Journal of Structural Engineering

This paper presents a finite element (FE) analysis to quantify the effects of weld-induced residual stresses on the flexural behavior of circumferentially butt-welded steel circular hollow section (CHS) members. FE modeling of the weld-induced residual stresses is first described. Nonlinear FE analysis in which the behavior of the CHS members subjected to bending is explored incorporating the residual stresses is discussed next. Two FE CHS tube models with different failure modes are developed in order to clarify the effects that the residual stresses have on the flexural behavior. Simulated results show that welding residual stresses can have significant effects on the initial stiffness and the ultimate strength of the CHS flexural members depending on the loading position; hence, they should be taken into account in assessing the behavior of steel CHS members with circumferential weld under bending.

A loading capacity analysis is conducted for prestressed continuous steel-concrete composite beams. On the basis of the basic theoretical framework of the solution of externally unbonded prestressed structures, formulas for calculating the three characteristic loads (crack, yield, and ultimate loads) of two-span prestressed continuous composite beams under symmetric concentrated loads are proposed and extended to general cases. The variation of tendon force is considered and the adopted limit equilibrium approach only requires the development of equilibrium equations, which avoids the solution of cumbersome simultaneous deformation compatibility equations. Furthermore, an elaborate finite element model is presented for simulating the nonlinear behavior of prestressed continuous composite beams by using the commercial finite element package. The numerical model considering both the material and geometric nonlinearities can fully reflect the complex behaviors of prestressed continuous composite beams during the whole loading process. The comparisons among the analytical, numerical, and experimental results demonstrate that the analytical method provides a convenient and reliable tool for a routine design practice and the finite element model provides an excellent numerical simulation for the nonlinear behavior of prestressed continuous composite beams.

The paper addresses the lateral-torsional buckling for a special steel truss structure which is adopted to reinforce a particular class of composite steel-concrete beams. The investigated instability phenomenon deals with a transitory ultimate limit state that may occur when the truss structure is being assembled and is bearing the precast floor system, before the concrete has hardened. During this transitory stage the truss beam works as a steel structure and may exhibit local or global instability phenomena. It provides the reinforcement of the final composite steel-concrete beam once the concrete has hardened. The main result of this paper is an analytical approach for the estimation of the elastic critical moment which is required to calculate the final design lateral-torsional buckling resistance moment in accordance with the technical code in force (the European standard for steel structures is considered in this paper). The simplified analysis this paper presents leads to a closed-form solution for the elastic critical moment, thus providing a simple and rapid tool for the calculation of the lateral-torsional buckling resistance moment. A comparison to a finite element analysis has been performed to demonstrate the correctness of the proposed analytical formulation.

A LiteSteel beam (LSB) is a new cold-formed steel hollow flange channel beam produced by using a patented manufacturing process involving simultaneous cold-forming and dual electric resistance welding. It has the beneficial characteristics of torsionally rigid closed rectangular flanges combined with the economical fabrication processes from a single strip of high-strength steel. Although the LSB sections are commonly used as flexural members, no research has been conducted on the shear behavior of LSBs. Therefore, experimental and numerical studies were conducted to investigate the shear behavior and strength of LSBs. In this research, finite-element models of LSBs were developed to investigate their nonlinear shear behavior, including their buckling characteristics and ultimate shear strength. They were validated by comparing their results with available experimental results. The models provided full details of the shear buckling and strength characteristics of LSBs and showed the presence of considerable improvements to web shear buckling in LSBs and associated postbuckling strength. This paper presents the details of the finite-element models of LSBs and the results. Both the finite-element analysis (FEA) and experimental results showed that the current design rules in cold-formed steel codes are very conservative for the shear design of LSBs. The ultimate shear capacities from FEAs confirmed the accuracy of proposed shear strength equations for LSBs on the basis of the North American specification and direct strength method (DSM) design equations. Developed finite-element models were used to investigate the reduction to shear capacity of LSBs when full-height web side plates were not used or when only one web side plate was used, and these results are also presented in this paper.

