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Real‐time hybrid simulation (RTHS) involves dividing a structural system into numerical and experimental substructures. The experimental substructure is challenging to model analytically and is therefore modeled physically in the laboratory. Analytical substructures are conventionally modeled using the finite element method. The two substructures are kinematically linked, and the governing equations of motion are solved in real‐time. Thus, the state determination of the analytical substructure needs to occur within the timestep, which is of the order of a few milliseconds. All structural systems are supported by a soil‐foundation system and any evaluation of the efficacy of response modification devices placed in the structure should consider soil‐foundation structure interaction (SFSI) effects. SFSI adds compliance to a structural system, thereby altering the natural frequencies. Additionally, nonlinear behavior in the soil can result in residual deformations in the foundation and structure, as well as provide added damping. These effects can occur under both wind and earthquake loading. To overcome the barrier of the large computational effort required to model SFSI effects in real‐time using the conventional finite element approach, a neural network (NN) model is combined with an explicit‐based analytical substructure and experimental substructure with dampers to create a framework for performing RTHS with SFSI effects. The framework includes a block of long‐short term memory (LSTM) layers that is combined with a parallel rectified linear unit (ReLU) to form a NN model of the soil‐foundation system. RTHS of a tall 40‐story steel building equipped with nonlinear viscous dampers and subjected to a windstorm are performed to illustrate the framework. It was found that a number of factors have an effect on the quality of RTHS results. These include: (i) the discretization of the wind loading into bins of basic wind speed; (ii) the extent of the NN model training as determined by the root mean square error (RMSE); (iii) noise in the restoring forces produced by the NN model and its interaction with the integration algorithm; and, (iv) the bounding of outliers of the NN model's output. Guidelines for extending the framework for the RTHS of structures subjected to seismic loading are provided. [ABSTRACT FROM AUTHOR]

The central difference is a popular algorithm used to integrate the equations of motion, yet suffers from two drawbacks: (1) it is only conditionally stable and requires a small-time step to maintain numerical stability; (2) it is non-dissipative, and high-frequency spurious oscillations may appear and compromise the accuracy of the solution. These drawbacks are detrimental to applying the algorithm to the real-time hybrid simulation of large, complex nonlinear structural systems. In this paper, the conventional central difference algorithm is modified to overcome these drawbacks, and the modified algorithm is applied to the real-time hybrid simulation of complex structural systems. [ABSTRACT FROM AUTHOR]

This study presents a finite element model for a hybrid self-centering damper considering the rate and temperature effects and explores the effects of different design parameters on the damper response. The damper, called as superelastic friction damper (SFD), consists of superelastic shape memory alloy (SMA) cables and a frictional energy dissipation mechanism. The experimental response of the SMA cables, frictional unit and overall damper at different loading frequencies and temperature are used to develop numerical model of the damper. Once a validated numerical model is obtained, parametric studies are carried out to evaluate force-displacement response of the damper when the design parameters are altered. The effects of damper design parameters on the equivalent stiffness, dissipated energy, equivalent viscous damping and self-centering capabilities of the damper are analyzed. Based on the findings, the recommendations for the design of the damper are presented. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the multidirectional cyclic lateral-load response of post-tensioned self-centering (SC) cross-laminated timber (CLT) shear walls. SC-CLT shear wall damage states are introduced and qualitatively defined in terms of the repairs needed to restore the lateral-load response of the SC-CLT wall. A comparison between SC-CLT wall damage states under unidirectional (in-plane) and multidirectional (in-plane and out-of-plane) lateral loading is presented. The experimental results show that the initiation of SC-CLT wall damage occurs at smaller story drifts under multidirectional loading compared to unidirectional loading. Engineering demand parameters (EDPs) are used to quantify the SC-CLT wall damage states. Uncertainty in the EDP value when a damage state occurs is considered and quantified. Using the experimental results, component (i.e., a CLT wall panel corner) and system (i.e., an entire SC-CLT wall) fragility functions are developed and presented. [ABSTRACT FROM AUTHOR]

