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In the presence of softening section constitutive relations, classical beam theories predict erroneous strain singularities, and the corresponding force/flexibility-based (FB) beam-column element formulations result in strain localization and loss of response objectivity, i.e., divergence, rather than convergence, with progressive mesh refinements. To address this challenge, various FB element formulations have been proposed in the literature. One of these formulations is the so-called "gradient inelastic" (GI) FB formulation, which is a two-node element formulation that eliminates the strain localization and achieves response objectivity through strain gradient nonlocality relations. Although a single two-node GI element can effectively simulate an entire beam or column, simulating such a member via multiple two-node GI elements in series (e.g., to apply intermediate point loads, to more accurately capture geometric nonlinearities, or to represent cross-section variation) would not lead to accurate response predictions. This is because, in a model with multiple two-node GI elements in series, the nonlocality relations are not enforced at the intermediate/connection nodes between adjacent elements. Instead, end member boundary conditions (BCs) are enforced at those connection nodes because the two-node GI formulation has been designed to simulate an entire member. To tackle this shortcoming, this paper proposes an innovative multinode GI FB element formulation. To enforce the nonlocality relations at the connection nodes, two different sets of mathematically admissible section strain compatibility conditions (CCs) are adopted. The multinode formulations using both sets of CCs are evaluated through several simulation examples, including beams and columns subjected to various loads. The evaluations demonstrate the ability of both element formulations to produce objective softening responses, while one set of CCs is found to more closely predict the responses of previously tested RC beams under midspan loading. [ABSTRACT FROM AUTHOR]

The present paper deals with the optimal design of nonprismatic beams, i.e., beams with variable cross section. To set the optimization problem, Euler-Bernoulli unshearable beam theory is considered and the elastica equation expressing the transverse displacement as a function of the applied loads is reformulated into a system of four differential equations involving kinematic components and internal forces. The optimal solution (in terms of volume) must satisfy two constraints: the maximum Von Mises equivalent stress must not exceed an (ideal) strength, and the maximum vertical displacement is limited to a fraction of beam length. To evaluate the maximum equivalent stress in the beam, normal and shear stresses have been considered. The former was evaluated through the Navier formula, and the latter through a formula derived from Jourawsky and holding for straight and untwisted beams with bisymmetric variable cross sections. The optimal solutions as function of material unit weight, maximum strength, and applied load are presented and discussed. It is shown that the binding constraint is usually represented by the maximum stress in the beam, and that applied load and strength affect the solution more than material unit weight. To maintain the generality of the solution, the nondimensionalization according to Buckingham Π -theorem is implemented and a design abacus is proposed. [ABSTRACT FROM AUTHOR]

Due to the strength reduction at the bent corners of fiber-reinforced polymer (FRP) stirrups, the high tensile strength of FRP reinforcement cannot be fully utilized in concrete structures. Although numerous experimental investigations have been conducted to study the reduced bent-corner strength, the mechanism of the strength reduction at bent corners has not been fully understood. This paper presents numerical research into the bent-corner strength of FRP stirrups with rectangular cross sections. Refined finite-element models are established based on Hashin's failure criterion and their validities are verified against six groups of 18 FRP stirrup specimens. Analyses of stress concentration in the elastic range and damage accumulation afterward are conducted to explain the mechanism of strength reduction at the bent corners of FRP stirrups. Further parametric analyses are conducted to quantitatively evaluate the contributing factors to the bent-corner strength and an empirical prediction equation is proposed. This research shows that the bond, compressive stress from concrete, and FRP material anisotropy are the critical contributors to the tensile stress concentration in the elastic range at the bent corners, which accounts for 7%, 48%, and 45% of the tensile stress concentration, respectively. The parametric analyses show that better bonding at the bent corners could potentially result in lower strength; the concrete properties and FRP width have limited influence on the bent-corner strength; and the bent-corner strength has logarithmic relation with the bent radius and thickness ratio of (R/t). This work reveals the critical mechanism of strength reduction of FRP stirrups at bent corners in concrete structures and potentially provides the opportunity for more efficient material use of FRP reinforcement. [ABSTRACT FROM AUTHOR]

Rebar buckling and fracture have been experimentally observed to cause significant degradation of the mechanical properties of reinforced concrete (RC) elements and should be accounted for in the analysis of RC structures. While data on the material properties of steel rebar are widely available, test data on rebar buckling remain scarce. This paper proposes a macromechanical model capable of predicting rebar buckling responses solely on the basis of steel material properties and the rebar geometry, even when rebar buckling experimental data are not available. This is achieved by computationally building the uniaxial stress-strain constitutive response at each analysis step through the equilibrium in the deformed configuration of the buckled rebar and through the cross-sectional constitutive response. The equilibrium in the deformed configuration is treated in a manner consistent with the corotational formulation, while the cross-sectional constitutive response is built through integration of a uniaxial damage plasticity model over the cross-section area. Rebar buckling is triggered by an initial imperfection, while low-cycle fatigue effects are reproduced through the uniaxial damage plasticity steel material model, without the need for cycle-counting algorithms. The proposed model is translated to a computer code, calibrated to a set of experimental data, validated to separate test data, and subsequently used in the analysis of an experimentally studied RC column that experienced rebar buckling under cyclic loading. Using the proposed model, the analysis captured the response of the RC column and the experimentally observed strength deterioration due to rebar buckling. [ABSTRACT FROM AUTHOR]

With the increasing demand for vertical isolation in the engineering field, the compressive behavior of low–shape factor (LSF) lead-rubber bearings (LRBs) needs to be further explored. However, the conventional simulation method of lead boundary conditions cannot accurately capture the actual compressive behavior of LSF bearings. Some scientific literature has reported this, but there is a lack of detailed studies to explain it. By comparing the experimental data and numerical simulation results of LSF bearings, the problem of this research is put forward. Thus, a refined numerical simulation method of lead boundary conditions is proposed in this paper based on the manufacturing process of LRBs. Assumed contact types, friction coefficients, and filling ratios of the lead plug are considered as variables of lead boundary conditions. The first shape factor and lead plug diameter are considered bearings variables. The compressive behavior of LRBs, including stiffness, deformation, and stress–strain distribution, are discussed through detailed analysis. The main study results indicate that contact types and filling ratios of lead plugs have a significant influence on the compressive behavior of LSF bearings, and the friction coefficient is negligible. The proposed refined simulation method is capable of capturing LRBs' mechanical behavior with better accuracy than the conventional model under different stresses, especially LSF bearings. However, the conventional numerical modeling method is recommended for LRBs with high shape factors (e.g., 30–50). Finally, an empirical formula is established based on refined simulation results. [ABSTRACT FROM AUTHOR]