Brace buckling in inverted-V-braced frames induces the vertical unbalanced force. The columns in the braced bay should be designed, per the capacity design concept, to remain elastic for gravity load actions and additional column axial forces that result from the brace buckling. However, owing to the difficulty in accumulating the buckling-induced column forces from different stories, empirical and often conservative approaches have been used in design practice. In this paper, three combination rules for a rational estimation of the column axial forces are proposed. The idea central to the three methods is to detect the stories with high buckling potential as precisely as possible by using pushover analysis and/or simple demand-to-capacity analysis. The vertical unbalanced forces in the stories detected as high buckling potential are then summed in a linear manner, whereas those otherwise are combined by following the SRSS (square root of sum of squares) rule. The accuracy and design advantage of the three methods is evaluated on the basis of extensive inelastic dynamic analyses. The mode shape-based method (MSBM), which is both simple and accurate, is recommended as the method of choice for practicing engineers among the three proposed.

This study proposes a method for estimating the properties of concrete creep and the unstrained length of cable, by using measured displacements and cable tension forces, in a concrete cable-stayed structure over time. The method is proposed on the basis of minimizing the differences between the actual measured responses of the structure and the predicted values that were calculated by using finite element analysis. The creep model uncertainty factor was introduced to characterize the incompleteness or inadequacy of the deterministic formulas of a given model. This factor was selected as the sole parameter to be estimated from the parameters relevant to the creep model. The numerical examples used in this analysis revealed good feasibility of this methodology for parameter estimation. These examples also supported the importance of considering concrete creep properties as a structural error factor in concrete cable-stayed structures. The estimated concrete creep properties may be used to reduce the long-term error associated with concrete creep in concrete cable-stayed structures.

Experimental results on cracking and deformation behavior of RC members subjected to short-term loading are frequently misinterpreted because shrinkage effect is not taken into account. Even at first loading, free shrinkage strain of concrete may well exceed the cracking strain. The shrinkage strain, restrained by reinforcement, significantly affects the cracking resistance and short-term deformations of RC members. Despite this, most known constitutive laws were derived by using the test data of shrunk RC members under the influence of tension stiffening coupled with shrinkage effect. In this paper, a numerical procedure has been proposed for eliminating shrinkage from moment-curvature and tension-stiffening relationships. The procedure is on the basis of the smeared crack approach and layer section model. It combines direct and inverse techniques of analysis of RC members. The inverse technique, earlier proposed by the first writer, aims at deriving tension-stiffening stress-strain relationships from experimental moment-curvature diagrams. The shrinkage effect was eliminated by assuming in the direct technique a positive (expansion) free shrinkage strain. On the basis of the proposed procedure, free-of-shrinkage tension-stiffening and moment-curvature relationships were derived by using test data of shrunk RC beams obtained by the writers and reported by other investigators. It was shown that negative portions of the tension-stiffening curves disappear after eliminating shrinkage.

Reinforced concrete (RC) structural walls are the primary lateral-load carrying elements in many structures designed to resist earthquakes. A review of the technical literature shows considerable uncertainty with regards to the effective stiffness of these structures when subjected to seismic excitations, which many design practices currently deal with by employing a stiffness reduction factor. In an attempt to obtain additional information regarding the stiffness of these structures, an analytical approach, combining the flexure and shear components of deformation, is proposed to evaluate the effective stiffness of the RC walls tested. Based on this proposed analytical approach, a comprehensive parametric study comprising 180 combinations was carried out and a simple equation for assessing effective stiffness of RC squat structural walls then proposed, on the basis of these parametric case studies.

The shear capacity of reinforced concrete (RC) members is often associated with sliding across inclined planes often referred to as critical diagonal cracks. However, quantifying the shear capacity of the RC member in terms of the sliding resistance of an inclined plane as a result of shear-friction has been found to be a very complex problem. This is because these sliding planes transcend both initially cracked and uncracked planes, their capacity is also a function of the separation between these sliding planes, and invariably the shear-friction sliding capacity overestimates the shear capacity of the member. In this paper, a structural mechanics model that incorporates shear-friction is developed for quantifying the various components of the shear resistance across a critical diagonal crack because of both longitudinal reinforcement and stirrups. It is shown that the shear resistance is less than would be anticipated from the direct application of shear-friction theory because the compressive force in the uncracked region of concrete is less than can be anticipated and because the shear resistance must provide shear forces to maintain equilibrium prior to resisting the direct shear force.