In light of the significant damage observed after earthquakes in Japan and New Zealand, enhanced performing seismic force‐resisting systems and energy dissipation devices are increasingly being utilized in buildings. Numerical models are needed to estimate the seismic response of these systems for seismic design or assessment. While there have been studies on modeling uncertainty, selecting the model features most important to response can remain ambiguous, especially if the structure employs less well‐established lateral force‐resisting systems and components. Herein, a global sensitivity analysis was used to address modeling uncertainty in specimens with elastic spines and force‐limiting connections (FLCs) physically tested at full‐scale at the E‐Defense shake table in Japan. Modeling uncertainty was addressed for both model class and model parameter uncertainty by varying primary models to develop several secondary models according to pre‐established uncertainty groups. Numerical estimates of peak story drift ratio and floor acceleration were compared to the results from the experimental testing program using confidence intervals and root‐mean‐square error. Metrics such as the coefficient of variation, variance, linear Pearson correlation coefficient, and Sobol index were used to gain intuition about each model feature's contribution to the dispersion in estimates of the engineering demands. Peak floor acceleration was found to be more sensitive to modeling uncertainty compared to story drift ratio. Assumptions for the spine‐to‐frame connection significantly impacted estimates of peak floor accelerations, which could influence future design methods for spines and FLC in enhanced lateral‐force resisting systems. [ABSTRACT FROM AUTHOR]

Damage caused by earthquakes to buildings and their contents (e.g., sensitive equipment) can impact life safety and disrupt business operations following an event. Floor isolation systems (FISs) are a promising retrofit strategy for protecting vital building contents. In this study, real‐time hybrid simulation (RTHS) is utilized to experimentally incorporate multi‐scale (building–FIS–equipment) interactions. For this, an experimental setup representing one bearing of a rolling pendulum (RP) based FIS is studied—first through characterization tests and then through RTHS. A series of tests was conducted at the Natural Hazards Engineering Research Infrastructure (NHERI) Experimental Facility at Lehigh University. Multiple excitations were used to study the experimental setup under uni‐axial loading. Details of the experimental testbed and test protocols for the characterization and RTHS tests are presented, along with results from these tests, which focused on the effect of different rolling surface treatments for supplemental damping, the FIS–equipment and building–FIS interactions, and rigorous evaluation of different RP isolation bearing designs through RTHS. [ABSTRACT FROM AUTHOR]

Direct integration algorithms are typically used to solve temporally discretized equations of motion to evaluate the performance of structures under dynamic loading. The stability of these direct integration algorithms are usually investigated for linear elastic structures. However integration algorithms are often applied to structures with nonlinear behavior. This paper presents a procedure based on discrete control theory to investigate the stability of direct integration algorithms applied to multidegree-of-freedom (MDOF) nonlinear structures. The discrete root locus approach is used to investigate properties of the poles of the discrete transfer function matrix representing the nonlinear structural dynamics and to assess the stability of the integration algorithm. The procedure is illustrated using a nonlinear shear building MDOF system to investigate the stability of popular direct integration algorithms, including the Newmark family of integration algorithms, the Hilber-Hughes-Taylor [alpha]-method, and two newly developed explicit integration algorithms. Stability limits are derived for the direct integration algorithms that are found to be conditionally stable. DOI: 10.1061/(ASCE)EM.1943-7889.0000083 CE Database subject headings: Algorithms; Transfer functions; Stability; Structural dynamics; Dynamic loads. Author keywords: Algorithm; Dynamics; MDOF; Transfer function; Stability; Nonlinear.

Real-time hybrid simulation combines experimental testing and numerical simulation by dividing a structural system into experimental and analytical substructures. Servohydraulic actuators are typically used in a real-time hybrid simulation to apply command displacements to the experimental substructure(s). Servohydraulic actuators may develop a time delay due to inherent actuator dynamics that results in a desynchronization between the measured restoring force(s) and the integration algorithm in a real-time hybrid simulation. Inaccuracy or even instability will occur in a hybrid simulation if actuator delay is not compensated properly. This paper presents an adaptive compensation method for actuator delay. An adaptive control law is developed using an error tracking indicator to adapt a compensation parameter used in the proposed compensation method. Laboratory tests involving large-scale real-time hybrid simulations of a single degree of freedom moment resisting frame with an elastomeric damper are conducted to experimentally demonstrate the effectiveness of the proposed adaptive compensation method. The actuator tracking capability is shown to be greatly improved and exceptional experimental results are still achieved when a good estimate of actuator delay is not available. DOI: 10.1061/(ASCE)ST.1943-541X.0000124 CE Database subject headings: Experimentation; Hybrid methods; Monitoring; Substructures; Adaptive systems. Author keywords: Experimentation; Hybrid methods; Tracking; Compensation; Adaptive systems.

