Journal of Structural Engineering

This paper offers importance vectors to identify the structural parameters that are most cost efficient to change to improve the reliability of an underperforming design. The methodology comprises the synthesis of reliability sensitivity measures and construction cost data to obtain practically useful parameter rankings. The results expose the change in the reliability per dollar spent on different parameters. A novel distinction is made between design parameters and tolerance parameters in order to include the possibility of improving the reliability by allocating resources to reduce variability in certain geometry and material parameters. The developments are intended as a tool for the emerging performance-based engineering approach. Sophisticated structural models are employed in conjunction with reliability analysis. In this paper, the costs are estimated based on RSMeans construction cost data, while the probabilistic models for material and geometry parameters are based on the JCSS model code. A detailed example of a six-story, three-bay reinforced concrete frame structure is presented to demonstrate the computation and interpretation of the cost-benefit importance vectors.

In structural reliability analysis, the uncertainties related to resistance and load are generally expressed as random variables that have known cumulative distribution functions. However, in practical applications, the cumulative distribution functions of some random variables may be unknown, and the probabilistic characteristics of these variables may be expressed using only statistical moments. In the present paper, in order to conduct structural reliability analysis without the exclusion of random variables having unknown distributions, the third-order polynomial normal transformation technique using the first four central moments is investigated, and an explicit fourth-moment standardization function is proposed. Using the proposed method, the normal transformation for independent random variables with unknown cumulative distribution functions can be realized without using the Rosenblatt transformation or Nataf transformation. Through the numerical examples presented, the proposed method is found to be sufficiently accurate in its inclusion of the independent random variables which have unknown cumulative distribution functions, in structural reliability analyses with minimal additional computational effort.

Life extension of existing deteriorating structures requires maintenance interventions, which allow partial or complete rehabilitation. However, such maintenance interventions have to be economically reasonable, i.e., maintenance expenditures have to be outweighed by expected future gains. If this cannot be achieved, the optimal lifetime of a structure has been exceeded. This paper discusses optimum maintenance planning (optimal sequences of maintenance times and rehabilitation levels) by maximizing the expected net present benefit rate throughout lifetime. The underlying formulation is as follows: (1) between maintenance interventions the deterioration due to Poisson shocks is modeled by a continuous-time Markov chain; (2) maintenance interventions can take place anytime throughout the lifetime and comprise the rehabilitation of all deterioration states above a certain minimum level; and (3) maintenance interventions are optimized by taking into account all life-cycle costs (construction, failure, inspection, and state-dependent rehabilitation costs as well as state- or time-dependent benefit rates). Numerical examples demonstrate that the proposed approach not only allows to optimize maintenance interventions, but also allows to determine optimal lifetimes and acceptable failure rates.

This study discusses the development of a procedure that can be used to assess the reliability of concrete masonry unit infill walls subjected to personnel-delivered blast loads. Consideration is given to maintain reasonable computational effort for both the structural analysis and reliability models. Blast load and wall resistance models are developed based on experimental and analytical data, and resistance is evaluated with a large strain, large displacement transient dynamic finite-element analysis. A sensitivity analysis is conducted to identify significant random variables and a reliability analysis conducted with a feasible level of computational effort. Reliability indices are estimated for two wall types and three design blast load levels in terms of wall failure as well as occupant injury, over various load frequency-of-occurrence times.

The objective of this study was to propose a seismic design methodology for moment-resisting frames to limit the extent of structural damage and distribute this damage uniformly along the height. This permits the efficient utilization of the energy dissipation capacity of most structural members to avoid undesirable dynamic responses, e.g., the formation of story mechanisms and/or the amplification of story drifts caused by $P$ –delta effect. The proposed methodology is based on the utilization of seismic design lateral load patterns to obtain a uniform distribution of story ductility ratios along the height. It is demonstrated that, on average, frame structures designed based on the proposed approach exhibit a more uniform distribution of story ductility and story drift ratios when compared to the distributions obtained using current U.S. seismic code provisions. Designs based on the proposed approach are expected to provide increased protection against global collapse and loss of life during a strong earthquake event.