Stiffness modifiers are used in linear structural analysis programs to recognize the low-strain amplitude nonlinear material stress–strain response and support the design of reinforced concrete structures. For example, stiffness modifiers are used to account for the reduction in stiffness that results mainly from crack development and propagation, bar bond slip between cracks, and bar bond slip of the longitudinal reinforcement developed beyond the column ends. This paper presents a primarily mechanistic model to compute the stiffness modifiers for reinforced concrete rectangular columns to support the seismic design of reinforced concrete structures. The model accounts for five modes of deformation found in columns and is calibrated against a database of test results reported in the literature on 38 large-section columns that exhibited ductile response. The paper also describes a simplified equation for use in seismic design. The simplified equation was calibrated through an extensive parametric analysis of the model. The mean, median and coefficient of variation of the ratios of the stiffness modifiers computed from the recorded response of test columns and the model output were 1.02, 1.00, and 0.18, respectively, and those calculated by the simplified equation for the same ratios were 0.95, 0.98, and 0.18, respectively. [ABSTRACT FROM AUTHOR]

Application of fiber-reinforced polymer (FRP) composites in the strengthening of reinforced concrete (RC) structures has become an increasingly accepted engineering practice. In particular, the use of externally bonded FRP wraps as a confining material for concrete can enhance both the compressive strength and the ultimate strain of concrete, making it suitable for strengthening and/or seismic retrofit of existing RC columns. This paper focuses on a recently developed analysis-oriented iterative FRP-and-steel confinement model for concrete and proposes a new optimization procedure to obtain an analytical expression for the corresponding monotonic envelope, reduce the associated computational cost, and increase the corresponding numerical robustness. Several analytical functions were evaluated in terms of their capability to fit the iteratively-generated stress-strain monotonic envelope for confined concrete. The newly proposed analytical formulation of the confined concrete stress-strain model was compared with the original iterative formulation in terms of computational cost for two examples of nonlinear seismic response analyses for: (1) an experimentally-tested concrete-filled FRP tube bridge column of a two-column bridge pier; and (2) a five-span bridge structure with FRP-retrofitted RC piers. It is found that the use of the newly proposed optimization-based analytical monotonic envelope can reduce by more than 30% the computational time associated with the original iteration-based monotonic envelope with negligible changes in the structural response prediction at both global and local levels. [ABSTRACT FROM AUTHOR]

As inelastic design for wind is embraced by the engineering community, there is an increasing demand for computational tools that enable the investigation of the nonlinear behavior of wind-excited structures and subsequent development of performance criteria. To address this need, a probabilistic collapse assessment framework for steel structures is proposed in this paper. The framework is based on the integration of a high-fidelity fiber-based nonlinear structural modeling environment with a wind-tunnel-informed stochastic wind load model to perform nonlinear time history analysis. General uncertainty is propagated using a stratified sampling scheme enabling the efficient estimation of reliabilities associated with rare events. The adopted models for simulating high-fidelity nonlinear structural behavior were found, in general, to be adequate for capturing phenomena, including progressive yielding, buckling, and low-cycle fatigue, that are essential for wind induced collapse analysis. In particular, the adopted fatigue model was found to be capable of predicting damage and potential fiber/section fracture associated with non–fully reversing stress-strain cycles that are characteristic of wind loading. Through illustration on a 45-story archetype steel building, critical discussions on the types of observed collapse mechanisms, the difference between along-wind and across-wind nonlinear behavior, reliabilities associated with first yield, and collapse are presented. A probabilistic description of the residual and peak story drifts is also provided through development of fragility functions. [ABSTRACT FROM AUTHOR]

There is no doubt that the strength and ductility of concrete structures can be enhanced significantly by lateral confining with steel or fiber reinforced polymer (FRP). Nevertheless, studying the mechanical response of eccentrically loaded confined concrete has received little attention compared to the study of a concentrically loaded concrete structure. To achieve a better fundamental understanding of the mechanical behavior of eccentrically loaded concrete under nonuniform confinement, an experimental study with analytical modeling was conducted in this work. A total of 100 concrete cubic specimens were tested with eccentric loadings and various lateral confinements from a fabricated apparatus that can simulate and generate passive confining pressure. Furthermore, this study successfully extended a well-established stress-strain model, originally proposed for eccentrically loaded FRP-confined circular concrete columns, to nonuniformly confined concrete cubes. The applicability and modification of this eccentricity-based model are verified and developed with the test results in this work. This paper concluded that there is good accuracy and agreement between the theoretical predictions and test results. [ABSTRACT FROM AUTHOR]

Although multiple-model structural identification (MM ST-ID) approaches appear to offer clear, conceptual benefits over singlemodel approaches, they have not yet been employed within a transparent scenario that will allow quantitative comparison, critique, and refinement. To fill this gap, the research reported in this paper aimed to (1) implement and compare current MM ST-ID approaches on a physical laboratory model to establish their accuracy and identify their merits and shortcomings, and (2) identify the ability to refine MM ST-ID methods by weighing observations based on their correlation with the desired predictions. The scenario implemented used modal parameters as the observations, and static displacements and strains as the desired predictions. The various MM ST-ID methods were evaluated based on how their prediction distributions agreed with the actual responses of the physical model. The results indicated that while all methods were successful in bounding the actual responses, the Bayesian updating approach proved to be the most efficient in terms of required number of simulations, and was able to produce prediction distributions with the smallest bounds (while still incorporating all measured responses). In addition, the mean of the MM ST-ID prediction distributions did not coincide with the model that had the largest weight (i.e., the highest likelihood), which indicates that single model approaches not only are unable to provide estimates of variability, but may produce biased predictions. Finally, through a second set of scenarios, the research reported in this paper showed how prediction distributions may be improved by weighing observations based on their correlation with the desired predictions. [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) structures are subjected to combinations of axial, shear, flexure, and torsional loading during an earthquake event. Comprehensive understanding of the behavior of shear panels is essential to predict the response of members under combined loading. This work focuses on the analytical modeling of RC shear panel elements. A baseline fixed strut-angle model (FSAM) was initially proposed for the shear behavior of RC panel elements, satisfying the three basic principles of mechanics: stress equilibrium, strain compatibility, and material constitutive behavior. The FSAM was later modified to consider the effect of dowel action and aggregate interlock through empirical relationships. Use of modified FSAM formulations for the estimation of the monotonic response of shear panels with different reinforcement ratios in longitudinal and transverse directions leads to loss of convergence after yielding of the transverse reinforcement. Thus, an accurate estimation of postyield response is not possible using the existing FSAM, which is critical for seismic design. This paper presents an improved algorithm for FSAM by employing experimentally validated constitutive relationships for the aggregate interlock mechanism. Comprehensive correlation studies considering parameters including the amount of reinforcement and concrete strength are reported to understand the improved FSAM. Predictions of the improved FSAM for shear panels with unequal reinforcement are better than those derived using the existing FSAM approach. [ABSTRACT FROM AUTHOR]