A building may suffer damage during an earthquake as a result of inelastic deformations developed in the members or connections. It is important that the structural integrity of the building be assessed to ensure the safety of the occupants. This assessment includes evaluating the ability of the structure to resist the demand from subsequent aftershocks and a major earthquake. In this paper a practical methodology to determine the low-cycle fatigue life of welded structural steel connections subject to inelastic cyclic loading is presented. The methodology is based on concepts of low-cycle fatigue and micromechanics, where an accumulated crack length based on a time history of strain and the corresponding triaxiality stress condition that develops in the structural component is calculated and used to establish the fatigue life. The methodology was used to predict the fatigue life of welded beam-to-column connection test specimens subjected to inelastic loading. A comparison with test results indicates that the methodology predicts reasonably well the relationship between number of cycles to fracture and the plastic rotation range observed in the test specimens. DOI: 10.1061/(ASCE)0733-9445(2009) 135:12(1472) CE Database subject headings: Predictions; Fatigue: Welds; Beam columns; Connections; Earthquake loads.

Real-time pseudodynamic (PSD) and hybrid PSD methods are a means to evaluate structural performance under dynamic loading when a component of a structural system cannot be accurately modeled and is load rate dependent. The real-time PSD and hybrid PSD testing of structures with full-scale elastomeric dampers is presented. A Bode diagram of the complete real-time testing system is used to design a velocity feed forward component that improves the tracking performance of the command displacements by the hydraulic actuator. To evaluate the accuracy of the tests, an on-line error tracking indicator is developed to monitor errors due to actuator lag or lead together with an amplitude indicator to reveal the amplitude errors in actuator measured displacements. The results from the real-time PSD and hybrid PSD tests are presented and evaluated using the error tracking indicators. The stability limits associated with a time delay for the tests are discussed and compared with delays that occurred in the tests. DOI: 10.106l/(ASCE)0733-9445(2009)135:9(1124) CE Database subject headings: Structural stability; Delay time; Hybrid methods; Dynamic tests; Damping.

A new beam-to-column connection for earthquake-resistant moment-resisting frames is introduced. The connection has a beam bottom flange friction device (BFFD) and posttensioned (PT) high-strength steel strands running parallel to the beam. The BFFD provides energy dissipation to the connection and avoids interference with the floor slab. The PT strands produce self-centering connection behavior. The connection behavior requires minimal inelastic deformation of the connection components and the beams and columns, and requires no field welding. A series of seven large-scale tests were performed to investigate the effect of the BFFD friction force, connection details, and the loading history on the performance of the connection under cyclic loading. The test results indicate that the BFFD provides reliable energy dissipation, and that the connection remains damage-free under the design earthquake. DOI: 10.1061/(ASCE)ST.1943-541X.0000006 CE Database subject headings: Connections; Beam columns; Post tensioning; Friction; Frames; Seismic design.

This paper presents research on seismic design of steel special moment resisting frames (SMRFs) with supplemental elastomeric dampers. The SMRF properties and damper design criteria are varied to illustrate their effects on the supplemental damper design. The SMRFs are based on current seismic design practices, and the supplemental dampers are designed using a simplified design procedure (SDP). The SDP allows the structure with dampers to be designed for specified performance objectives, where the structure is intended to reach selected performance levels, defined in terms of story drift and member response, at specified seismic input levels. Nonlinear dynamic time history analyses show that SMRFs with dampers designed using the SDP achieve the specified seismic performance objectives, and that elastomeric dampers are more effective when used in a more flexible SMRE The SDP permits various seismic performance objectives and corresponding design criteria to be specified. Criteria that allow some inelastic behavior (R=2), but limit the story drift to 1.5% under the design basis earthquake are shown to lead to the most effective damper design. DOI: 10.1061/(ASCE)0733-9445 (2009) 135:5 (489) CE Database subject headings: Damping; Seismic design; Rehabilitation; Steel frames.