To accurately predict the inelastic earthquake response of a structure by pushover analysis, the prescribed load profiles should be consistent with the actual force profiles which develop maximum design values during the time-history response of the structure. In the present study, a new modal combination method, factored modal combination (FMC), was developed to predict the earthquake load profiles of building structures affected by higher dynamic modes. In the FMC, multiple story load profiles are predicted by combining the modal spectrum responses multiplied by the modal combination factors. Based on the results of parametric studies on moment-resisting frames and walls, the modal combination factors were defined according to the hierarchy of each mode affecting the dynamic responses of structures. The FMC was applied to prototype buildings with and without vertical irregularity. The results showed that the FMC predicted the actual story load profiles which developed during the elastic and inelastic time-history responses of the structures.

In this research, the seismic performance of a proposed efficient energy dissipating steel-braced frame (EEDBF) in relation to that of a moment-resisting frame (MRF) and chevron braced frame (CBF) is studied. The frame is intended to combine the advantages of MRF and CBF and eliminate most of the disadvantages pertinent to these frames. Nonlinear static pushover, time history, and damage analyses of the three frames are conducted to assess the performance of the EEDBF compared to that of MRF and CBF. The analyses results revealed that the EEDBF has a more stable lateral force-deformation behavior compared to CBF. The energy dissipation capacity of the EEDBF is comparable to that of the MRF. The drift of the EEDBF at small to medium intensity ground motions is comparable to that of the CBF and smaller than that of the MRF. At high intensity ground motions, the drift of the EEDBF is smaller than those of both CBF and MRF. Furthermore, the EEDBF is found to experience less damage compared to other frames.

Characteristics of near-fault ground motions warrant special consideration due to their severe and impulsive effects on structures. These characteristics are unique compared to far-field ground motions, upon which nearly all seismic design criteria are based. The objectives of this study were to explore the shake table response of reinforced concrete, to investigate near-fault ground motion effects on reinforced concrete bridge columns subjected to near-fault ground motions, and to provide a framework for the evaluation of bridge columns near active faults. Two large-scale columns were designed and tested under a near-fault ground motion on a shake table at the University of Nevada, Reno. One column represented the current California Department of Transportation far-field design, and the other was based on the American Association of Highway and Transportation Officials provisions. The most unique measured response characteristic in both columns was the large residual displacements even under moderate motions. A new hysteresis model was developed to capture this effect and was incorporated in an analytical model. Based on this finding, a framework for the evaluation of reinforced concrete bridge columns with respect to the control of residual displacement was proposed.

Lightweight, portable air-pressurized beams and arches serve as primary load-carrying members for a number of civilian and military structures. These members are made from synthetic fibers that are woven or braided into a circular cross section. The pressurized air provides structural capacity by pretensioning the fabric and through its behavior as a confined gas. In this paper, a beam finite element is developed for the analysis of pressurized fabric beams based on virtual work principles. Work done by internal pressure due to deformation-induced volume changes is included in the formulation. A nonlinear moment-curvature relationship accounts for fabric wrinkling, and shear deformations are incorporated. A mixed-interpolation Timoshenko beam element is used to discretize the virtual work expression. A numerical method for determining the moment-curvature relationship of an inflated beam made from a fabric obeying a nonlinear stress–strain relationship is developed. Results of experiments on pressurized fabric beams loaded in three- and four-point bending are presented, and the finite-element model is shown to accurately predict experimentally observed load-deflection response for a range of pressures. Simulations demonstrate that in addition to prestressing the fabric, the pressurized air significantly increases beam capacity as the beam volume decreases due to deformation.

A procedure for designing low-weight space trusses based on the innovative Big Bang–Big Crunch (BB–BC) optimization method is developed for both discrete and continuous variable optimization. BB-BC optimization is a population-based heuristic search consisting of two parts: The Big Bang where candidate solutions are randomly distributed over the search space; and a Big Crunch where a contraction operation estimates a weighted average or center of mass for the population. Each sequential Big Bang is then randomly distributed about the center of mass. The objective of the optimization is to minimize the total weight (or cost) of the structure subjected to material and performance constraints in the form of stress and deflection limits. Designs are evaluated for fitness based on their penalized structural weight, which represents the actual truss weight and the degree to which the design constraints are violated. BB-BC optimization has several advantages over other evolutionary methods: Most significantly, a numerically simple algorithm with relatively few control parameters; and the ability to handle a mixture of both continuous and discrete design variables. Low-weight design and performance comparisons for several benchmark-type truss structures are presented between the BB-BC procedure and various classical and evolutionary optimization methods.