Nonlinear finite-element models were developed, verified, and used to simulate the measured response of twelve 3.54 m (11.6 ft) tall posttensioned masonry walls with $810\times 100\mathrm{mm}$ ($32\times 4\mathrm{in}.$) cross sections that were tested to failure under uniform lateral loading in a laboratory. Variables included masonry type, wall slenderness, degree of tendon restraint, and prestress magnitude. The force-displacement relations obtained from the experimental tests of the walls were simulated using models developed by using an extended version of the program Dynamic Response Analysis of Inelastic Two-Dimensional Structures (DRAIN-2DX), and the results illustrate the influence of tensile strength, masonry type, magnitude of prestress, and tendon restraint on wall response to out-of-plane lateral loading. The computations indicate that load-displacement response of posttensioned masonry (PTM) walls is influenced by masonry tensile strength and that the stiffness and number of tendon restraints have a direct effect on peak load capacity and postpeak unloading slope. Tendon stress and masonry compression strain responses to applied loading were also investigated using finite-element analysis.

High-performance fiber-reinforced concrete (HPFRC) materials have a unique strain-hardening behavior in tension that translates into enhanced structural response, especially under reversed cyclic loading. Recent experimental research has shown that the use of HPFRC to replace regular concrete in components subjected to high cyclic deformation demands can lead to significant benefits, such as relaxation of detailing requirements and reduction in the amount of reinforcing steel. A structural system that is a good candidate to benefit from HPFRC is reinforced concrete coupled walls, where coupling beams and plastic hinge zones undergo large cyclic deformation demands during the design seismic event. This paper discusses the seismic performance of a prototype 18-story coupled-wall system in which the wall plastic hinge zones and the coupling beams are made of HPFRC materials instead of regular reinforced concrete. Computational simulation models are used to investigate system performance under various hazard levels, and system response is evaluated through various parameters including interstory drift, rotation, and distortion of critical structural parts. The simulation results show that the use of HPFRC in place of regular concrete leads to good overall seismic response with enhanced plastic hinging behavior in the wall piers and crack control in the coupling beams and piers.

The primary objective of this paper is to present a general review of the different macromodels used for the analysis of infilled frames. A number of distinct approaches in the field of analysis of infilled frames since the mid-1950s have yielded several analytical models. These studies stressed that the numerical simulation of infilled frames is difficult and generally unreliable because of the very large number of parameters to be taken into account and the magnitude of the uncertainties associated with most of them. In this paper, the advantages and disadvantages of each macromodel are pointed out, and practical recommendations for the implementation of the different models are indicated.

In this study, a novel approach using a modified time series analysis methodology is used to detect and locate structural changes by using ambient vibration data. In addition, it is shown that the level of the damage feature gives important information about the relative change of the damage severity, although direct damage quantification is not achieved. In this methodology, random decrement (RD) is used to obtain pseudofree response data from the ambient vibration time histories. Autoregressive models with exogenous input (ARX models) are created for different sensor clusters by using the pseudofree response of the structure. The output of each sensor in a cluster is used as an input to the ARX model to predict the output of the reference channel of that sensor cluster. After creating ARX models for the healthy structure for each sensor cluster, these models are used for predicting the data from the damaged structure. The difference between the fit ratios is used as the damage feature. The methodology is first applied to experimental ambient vibration data from a steel grid structure, in which different damage scenarios, such as local stiffness loss and boundary condition change, are simulated. The results show that damage was detected and located successfully for most of these cases. Moreover, it is observed that the relative extent of the damage is also estimated by using the method. Then, output-only data from the Z24 bridge is used for further verification of the methodology with real-life data where different levels of pier settlement were applied as damage. It is shown that the approach is successful in damage identification and localization with a minimum number of false alarms. The potential and advantages of the methodology are discussed on the basis of the experimental results. Limitations of the approach are also addressed, along with future research directions.