Direct integration algorithms are often used to solve the temporally discretized equations of motion for structural dynamic problems. Numerous studies have been conducted to investigate the stability of integration algorithms for linear elastic structures. Studies involving the stability analysis of integration algorithms for nonlinear structures are limited. This paper utilizes discrete control theory to investigate the stability of direct integration algorithms for nonlinear structural dynamics. The direct integration algorithms are represented by a closed-loop block diagram, where the nonlinear restoring force of the structure is related to a varying feedback gain. The root locus method is used to analyze the stability of the closed-loop system for various degrees of nonlinear structural behavior. The well-known methods of the Newmark family of integration algorithms and the Hilber-Hughes-Taylor a method, as well as a newly developed integration algorithm, referred to as the CR integration algorithm, are analyzed using the proposed method. It is shown that the stability of an integration algorithm under nonlinear structural behavior is dependent on the poles and zeros of its open-loop discrete transfer function. An unconditionally stable integration algorithm for linear elastic structures is shown not to always remain stable under nonlinear structural behavior. DOI: 10.1061/(ASCE)0733-9399(2008)134:9(703) CE Database subject headings: Algorithms; Dynamics; Transfer functions; Stability; Structural dynamics; Structural behavior.

In structural dynamics, integration algorithms are often used to obtain the solution of temporally discretized equations of motion at selected time steps. Various time integration algorithms have been developed in the time domain using different methods. In order for an integration algorithm to be reliable it must be stable and accurate. A discrete transfer function is used to study the properties of integration algorithms. A pole mapping rule from control theory in conjunction with a discrete transfer function is used to develop new integration algorithms for obtaining solutions to structural dynamics problems. A new explicit integration algorithm, called the CR (Chert and Ricles) algorithm, is subsequently developed based on the proposed method. The properties of the algorithm are investigated and compared with other well established algorithms such as the Newmark family of integration algorithms. By assigning proper stable poles to the discrete transfer function the newly developed CR explicit algorithm is unconditionally stable and has the same accuracy as the Newmark method with constant acceleration. In addition, the CR algorithm is based on expressions for displacement and velocity that are both explicit in form, making it an appealing integration algorithm for solving structural dynamics problems. DOI: 10.1061/(ASCE)0733-9399(2008) 134:8(676) CE Database subject headings: Algorithms; Structural dynamics; Transfer functions; Stability; Discrete elements.

The seismic performance of a composite moment resisting frame (MRF) comprised of concrete filled tube (CFT) columns and wide flange beams was investigated experimentally. The four-story composite MRF test structure was designed using performance-based design concepts. The performance objectives include achieving: (1) the operational performance level under the frequent occurrence earthquake; (2) the life safety performance level under the design basis earthquake; and (3) the collapse prevention performance level under the maximum considered earthquake. The hybrid pseudodynamic test method was used to subject the test structure to these various seismic input levels. P-[DELTA] effects associated with the gravity frames in the prototype building were included analytically in the tests. Results from the tests indicated that the structural performance under the simulated seismic loading was consistent with the expected performance for all three earthquake levels, indicating that effective seismic performance of composite MRFs with CFT columns can be achieved. DOI: 10.1061/(ASCE)0733-9445(2008)134:2(279) CE Database subject headings: Composite structures; Earthquake resistant structures; High strength concrete; High strength steel; Frames; Hybrid methods; Seismic effects; Pseudoynamic method.