Tension field theory assumes that after elastic shear buckling has occurred, compressive stresses in the web plate do not increase; therefore, postbuckling shear strength solely results from the development of tensile stresses within a defined diagonal tension field. Finite-element analyses have found this fundamental assumption to be invalid and a new theory (compression theory) was proposed that bases the development of postbuckling shear strength mostly on the compressive response of the plate (tension plays a secondary role). Previous work has validated compression theory against a wide variety of published experimental data at ambient temperature. This paper shows that compression theory can predict the ultimate postbuckling shear strength of steel web plates up to temperatures of 1,100°C. This finding is significant because steel plate girders subjected to fire loading are highly susceptible to web shear buckling. Experimentally validated finite-element models are used to develop temperature-dependent multiplicative parameters to permit the use of compression theory to calculate ultimate postbuckling shear strength at elevated temperatures. Comparisons with published experimental data show that predictions of ultimate postbuckling shear strength from compression theory closely agree with the literature. [ABSTRACT FROM AUTHOR]

To support the repair and maintenance of steel eyebars in bridges, tension stresses are sometimes estimated from the observed member natural frequencies. Although the ends of such eyebars are often essentially fixed, a pin-ended model has typically been used to predict tensions because closed-form solutions do not exist for any other end conditions. An approximation that reflects vibrating stretched wires with fixed ends has also been used, but this still incurs considerable error at lower member slenderness and stress levels. To provide for better estimates of eyebar tensions from observed vibrations, this paper presents exact tabular solutions for determining the direct tension stress in undamped steel bars, generalized for any length or thickness, where both ends of the member are fixed or where one end is fixed and the other pinned. Approximation equations derived from these exact solutions are also developed. Examples demonstrate the application of these new tools and the potential increase in accuracy over the former methods. [ABSTRACT FROM AUTHOR]

The bond of reinforcing steel bars is closely related to the stress state in the surrounding concrete. However, most of the current studies are limited to static monotonic loading and only few studies on the dynamic bond behavior of deformed bars under lateral pressures are reported in the literature. The purpose of this paper is to present an experimental investigation on the effect of the loading rate on the bond behavior of deformed bars subjected to biaxial lateral pressures. The test results indicate that the bond parameters are influenced by lateral pressures, loading rate, bar diameter, and concrete strength. Finally, an empirical bond stress-slip relationship for deformed steel bars is proposed and validated against experimental results. [ABSTRACT FROM AUTHOR]

A finite-element (FE) model to simulate nonlinear structural behavior of posttensioned composite beams with partial shear connectors is introduced in this paper. Stress-strain material models for concrete and steel are implemented in the FE formulations and the nonlinear load-slip relation of shear connectors is also considered. With the load-slip relation, a nonlinear numerical formulation to consider the slip effect along the interface of posttensioned composite beams is derived. A solution procedure, which includes modeling of the section in prestressed concrete (PSC) structures, tendon behavior for consideration of sequential loading conditions, and a unique section analysis methodology to consider the partial shear connection of posttensioned composite beams, is also proposed. Numerical results yielded by the proposed model are compared with experimental results for verification of the model's performance. The results show satisfactory structural behavior of partially composite posttensioned beams, and thus the applicability of the proposed FE model is validated. [ABSTRACT FROM AUTHOR]

Low yield point (LYP) structural steels had been developed and used for the seismic energy dissipating, however, the fracture of various LYP steel energy dissipation components may lead to progressive collapse of structures. To address this issue, the fracture behavior of LYP steels under various stress states were comprehensively investigated using experiments, theoretical analysis, and numerical simulations in this work. There are three meaningful contribution points. First, an improved weighted average (WA) method and dichotomy-based optimization are proposed to calibrate true stress–strain relation after necking onset. The maximum deviation for the prediction of full-range tensile load-displacement behavior based on the proposed WA method is less than 3%, which is better than the existing WA methods. Second, a modified ductile fracture model is developed to identify the fracture of LYP steels under various stress states. Comparison results among existing fracture models, the proposed fracture model and tests indicate that the fracture plastic strain can be more reasonably predicted by the proposed ductile fracture model than the existing models. Thirdly, an advanced ductile fracture prediction framework is proposed to simulate the complete tensile behavior of structural steels. Based on the proposed framework and object-oriented programming technique, the complete tensile behavior can be accurately and conveniently predicted by employing the proposed WA model, proposed ductile fracture model and developed user defined material subroutine. The verified WA method and ductile fracture model would contribute to predicting the failure of structural steel members, connections, welds and structures. It is meaningful to prevent casualties and property losses resulting from the subsequent structural progressive collapse. [ABSTRACT FROM AUTHOR]

Ultrahigh toughness cementitious composite is a cementitious material that exhibits the characteristic of pseudo strain hardening under uniaxial tension, and has potential use in structures that experience repeated or fatigue loads. A series of quasi-static, dynamic, and fatigue tests were performed to evaluate the frequency effect on the compressive fatigue behavior of this material; in this paper the results are reported. It is found that the fatigue life and deformation pattern are influenced by loading frequency. The relationship between the secondary strain rate, the secondary strain rate per cycle, and the fatigue life is illustrated and equations applied to predict the fatigue life of specimen are developed. The failure strain of each loading frequency is found to follow the Weibull distribution. Based on the distribution of the static failure strain at the same stress level, a probabilistic model of the fatigue failure strain is proposed to describe the frequency effect, which gets a considerable agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Limited research has been conducted on the fire resistance of high-strength steel (HSS) T-joints after an earthquake. The aim of this study is to assess postearthquake fire resistance and determine the optimal design dimensions of HSS-circular hollow sections-square hollow sections (CHS-SHS) T-joints considering the nonuniform temperature field effect through parameter analysis. The computational models of HSS T-joints developed in this study were validated against existing test results. Twenty four computational models related to HSS-CHS-SHS T-joints were established using the finite element software ANSYS; further, a sequential temperature-displacement analysis method was used to perform a parameter study. Subsequently, parameter analysis was conducted on HSS-CHS-SHS T-joints by varying parameters such as chord-width-to-thickness ratio (γ), brace-to-chord-thickness radio (τ), brace-to-chord-width ratio (α), chord-length-to-height ratio (β), and damage variable (D˜). The results indicate that the failure of HSS-CHS-SHS T-joints is attributed to plasticization of the chord surface, and that parameters α and D˜ have a significant influence on the postearthquake failure mechanism and catastrophic process. Moreover, an empirical theoretical model for determining the bearing capacity of HSS-CHS-SHS T-joints after exposure to fire is proposed and validated. The HSS-CHS-SHS T-joint is widely used in offshore platforms, building frames, transmission towers, etc. Secondary fires occur frequently after an earthquake, which typically cause huge casualties and economic losses. Since earthquakes may produce residual stress and deformation in HSS-CHS-SHS T-joints, the failure mechanism of HSS-CHS-SHS T-joints under fire cannot be used to accurately evaluate their reliability and safety under a postearthquake fire. Thereby, the investigation of this study can provide valuable reference for design, repair, and reinforcement of HSS-CHS-SHS T-joints. [ABSTRACT FROM AUTHOR]