This paper describes a test program on full-scale fabricated simply supported I-beams with central concentrated load and an effective lateral brace at the midspan of the top compression flange that was performed mainly with the aim of experimental verification and investigation of distortion in doubly symmetric I-section beams. The experimental ultimate loads were compared with the buckling loads of the specimens determined by using the so-called Southwell, Modified, and Massey extrapolation techniques, and in most of the cases very little discrepancy was found between the maximum test and extrapolated loads. Moreover, the experimental beam strengths were compared with the design strengths predicted by the American AISC/LRFD and the Australian AS4100 specifications. Generally, the two specifications provide unconservative predictions in the inelastic range of structural response as the beam length decreases and inelastic behavior becomes more intense. However, the AS4100 specification provides conservative predictions in comparison with the AISC/LRFD specification.

This paper presents finite-element (FE) modeling and nonlinear analysis of steel-concrete composite frames subjected to very large cyclic loading. A three-dimensional FE model is developed to carry out the nonlinear analysis. Modeling details of the steel frame, the reinforced concrete (RC) slab, the interactions between the RC slab and the frame beams, and the associated constitutive relationships for cyclic loading are presented. A composite frame previously tested by the writers is analyzed using the proposed FE model. Good correlation is observed between the experimental and analytical hysteresis curves up to rotation amplitude of $0.04\mathrm{rad}$ . One of the notable observations during the loading to large rotations is the fracture at steel beam ends. To simulate such behavior, a simplified fractured steel connection model is introduced. The proposed nonlinear model is found to be capable of capturing the fracture behavior of beam ends to an acceptable accuracy.

This paper presents a test program on cold-formed stainless steel tubular sections subjected to combined bending and web crippling. A series of tests was performed on square and rectangular hollow sections fabricated by cold-rolling from high strength material of duplex and high strength austenitic stainless steel sheets. The concentrated load was applied by means of a bearing plate which acted across the full flange width of the section. The flanges of the hollow section specimens were not fastened to the bearing plates during testing. The specimens were tested using the interior-one-flange loading condition specified in the American Specification and Australian/New Zealand Standard (AS/NZS) for cold-formed stainless steel structures. Different specimen lengths were tested to obtain the interaction relationship between moment and concentrated load. The combined bending and web crippling test results were compared with the design strengths obtained using the current American Specification and AS/NZS for cold-formed stainless steel structures. Generally, it is shown that the American Specification and AS/NZS Standard conservatively predicted the strengths of cold-formed high strength stainless steel square and rectangular hollow sections subjected to combined bending and web crippling.

The midply shear wall system is a new concept in shear wall design. Test results presented here demonstrate that midply walls have superior survival characteristics under earthquake loading, and have a dynamic load-carrying capacity of more than two and a half times that of comparable standard shear walls. To prevent brittle failure at the end studs due to high-tension forces, steel rods are used at each end of the midply wall in lieu of hold-down connectors. Seismic damage accumulation was identified experimentally for the first time in wood shear walls under repeated earthquake loading. Further understanding of this phenomenon is important for seismic safety evaluation of buildings which experience multiple earthquakes during their service life. This study and the previous work on the static and cyclic performance of midply walls provide design engineers with experimental and conceptual evidence of the suitability of midply walls in wood construction applications where the architecture requires superior shear resistance.

The Saffir–Simpson scale for categorizing hurricane intensity and damage potential is increasingly being used by hurricane forecasters and emergency managers. The hurricane intensity categories are associated in the scale with 1-min wind speeds. For structural engineering purposes the ASCE 7 Standard defines these 1-min speeds as speeds at $10\mathrm{m}$ over open water. This technical note provides estimates of the ratio of peak 3-s wind speeds at $10\mathrm{m}$ over open-terrain exposure—the speeds used in the ASCE 7 wind map—to 1-min speeds at $10\mathrm{m}$ above open water. Based on the ASCE 7 power-law model, the estimated ratio is 1.03. Based on the logarithmic law model, depending upon assumptions pertaining to the surface roughness for flow over open water, and upon the estimation method, the ratio varies from 1.03 to 1.12.