genetic algorithms are combined with Latin hypercube sampling to optimize the maintenance strategies in a full probabilistic context. Owing to the scarce information on the structural performance effects, due to the interaction among different types of maintenance actions, special attention is paid to the assumptions with respect to the combination of effects of different maintenance types. Examples of application, based on data collected on reinforced concrete bridge crossheads in the United Kingdom, are presented. The benefits of the combination of preventive and essential maintenance actions over single maintenance type strategies are clearly indicated; namely, the reduction in cost for similar condition and safety indices. The use of alternative probabilistic indicators in bridge maintenance, such as the lower percentiles of performance, is also analyzed.

During the past decade, a variety of bridge maintenance management methodologies have been developed to cost-effectively allocate limited budgets to deteriorating highway bridges for performance enhancement and lifespan extension. In most existing research and practice, however, bridges are treated individually rather than collectively as integral parts of a transportation network. In addition to safety issues, failure of bridges renders a highway network inaccessible, either partially or completely. This may lead to considerable economic consequences, ranging from agency costs caused by repair/reconstruction to user costs as a result of disrupted network service (e.g., congestion and detour). Therefore, it is both natural and important to maintain the satisfactory long-term performance of not only individual bridges but the highway network. In this paper, a novel analytical/computational framework for network-level bridge maintenance management using optimization is presented. This framework integrates time-dependent structural reliability prediction, highway network performance assessment, and life-cycle cost analysis. Failure occurrences of individual bridges and their effects on the overall performance of the highway network are evaluated probabilistically. The maintenance resources are prioritized to deteriorating bridges through simultaneous and balanced minimization of three objective functions, i.e., maintenance cost, bridge failure cost, and user cost. Each of these cost metrics is computed as the present value of the expected economic expenditure accrued over the specified time horizon. The resulting multiobjective optimization problem is solved by a genetic algorithm. An application example is provided for maintenance management of deteriorating reinforced concrete deck slabs of an existing bridge highway network in Colorado. It is shown that the proposed methodology is effective for enhancing the bridge maintenance management practice at network-level through probabilistic quantification and preferred balance of various life-cycle costs.

This technical note investigates the accuracy of a simplified hand method recommended in several design guidelines for practicing structural engineers for calculating the lateral stiffness of shear walls with openings. Parametric studies are performed in which the location and size of the opening as well as the aspect ratio of the wall are varied. Toward this goal, a special-purpose finite-element algorithm is developed and implemented in the computing package MATLAB. Results from finite-element analysis are compared to those of the hand method. It is found that the hand method, despite its complexity, is highly approximate and often enormously overestimates the stiffness of shear walls with openings.