A beam string structure is an efficient hybrid system comprising beams, cables, and struts. This study proposes an active beam string structure that adapts to external loading for deflection control, achieved by replacing passive struts with telescopic ones. An optimization-based model is created to minimize deflection, with the maximum deflection of beams serving as the optimization objective. A deflection control framework is constructed by using a hybrid genetic algorithm and back-propagation neural network. The former combines the strengths of the genetic and gradient descent algorithms, and the latter trains a prediction network applying mechanical responses, resulting in quick output of control schemes. To assess the control framework's performance, a scaled model is designed and fabricated, including a measuring system for deflection and stress, an actuating system with telescopic struts, and a PC-based decision-making system. Experimental and numerical studies are carried out for the model. The control schemes using the hybrid genetic algorithm and back-propagation neural network successfully reduced the deflection responses by at least 80% in simulations and experiments. The results validate the accuracy of the algorithm and reliability of the network, further demonstrating the effectiveness of the control framework. In addition, the deflection control process also optimizes the internal forces of the beam, with a maximum decline rate of stress response approaching 60%. [ABSTRACT FROM AUTHOR]

This paper investigates the fatigue behavior of reinforced concrete beams at low temperatures compared to similar beams tested in fatigue at room temperature. Four large-scale steel-reinforced beams (200 × 400 × 4,200 mm) were fabricated and tested. All of the beams had temperature differentials over their depth to simulate solar radiation and in-service temperature of the bridges. Two beams had stirrups while the other two had no shear reinforcement. One beam of each type was tested at room temperature and the other at low temperature. The beams were cyclically loaded to failure with a stress range representative of the ratio of live to dead loads found in most bridges. This study showed that the fatigue life of the reinforced concrete improved at low temperature and this improvement was much more pronounced when shear reinforcement was not present. In fact, low temperature changed the mode of failure of the reinforced concrete beams without stirrups from shear to flexural fatigue failure as a result of the higher shear strength and stiffness of the reinforced concrete, the lower number of cracks with smaller widths, and improved fatigue properties of steel at low temperature. The results showed that the fatigue life of the reinforced concrete beam without shear reinforcement at room temperature primarily depended on the concrete and concrete-to-steel bond while fatigue lives of the other beams depended on the stresses in the tensile reinforcement. The strains and stiffness degradation were much lower in the low-temperature beams compared to their room-temperature counterparts. The findings of this study showed that the contribution of concrete to the performance of the beam under either static or cyclic load becomes more pronounced at low temperatures. [ABSTRACT FROM AUTHOR]

The direct strength method (DSM) design rules for cold-formed steel members in shear have been incorporated recently into the North American specification for the design of cold-formed steel structural members and are being implemented in the Australian cold-formed steel structures standard. The method, which was calibrated for unperforated members only, requires two inputs including the buckling load Vcr and the shear yielding load Vy. For members with square web cutouts, Vcr can be computed by either the spline finite strip method or the tabulated values based on the shear buckling coefficients kv as in previous studies or the finite element method. However, Vy has not been accurately formulated including holes. This paper represents a practical model to obtain Vy for members with central openings subjected predominantly to shear. The model ranges from very small holes where traditional shear yielding predominates to large holes where Vierendeel action dominates. The model is verified with the DSM design formulas using the predominantly shear tests recently conducted at the University of Sydney and Queensland University of Technology with both square and circular web openings and for shear spans with aspect ratio of 1.0. [ABSTRACT FROM AUTHOR]

In moderate seismic regions, engineers are permitted to use a response modification coefficient, R, of 3 in the design of steel systems without any specific seismic detailing or proportioning requirements. Recent research, however, has raised questions regarding the reliability of concentrically braced frame (CBF) systems designed using the R ¼ 3 provision because their performance under the maximum considered earthquake seismic hazard is inherently dependent on reserve capacity--lateral force-resisting capacity outside the primary seismic force-resisting system (SFRS). In a previous study by the authors, two full-scale two-story CBF tests were conducted to better understand seismic behavior and associated reserve capacity after significant damage in the primary SFRS. These tests were (1) an R ¼ 3 chevron; and (2) an R ¼ 3.25 split-x ordinary concentrically braced frame (OCBF). To complement the experimental program, a numerical parametric study was conducted to further distinguish the influence that two design parameters--system type and system configuration--have on overall system behavior including non-ductile limit states and reserve capacity. Although the prototype frames studied were designed for the same building and location, they exhibited a wide variety of limit states. The occurrence of particular classes of mechanisms and their associated reserve capacities was influenced by the system type and system configuration. Specific reserve capacity mechanisms depended on the location of damage within the primary SFRS. Predicting these damage locations is difficult because they depend on as-built weld overstrength. This paper explores the classes of reserve capacity mechanisms that were observed both experimentally and in the companion numerical simulations. [ABSTRACT FROM AUTHOR]

A new type of lightweight shear walls made with a laminated bamboo (glubam) stud frame and ply-bamboo sheathing panels is introduced in this paper. The lateral loading experiments of such shear walls and corresponding similar lightweight wood shear walls with three different panel-frame nailing connections and two thicknesses of sheathing panels were performed, in accordance with international standards. Test results demonstrate that the lightweight glubam shear walls have similar performance as conventional lightweight wood-frame shear walls, with higher initial stiffness and peak load, but less ductility. An increase in nail diameter can improve the load-bearing capacity of the shear walls until the failure mode of panel-frame connection changes from yielding of the nails to pull-through of nails from the ply-bamboo panels. Increased panel thickness can increase the bearing capacity and deformability of the shear walls when the failure pattern of panel-frame connection changes from panel failure to nail yielding. A new uplift hold-down device and improved design of edge studs were used to increase the ductile performance of glubam shear walls. [ABSTRACT FROM AUTHOR]

Longitudinal cracks in wood beams of heavy timber structures are commonly seen and can impair the load-carrying capacity of such beams. This paper presents the experimental and numerical modeling results on 35 pieces of full-scale wood beams with saw kerfs that simulate lengthwise checks. The influences of the checks and self-tapping screws used for retrofitting purposes were identified based on the normalized load-carrying capacity of the tested beams. It was found that a longitudinal check (up to 2/3 of the beam span) may cause a resistance loss up to 46.2% compared to an unchecked beam, and the use of self-tapping screws may increase the resistance to no less than 86% of that of an unchecked beam. A three-dimensional (3D) finite-element method based model was developed and verified to further quantify the stress intensity factor at the check tip. Based on the modeling and test results, a simple formula was established for prediction of the stress intensity factor, considering the geometry of the checks and the contribution of the self-tapping screws. The generated formula was verified with good agreement and therefore can be used for determination of the shear resistance of checked wood beams and the required screw spacing for retrofitting purposes. [ABSTRACT FROM AUTHOR]