Nondestructive dynamic methods are a useful tool in structural engineering. Their purpose is twofold. First, they provide guidance on the validation of a mathematical model of the structure under investigation. Second, repeated tests over time can indicate the emergence of possible damage occurring during the structure’s lifetime. Recent technological progress has generated extremely accurate and reliable experimental methods, enabling a good estimate of dynamic behavior of a structural system. Although experimental techniques are now well-established, the interpretation of measurements still lags somewhat behind. This particularly concerns structural identification due to its nature of inverse problem. In addition, when identification techniques are applied to the study of real-world structures, additional obstacles arise given the complexity of the structural behavior, the inaccuracy of the analytical models used to interpret experiments, measurement errors, and incomplete field data. This study discusses some of these aspects with reference to a series of harmonic forced-vibration tests carried out on the new headquarters of the Hypo Bank (Tavagnacco, northeast Italy). The special structural typology of the tilted building and the complex distribution of the resisting structural members have made interesting, and at the same time quite complicated, the interpretation of the real dynamic behavior of the construction. The present paper (Part I) is devoted to the experimental programme. The use of the measured dynamic data in the calibration of numerical models of the Hypo Bank complex is presented and discussed in Part II.

The Hypo Bank headquarters complex, a structure formed by two adjacent seven-story buildings, was subjected to a series of harmonic forced vibration tests in August 2005. In Part I of this research, the natural frequencies, damping factors, and three-dimensional shapes of the first vibration modes of each building were estimated. In this paper, the experimental characteristics are compared with those predicted by a preliminary finite-element model. The properties of vibration modes with prevailing amplitudes along the longer side of the complex are described with poor precision. This large disagreement has suggested that, in spite of the presence of the seismic joint between the two buildings, some coupling through the joint should be considered to predict the dynamic behavior of the whole complex reasonably well.

This paper presents the results of experimental tests undertaken to resolve questions regarding the viability of multistory prestressed timber structures, specifically the reduction in the prestress load over time. Reduced- and large-scale specimens were tested for 1 year in controlled and uncontrolled heated indoor conditions. Specimens included beams and two-bay frames made from radiata pine laminated veneer lumber (LVL) with a box-shaped cross section. The prestress load was applied through the center of the section by using an unbonded tendon. All relevant quantities such as load in the prestress tendon, deflection, timber moisture content, environmental temperature, and relative humidity were monitored throughout the test. Functions were fitted to the one-year experimental curves and then used to indicate more clearly the trends of the prestress losses during the testing. For a beam in which all timber is loaded parallel to the grain, a reduction in prestress of 1.4% was found after 1 year, whereas for a frame in which 11% of its length is loaded perpendicular to the grain, the loss increased to 7% because of the presence of columns. Furthermore, an attempt was made to separate the contributions made to prestress losses by key factors, namely creep and mechano-sorption of the LVL that is parallel and perpendicular to the grain; relaxation of the prestressing strand; and shrinkage/swelling on account of environmental variations. The most important factor is the proportion of timber that is loaded perpendicular to grain compared with the timber loaded parallel to grain.

This paper investigates the long-term behavior of beams and frames made from laminated veneer lumber (LVL) prestressed with unbonded tendons. A formula for the evaluation of the prestress losses during the service life was derived from the integral equations of wood and tendons using the age-adjusted effective modulus method. All phenomena affecting the time-dependent behavior of the system, such as creep, mechano-sorption, thermal and moisture strains of the wood, and relaxation and thermal strains of the steel tendon, were considered. The formula was validated on experimental results showing the possibility to adequately predict the losses, although with some approximation. The formula was simplified further by removing some parameters that have little or almost no effect, and rewritten in a nondimensional format, leading to an easy-to-use closed-form solution suitable for implementation in codes of practice. The most important parameters governing the prestress losses were the proportion of the timber member’s length loaded perpendicular to the grain, and the ratio between the axial stiffnesses of the tendon and timber frame. An increase in these parameters significantly raises the amount of prestress loss. Possible methods for the mitigation of losses in a frame are finally presented and briefly discussed.