The mechanism method, based on the particularization of limit design to masonry structures proposed by Heyman, is certainly a very suitable procedure for the analysis of no-tension material arches, especially because of its simplicity. The literature largely treats the method under the hypothesis of infinite compressive strength of the material, whereas fewer studies are devoted to finite compression strength. In this paper the latter case is revisited with reference to masonry arch bridges, assuming a rigid-perfect plastic model for the material and using a nondimensional formulation. In this way, results are derived and discussed for a wide range of parameters. The results are compared with those obtained using Heyman's hypotheses. [ABSTRACT FROM AUTHOR]

Forum papers are thought-provoking opinion pieces or essays founded in fact, sometimes containing speculation, on a civil engineering topic of general interest and relevance to the readership of the journal. The views expressed in this Forum article do not necessarily reflect the views of ASCE or the Editorial Board of the journal. [ABSTRACT FROM AUTHOR]

In recent years, significant advances have been accomplished in the fields of modeling, analysis, and design of deteriorating civil engineering systems, and novel approaches to life-cycle assessment, maintenance planning, and optimal design of structural systems have been proposed. Despite these advances, life-cycle concepts are far from being explicitly addressed in design and assessment codes and effectively implemented into practice. There is, therefore, a need to promote further research in the field of life-cycle performance of structural systems under uncertainty and to fill the gap between theory and practice by incorporating life-cycle concepts in structural design and assessment codes. An effort is currently ongoing within the Structural Engineering Institute (SEI)/ASCE to meet this need. This paper is part of this effort and is aimed at presenting a review of the main principles, concepts, methods, and strategies for life-cycle assessment and design of deteriorating structural systems under uncertainty. General criteria for deterioration modeling are first presented, with emphasis on the effects of corrosion and fatigue in steel structures and chloride-induced corrosion in concrete structures. The time-variant structural performance is then investigated with reference to a set of probabilistic performance indicators, and the structural lifetime associated with a reliability target is formulated. The role of inspection and monitoring, the effects of maintenance and repair interventions, and the definition of cost-effective maintenance strategies are discussed. The concepts of life-cycle performance assessment and maintenance planning are used to formulate the life-cycle reliability-based design problem in an optimization context. [ABSTRACT FROM AUTHOR]

This paper attempts to enhance (1) the flexibility-corotational beam formulation for simulating highly geometrically nonlinear problems, and (2) the arc length method of integration to develop a robust and economical solution scheme for random quasi-static loading input. The existing flexibility-corotational formulation of a three dimensional beam is improved by (1) updating the rotation matrices by using the quaternions, and (2) proposing novel methods for tracking the displacement compatibility. Further, the cylindrical arc length method is modified by (1) introducing capabilities for handling random quasi-static loading, and (2) proposing automatic adjustment of the arc length. By using these developments several classical geometric nonlinearity problems are solved and path independency for buckling and post-buckling path with the random quasi-static loading is verified. The proposed variation in the arc length solution scheme is also shown to be better than the implementation of the arc length method in the state of the art commercial software. [ABSTRACT FROM AUTHOR]

The article analyzes the study which focuses on the strain rate effect on steel reinforcement-concrete bond characteristics for static and low-cycle dynamic loading conditions. It notes that the researchers provides a detailed paper that reports experimental findings on the effect loads such as those produced by blast. The study shows that the slope of the strain profile is a measure of the bond stress between rebar and concrete.

Exceptional mechanical properties of ultra-high performance concretes (UHPC) offer strong strengthening capacities in bending and shear when used as overlay on normal strength concrete (NSC) structures. Nonetheless, lack of simple and intuitive design models for hybrid elements in design guidelines refrain designers from using UHPC overlays for structural applications. Thereby, a simplified sectional analysis model for NSC-UHPC hybrid elements was developed based on the philosophy of the Canadian Bridge Design Code CSA-S6. By using a new average stress distribution for NSC in hybrid elements that considers the strain at the extreme compressed fiber, equilibrium of forces can be solved by a second-degree equation with direct computation. The simplified model provides the complete moment–curvature behavior of hybrid elements for design purposes, thus allowing verifications in service and ultimate state conditions. An empiric equation is also proposed to evaluate the maximum bending capacity of hybrid elements for predesign. It only uses an approximation of a lever arm between forces in the hybrid cross section and thus offers a quick and easy way to evaluate the bending capacity. Both tools were validated on a detailed and iterative sectional analysis program and with results of four international experimental campaigns. The simplified sectional analysis model and empirical equation showed very good accuracy at reproducing the behavior of a wide range of NSC-UHPC hybrid elements configurations. [ABSTRACT FROM AUTHOR]

The high-frequency force balance (HFFB) test is a widely used method for estimating lateral turbulent wind loads on tall buildings in a wind tunnel. The method relies on use of a highly accurate force sensor placed at the base of the tall building model to measure base forces and overturning moments. Because the turbulent wind load excitation is broadband, it is important to adequately design the connection detail between the model and the sensor. Accurate design is needed to avoid contaminating the measurement of the lateral wind loads by the potential spurious resonant effect, which is induced by flexibility in the connection detail. This design often relies on the experience of the wind tunnel modeler. On occasion, because of the limitations of test setup and environment, the connection detail between the building model and the HFFB force sensor may not be sufficiently stiff. Consequently, resonant response caused by the flexible connection may affect the power spectral density function of the measured wind loads. One of the methods to address this problem is tofilter the HFFB output signal and to remove undesirable response within a specific frequency interval by conventional digital filter methods. However, this may cause the removal of important features of the wind loading. This paper presents a simple yet efficient method, based on classical dynamic theory, to eliminate the spurious effect. The method is applied to the "correction" of an experimental wind load spectrum. [ABSTRACT FROM AUTHOR]

This paper presents a formal optimization methodology for the design of seismic retrofitting of three-dimensional irregular buildings. The damping coefficients of viscous dampers potentially allocated in given feasible locations as well as the stiffness of their supporting braces are adopted as design variables. The objective function minimizes a cost function of the dampers while constraints are added to limit various responses of interest to allowable values under a filtered white noise excitation (e.g., interstory drift at each location separately, total acceleration at each location separately, force of each damper, stress in each supporting brace, force/forces of each structural member, base shear, overturning moment at the base, side constraints on dampers' forces and braces' cross sections, etc.). A first-order optimization method is adopted for that purpose. The constraints on various normalized responses are condensed to a single constraint on their maximum value and the gradient required is efficiently derived analytically using the adjoint analytical method. Thus, a computational effort at the order of a single additional analysis is required for the evaluation of the gradient of the constraint regardless of the number of design variables considered or responses to be constrained. This efficient scheme enables a study on the effect of limiting the brace size with and without a limit on its stresses. [ABSTRACT FROM AUTHOR]