A systematic approach to determining reliability-based acceptance criteria for deteriorating elements in structural systems is proposed as a basis for calibration of safety factors in codes and standards and for verifying acceptability of inspection and maintenance strategies for specific structures. The goal is to establish deterioration acceptance criteria for the elements of a structural system in compliance with criteria formulated for the system. Existing methods significantly overestimate the deterioration reliability of redundant structural systems because they neglect the joint effect of deterioration failures of different elements. To more realistically capture the load-sharing behavior of deteriorating redundant structural systems, it is proposed to establish deterioration acceptance criteria on the basis of easily computable, idealized structural systems, which are calibrated to the characteristics of the real structure. The approach is validated on an example structural system and is found to represent a significant improvement over current methods. The paper concludes with a study of the main factors influencing acceptance criteria of deterioration reliability.

Using steel pedestals has become an effective method for elevating simply supported bridges in the United States. However, a method is needed to estimate their lateral load capacity and probability of failure subjected to different levels of applied loads. This paper shows the development of probabilistic models for the lateral load capacity of steel pedestals. The capacity models consider the prevailing uncertainties, including statistical uncertainty and model errors due to inaccuracy in the model form or missing variables. The proposed capacity models are used to estimate the conditional failure probability (or fragility) of an example steel pedestal for given sets of lateral and vertical loads. Because of the discontinuity in the limit state function, Monte Carlo simulation is used to estimate the fragility. Results show that for the studied steel pedestal, increasing vertical load decreases the probability of failure subjected to lateral loads. To investigate the effect of different random variables on the results, a sensitivity analysis is also conducted for the example steel pedestal.

Eight building seismic rubber bearings (four uninsulated lead rubber bearings, one insulated lead rubber bearing, two uninsulated natural rubber bearings, and one insulated natural rubber bearing) were experimentally investigated in fire tests following the ISO834 standard heating process. The effects of vertical load and the duration of fire on the failure mode and residual mechanical properties of the rubber bearings were analyzed. The experimental results show that (1) the fire resistance of rubber bearings without insulation fails to exceed 1.5 h; (2) the duration of fire exposure has significant influence on the mechanical-properties degradation of rubber bearings; (3) the effect of vertical load, to which rubber bearings were subjected during the fire, on the residual mechanical properties and fire resistance is inevident; and (4) the proposed insulation is successful in preventing rubber bearings from significant degradation in mechanical properties in fires.

Duration effects on the design wind loads for glass are examined with respect to ASCE 7-10. Pressure time histories are created by combining variations of wind speed and wind direction from a “design cyclone” with pressure coefficients obtained from wind tunnel tests of low-rise buildings. The results indicate that the assumption of a 3-s gust duration in ASCE 7-10 may underestimate appropriate values for design loads for glass, with the degree of underestimation varying, depending primarily on the probability of exceedence of the peak pressures. On the basis of these results, modifications to the peak pressures specified in ASCE 7-10 are suggested that maintain the time duration of 3-s, consistent with current glass-design provisions.

The estimation of the extreme of non-Gaussian load effects for design applications has often been treated tacitly by invoking a conventional peak factor on the basis of Gaussian processes. This assumption breaks down when the loading process exhibits non-Gaussianity, in which a conventional peak factor yields relatively nonconservative estimates because of failure to include long tail regions inherent to non-Gaussian processes. To realistically capture the salient characteristics of non-Gaussian load effects and incorporate these in the estimates of their extremes, this study examines the peak factor for non-Gaussian processes, which can be used for estimating the expected value of the positive and negative extremes of non-Gaussian load effects. The efficacy of previously introduced analytical expressions for the peak factor of non-Gaussian processes on the basis of a moment-based Hermite model is evaluated and the variance of the estimates in standard deviation is derived. In addition, some improvements to the estimation of the peak factor and its standard deviation are discussed. Examples, including immediate applications to other areas, illustrate the effectiveness of this model-based peak factor approach.