To explore how steel-plate composite (SC) walls response to in-plane loading is essential for SC structural design, especially combined with thermal loading. This work focuses on the intermediate high temperature (≤400°C) that is often considered during design process. Individual mechanical behavior of steel plate and concrete in SC walls under intermediate high temperature are studied. Thermoplasticity is applied to describe steel plate as 2D element, by which plastic flow combined with mechanical property degradation can be considered. High-temperature mechanical behavior of concrete is very complicated. In this study, strain of concrete at high temperature is decomposed into transient thermal strain, free thermal strain, and mechanical strain. By elaborating the physical meaning of transient thermal strain and selecting mathematical models for every strain component, deformation of high-temperature concrete can be calculated. To study the in-plane behavior experimentally, an SC panel test with in-plane shear and thermal loading was conducted. Steady thermal state and transient thermal state were considered in two specimens, respectively. Comparison between test data and theoretical calculation results shows good agreement. Verifications from other researchers' tests show that the model can be applied to general situations of in-plane loading combined with intermediate high temperature. [ABSTRACT FROM AUTHOR]

Concrete-filled thin-walled helical corrugated steel tubes (CFCST) are innovative composite members suitable for rehabilitating or reconstructing piers/columns under earthquake or long-term corrosion damage. The strength and ductility of the concrete can be effectively improved by the lateral confinement of the thin-walled corrugated steel tube (CST). However, for such a member, the previously proposed monotonic model is not applicable to large helical angles, and no existing cyclic model can be directly employed. Hence, some key indicators, including the nominal hoop stress, equivalent confining stress, peak stress, peak strain, plastic strain, and stress deterioration, are examined based on an enlarged experimental database and the working mechanism of CFCST. Subsequently, a systematic constitutive model with an updated monotonic stress–strain relationship and rational unloading/reloading rules was developed for the CST-confined concrete in this study. The accuracy of the proposed model was validated by the experimental results of both the monotonic and cyclic responses of CST-confined concrete specimens. [ABSTRACT FROM AUTHOR]

Collapse in steel structures is often controlled by loss of load carrying capacity of steel columns due to interactive buckling, which involves interactions between local and global (i.e., lateral and lateral torsional) buckling. Commonly used concentrated plastic hinge or fiber-based elements do not simulate the physics of this response, potentially leading to inaccuracy in performance assessment. A nonlinear fiber-beam-column element [termed the Torsion Fiber Element (TFE)] to simulate monotonic interactive buckling in steel beam-columns is presented. The element, implemented in the OpenSees platform, incorporates St. Venant as well as warping torsion through enrichment of strain interpolation functions, in addition to axial and flexural deformation modes. Local buckling is represented through a softening multiaxial constitutive relationship. The efficacy of this approach is examined by comparing its results against those obtained from continuum finite element simulations as well as experimental data on beam-columns subjected to monotonic loading. The comparisons indicate that the element can functionally represent the physics underlying interactive buckling, resulting in effective prediction of the overall monotonic load-deformation response, as well as internal deformation and stress fields. Limitations of the element in its current form are summarized, along with prospective improvements. [ABSTRACT FROM AUTHOR]

The use of unstressed Grade 1860 MPa seven-wire steel strands as longitudinal reinforcement has the potential to reduce steel congestion in reinforced concrete members. However, no studies which use unstressed steel strands as the longitudinal reinforcement in concrete columns have been reported in the literature. Thus, in this study, five large-scale column specimens: four of which used unstressed steel strands and one which used conventional Grade 420 MPa deformed bars as longitudinal reinforcement were subjected to cyclic loading under constant axial load to investigate the effectiveness in using strands. Columns with unstressed steel strands as longitudinal reinforcement exhibited drift capacities ranging between 4.96% and 5.70%, which is well over the conventional expectation of 3.50% drift requirement for columns in special moment frames. Also, the specimens exhibited lower energy dissipation but enhanced recentering capability owing to the fact that the strands have significantly higher tensile yield capacity compared to deformed bars. A reduction in effective elastic stiffness was also observed as the postcracking stiffness of specimens with strands dropped due to the low longitudinal reinforcement ratio of 0.78% provided. The test results indicated the relative ineffectiveness of strands in compression than in tension, where the strand bulged and unwound once the surrounding confinement deteriorated. Despite this, strands did straighten up and continued to sustain tensile stresses under load reversal. Based on the experimental observations, two constitutive models, one which considers strain hardening and the other which assumes an elasto-perfectly plastic response in tension was proposed for strands. The constitutive model with strain hardening was able to produce reasonably accurate estimates of flexural strength when used in a detailed moment-curvature analysis. Both constitutive models when used in a simplified analysis, which used equivalent stress block for concrete, was able to consistently produce conservative estimates of flexural strength, which is suitable for design purpose. [ABSTRACT FROM AUTHOR]

The high cost of manufacturing high-performance fiber-reinforced cementitious composites (HPFRCCs) and low confidence in predicting their mechanical properties have limited their widespread use as construction materials. Also, the lack of reliable numerical stress-strain cyclic behavior models prevents the analysis and study of potential industrial applications. In this research, a new uniaxial fiber-reinforced cementitious composite constitutive model (FRCC model in OpenSees) is developed and validated for use in monotonic and cyclic analyses consisting of fiber section finite elements. The model is validated at the material and structural level through correlations of the numerical results with the limited laboratory test data available in the literature, as well as some new experimental results. The quantification of the uncertainties in structural response parameters due to model input parameter uncertainties is conducted via sensitivity analysis. The validation results make the model useful for performance-based assessment and design of civil engineering structures that use HPFRCCs. [ABSTRACT FROM AUTHOR]

This study numerically investigated the elevated temperature resistance of earthquake-damaged square hollow section (SHS) T-joints subjected to brace axial compressive load. The residual stresses and deformations due to the earthquake were obtained from a pushover analysis. The extensive numerical simulations were analyzed using nonlinear finite-element software to examine the effects of the β ratio, γ ratio, damage level (Dˇ), and initial applied load (nf) on the fire resistance of SHS T-joints after verification of the numerical simulations based on the available test results in the literature. The results indicated that the fire resistance of SHS T-joints with axial compressive load in the brace member decreased significantly due to the residual stresses and deformations from the earthquake. This situation worsened in all numerical models as the damage level increased. The reduction rate of the critical temperature at various damage rates was independent of the initially applied load ratio, but the loading rate directly affected the fire resistance of the joint regardless of the earthquake. As the β ratio decreased, the yielded area at the brace-to-chord intersection region increased. Thus, the critical temperature of the joints at a constant damage level was inversely proportional to the β ratio. Joints with a large γ ratio had larger residual stresses and deformations than those of corresponding joints with a small γ ratio. Therefore, the fire resistance of the joints after the earthquake decreased as the chord-wall thickness decreased. The critical temperature of the joints with either a large β or γ ratio was limited by the deformation limit at high damage levels. The fire resistance of the SHS T-joints after the earthquake decreased due to residual stresses and deformations. That effect increased even more in joints with high β and γ ratios. [ABSTRACT FROM AUTHOR]

An analytical procedure is used to study the effects of moment gradient and load height on the elastic stability of wide flange steel beams. Lateral torsional buckling is the limit state considered. Solutions are developed for a series of general moment functions which are produced by continuous load types with possible end moments. For each load type, an equivalent uniform moment factor is developed. Additionally, a load height factor is developed to modify the equivalent uniform moment factor for these load types where loading is applied above the shear center. Solutions are processed numerically using a Taylor series polynomial approximation. Results are presented in terms of an equivalent uniform moment factor and a load height factor. Comparison with national code procedures for moment factor show discrepancies that are conservative in some circumstances by up to 51% and unconservative in others by up to 8%. The largest discrepancies occur under the effect of reverse curvature bending and load position above the shear center. Results for load position show that members loaded above their shear center are more susceptible to lateral torsional buckling than those loaded at their shear center. Some design examples are presented using the load height factors developed. [ABSTRACT FROM AUTHOR]

Similar to the definition of the moment-independent importance measure on model failure probability, the authors define the moment-independent importance measure concerning the importance sampling method. After combining the importance sampling procedure, the sampling efficiency is improved and the cost of computation is cheaper. After the moment-independent importance measures of all inputs are obtained, a new method called conditional importance sampling is proposed to calculate the failure probability and those inputs with high importance measures are chosen to be conditional variables. And state-dependent parameter method is employed in the new method. A numerical example and two structural engineering examples are used to demonstrate that the failure probability estimated by the proposed conditional importance sampling method is accurate and converges faster than those estimated by the traditional importance sampling method. [ABSTRACT FROM AUTHOR]

Buckling-restrained brace frames (BRBFs) have been a popular seismic force-resisting system in the US since it was first introduced to the AISC Seismic Provisions and ASCE 7 in 2005. The steel core of a buckling-restrained brace (BRB) yields in tension and buckles plastically in a high-mode buckling pattern in compression under cyclic loading. Prequalification testing of BRBs in conformance with the AISC 341 requirements relies on a highly idealized loading protocol that is not representative of the expected response during an earthquake. Although testing has demonstrated that BRBs have excellent energy dissipation capacity, a low-cycle fatigue model will permit designers to evaluate the remaining life of BRBs after a seismic event. To develop and validate such model, 18 full-scale BRBs in groups of three different yielding strengths (1,110; 2,220; and 3,330 kN) were cyclically tested to fracture with the following loading conditions: (1) symmetric cycles with constant strain amplitudes (±0.25% to ±3%), (2) asymmetric cycles with constant strain amplitudes and non-zero-mean strains, (3) symmetric cycles with variable strain amplitudes, and (4) simulated earthquake responses. Two low-cycle fatigue models for BRBs (a standard model and an alternative model for ease of practical use) were developed based on 11 symmetric, constant-amplitude tests. An assessment procedure combining the proposed fatigue models, rainflow cycle counting method, and Miner's damage index was proposed to predict the fatigue life of BRBs subjected to random cyclic loading histories, the accuracy of which was verified by the variable-amplitude test results. [ABSTRACT FROM AUTHOR]

This study examined the effects of the unit weight of concrete (ωc) on the flexural performance of shear walls with different amounts (Ash) of transverse reinforcement in the boundary elements. Six shear wall specimens with barbell-shaped cross sections were tested up to failure under a constant axial load and cyclic lateral loads. Backbone curves determined using the cyclic lateral load–displacement of shear walls were compared with predictions obtained using the two-dimensional (2D) nonlinear laminar approach to examine the effect of ωc on the confinement pressure provided by the transverse reinforcement. The test results indicated that lightweight aggregate concrete (LWAC) shear walls exhibited a smaller displacement ductility ratio and cumulative work damage indicator than their normal-weight concrete (NWC) shear wall counterparts. The conservatism of the ACI 318-19 approach using the equivalent stress block for estimating the moment capacity of shear walls was somwhat affected by ωc or Ash. Meanwhile, the confinement by transverse reinforcement in the boundary elements tended to decrease with a decrease in ωc , which resulted in earlier damage to the longitudinal reinforcement and a faster reduction in the applied load at the post-peak performance for LWAC shear walls than for NWC ones with the same Ash. The current 2D nonlinear laminar approach can accurately estimate the overall flexural performance of shear walls made using different types of concrete. [ABSTRACT FROM AUTHOR]

An analytical model is proposed to predict the stress–strain relationship of confined concrete subjected to reinforcement corrosion. The stress–strain graph of confined concrete with corroded reinforcement was predicted using estimation of four key parameters: maximum compressive stress, strain at maximum compressive stress, modulus of elasticity of concrete, and ultimate compressive strain of concrete. To validate the model, analytically predicted stress–strain curves of confined concrete of circular RC columns damaged due to corroded reinforcement were compared with those obtained from full-scale experimental tests with varying degrees of corrosion. The results of the comparison indicate good agreement. The RC columns had identical size, the same longitudinal steel reinforcement and spiral confinement type but with different spiral pitches providing varying levels of confinement. The corroded RC columns were categorized into four corrosion groups: noncorroded, low corrosion, high corrosion, and mechanical pitting corroded. The presented model is based on a modified version of the Mander's universal model for RC columns. [ABSTRACT FROM AUTHOR]

This paper presents a new predictive formula for the shear capacity evaluation of reinforced concrete members subjected to combined axial, bending, and shear forces. The formula is based on a tensorial approach to concrete shear resistance that is very similar to the one used in the shear-enhanced fiber beam element. The total shear resistance is broken down into the contribution of concrete and contribution of shear reinforcement. The concrete contribution to the shear resistance is calculated using a normal-shear stress failure envelope. Normal (longitudinal) stresses are calculated from axial and bending forces acting on the concrete member. In the formulation, a number of simplifications are made to keep the formula as simple as possible but still sufficiently accurate. The resulting formulation, although capable of accounting for all of the major variables that influence the shear strength, including size effect, remains particularly simple and with a compact notation. The predictions of the proposed formula are compared with those used by the American Concrete Institute (ACI), the European Committee for Standardization (CEN) Eurocode, and the International Federation for Structural Concrete (fib) model code, and its accuracy is checked against a vast experimental database available in the literature. Results and comparisons are very encouraging and confirm the soundness of the underlying mechanical model. The capability of this model to provide a unified approach for reinforced and unreinforced members opens up the possibility to extend the application of the proposed formula to engineered cementitious composites, such as fiber-reinforced concrete. [ABSTRACT FROM AUTHOR]

The advancement in structural health monitoring (SHM) technology has been evolving from monitoring-based diagnosis to monitoring-based prognosis. The structural stress response derived by the measured strain data is increasingly used for structural condition diagnosis and prognosis because it can be directly used to indicate the safety reserve of a structural component or provide information regarding the load-carrying capacity of the whole structure. Therefore, accurate forecasting of structural stress responses is an essential step for the reliable diagnosis and prognosis of structural condition. For a large-scale, complex structure subjected to multisource effects such as live loads and environmental loads, its stress evolution is a typically nonlinear dynamic process. Moreover, the online monitoringderived stress data extracted from an SHM system are extremely massive. This arouses a strong demand for developing a computationally efficient and accurate method for forecasting structural stress responses. In this work, we propose the use of a Bayesian modeling approach with Gaussian processes (GPs), which allows for probabilistic forecasts of structural stress responses and has great capability of modeling the underlying nonlinear dynamic process. Although powerful for characterizing dynamic nonlinearity of structural stress responses, the conventional GP-based Bayesian modeling approach remains computationally intensive because of the massive stress data increasingly gathered by the monitoring system. We propose a moving window strategy to substantially shrink the size of training data, thus leading to a reduced-order GP model and effectively alleviating the high computational cost. The feasibility of the reduced-order GP-based Bayesian modeling approach is illustrated by using the real-time monitoring-derived stress data acquired from a supertall structure. Its performance is compared with the full GP-based Bayesian approach, and the comparison results indicate that the proposed approach holds higher computational accuracy and efficiency for stress response forecasts than the traditional method. [ABSTRACT FROM AUTHOR]

The article focuses on the Technical Council on Life-Cycle Performance, Safety, Reliability and Risk Structural Systems established by the Structural Engineering Institutes (SEI)-ASCE Technical Activities Division. It was established due to the need for risk-informed approaches to civil infrastructure design and management. The council promotes important directions for research and applications for life-cycle performance assessment and risk-informed decision making in structural engineering.

A large number of experimental curvatures in plastic hinges of concrete members are used alongside analytical moment-curvature relations to backestimate strains in the bars, the extreme concrete fibers of a section, or its confined core at the ultimate conditions of concrete sections in flexure with axial loads. Strain limits derived from these ultimate strains can support fiber models for prismatic members or models with finite-length plastic hinges at the ends. The measurements come from circular or rectangular columns (some tested diagonally), walls, or beams. The ultimate strains derived for steel bars and concrete are not local material properties (especially for cyclic loading): they depend on the geometric features of the section as a whole and of the immediate vicinity of the most critical point in the section. The size effects are clear: (1) concrete ultimate strains increase in a small compression zone, (2) the monotonic ultimate strain of tension bars increases with decreasing number of bars in the tension zone, (3) the ultimate strain of steel in cyclic tests increases with the increasing number of bars in the compression zone, and (4) the ultimate strain of confined concrete is larger at a section corner in biaxial bending than along the full side of a rectangular compression zone in uniaxial flexure, while at the perimeter of a circular section, it is in between these two extremes. The cyclic ultimate strain of steel in tension increases as the bar diameter-to-stirrup spacing ratio increases, thanks to the delay of bar buckling in previous compression half-cycles. The ultimate strains derived apply both as mean values in a plastic hinge and at the end section of prismatic members. Compared to experimental ultimate curvatures, those computed from the proposed ultimate strains do not have bias and exhibit much less scatter than those obtained from arbitrary ultimate strains specified in some codes. These code predictions are, in general, unsafe. [ABSTRACT FROM AUTHOR]

A method is presented to calculate the elastic flexural and warping normal stresses in simple span cold-formed C- and Z-sections, with end torsional restraints and one flange attached to the sheathing. The method utilizes a displacement compatibility approach to calculate the normal stresses at transition points in the cross section, i.e., web-flange intersection, flange-stiffener intersection, and the end of the flange stiffener. The method incorporates the stiffness of the diaphragm and the torsion-restraining effects of the sheathing. The example provided shows that the distribution of stresses across the cross section is sensitive to the restraining effects of the sheathing and can deviate significantly from the typically assumed stress distribution of constrained bending. The procedure for calculating the stresses and deformations and system forces is outlined, and trends observed by the authors are discussed. [ABSTRACT FROM AUTHOR]

It is often assumed that, at a given lateral strain, the axial compressive stress and strain of fiber reinforced polymer (FRP)--confined concrete are the same as those of the same concrete when it is actively confined under a confining pressure equal to that supplied by the FRP jacket. An experimental program was undertaken to assess the validity of this assumption, in which 63 actively confined and FRP-confined normal-strength (NSC) and high-strength concrete (HSC) specimens were tested under axial compression. The axial stress-strain and lateral strain-axial strain curves obtained from the two different confinement systems were assessed. The results indicate that, at a given axial strain, lateral strains of actively confined and FRP-confined concretes correspond when they are subjected to the same lateral confining pressure. At the points of intersection on the lateral strain-axial strain curves, FRP-confined NSC exhibits only slightly lower axial compressive stresses compared with those of actively confined NSC. Conversely, the difference between the axial stresses of actively confined and FRP-confined HSC is found to be significant, indicating that the compressive behavior of confined HSC is more sensitive to the application path of confining pressure than the behavior of confined NSC. By using the combined results of the present study and two comprehensive experimental data- bases of actively confined and FRP-confined concretes, an expression has been developed for the prediction of the difference in the confining pressures that results in differences in the axial stresses between actively confined and FRP-confined concretes. [ABSTRACT FROM AUTHOR]

Analytical models are proposed to predict the compressive resistance of discontinuous bracing members in steel X-braced frames when governed by flexure of the connecting plates at the mid-connections. The connections are bolted single shear splice connections commonly used for HSS bracing members. An incremental analysis procedure is developed to predict the axial load-deformation response of the brace-connection assembly including geometric and material nonlinearities. The buckling strength depends on the thickness of the connecting plates, the length of the connection, the clear distance between the continuous and discontinuous bracing members at the brace intersection, and the length of the discontinuous brace segments. The method is validated against test results from full-scale X-bracing specimens. It is also used to verify the design procedure proposed in the AISC Design Guide 24 for Hollow Structural Section Connections for a series of 17 X-braces covering a wide range of properties. The AISC design procedure generally over-predicts the buckling resistance. A new design procedure is proposed that better reflects the buckling response of discontinuous braces in X-bracing. [ABSTRACT FROM AUTHOR]