Rate-independent and rate-dependent models are presented for the hysteretic shear stress-strain behavior of elastomeric damping materials. A rate-independent hysteretic model, called the general asymptote and power function (GAPF) model, is presented that simulates different types of hysteretic behavior depending on the selected asymptote function. A rate-dependent hysteretic model, formed from a parallel combination of the GAPF model and a dashpot, is also presented which simulates loading frequency dependent behavior in addition to strain amplitude dependent behavior. Closed-form expressions for the shear stress as a function of shear strain are provided for each model. The models are calibrated for three different damping materials, and good correlation between experimental and analytical hysteretic behavior is observed. The models are investigated under variable cyclic loading. To prevent unrealistic stress values (overshooting) after a small strain reversal followed by reloading, a sequential asymptote model is introduced, based on the GAPF model. The hysteretic models were incorporated into a finite-element program within an elastomeric damper element, and the results of nonlinear time history analyses of a building structure with elastomeric dampers under simulated earthquake loading are presented to illustrate behavior of the hysteretic models under several loading histories. DOI: 10.1061/(ASCE)0733-9399(2007)133:11(1162) CE Database subject headings: Hysteresis; Energy; Damping; Elastomer; Loading history.

As the use of buckling-restrained braced frames (BRBFs) has increased in the United States, the need has grown for knowledge about member and system behavior under seismic loads and for implementing this knowledge into design provisions. In particular, methods for designing BRBFs and predicting seismic response require validation. To address this need, along with the need for experiments demonstrating system-level BRBF performance, a research program composed of numerical and large-scale experimental simulations was initiated at the ATLSS Center, Lehigh University. This paper describes the nonlinear dynamic analyses that were conducted as part of this research program. Numerical simulations of BRBF response were conducted using ground motion records scaled to two seismic hazard levels. The performance of the prototype BRBF was acceptable and performance objectives were met in terms of structural damage. It is shown that the currently accepted deflection amplification factor underestimates mean inelastic lateral displacements under design-level earthquakes and the system overstrength factor may be unconservative. The current method for predicting BRB maximum ductility demands is also shown to be unconservative and a more rigorous method for predicting BRB maximum ductility demands is provided. DOI: 10.1061/(ASCE)0733-9445(2007) 133:9(1195) CE Database subject headings: Steel frames; Dynamic analysis; Buckling; Bracing; Energy dissipation; Design standards; Building codes; Seismic effects.

As buckling-restrained braced frames (BRBFs) have been used increasingly in the United States, the need for knowledge about BRBF behavior has grown. In particular, large-scale experimental evaluations of BRBFs are necessary to demonstrate the seismic performance of the system. Although tests of buckling-restrained braces (BRBs) have demonstrated their ability to withstand significant ductility demands, large-scale BRBF tests have exhibited poor performance at story drifts between 0.02 and 0.025 rad. These tests indicate that the large stiffness of the typical beam-column-brace connection detail leads to large flexural demands that cause undesirable failure modes. As part of a research program composed of numerical and experimental simulations, a large-scale BRBF with improved connection details was tested at the ATLSS Center, Lehigh University. During multiple earthquake simulations, which were conducted using a hybrid pseudodynamic testing method, the test frame sustained story drifts of close to 0.05 rad and BRB maximum ductility demands of over 25 with minimal damage and no stiffness or strength degradation. The testing program demonstrated that a properly detailed BRBF can withstand severe seismic input and maintain its full load-carrying capacity. DOI: 10.1061/(ASCE)0733-9445(2007) 133:9(1205) CE Database subject headings: Steel frames; Seismic effects; Experimentation; Pseudodynamic method; Buckling; Bracing; Energy dissipation; Connections.

A posttensioned (PT) steel moment resisting frame is a self-centering earthquake resistant steel frame that uses posttensioning steel to compress the beam flanges against the column flanges at the connections. The posttensioning contributes to the moment capacity of the connections and provides an elastic restoring force that returns the frame to its pre-earthquake position. This paper describes the behavior and design of PT frames and PT frame systems, where a PT frame system is a PT frame with the collector elements that connect it to the floor system. The interaction of the floor system with the PT frame produces axial forces in the beams that add to those from the posttensioning. This paper outlines a performance-based seismic design approach for PT steel frame systems. Seismic performance levels, seismic input levels, structural limit states and capacities, and structural demands for PT frame systems are defined. The design objectives are outlined, design criteria are given, and a step-by-step design procedure is given. The design approach is evaluated via comparisons with time-history analysis results. DOI: 10.1061/(ASCE)0733-9445(2007) 133:3(389) CE Database subject headings: Steel frames; Earthquake resistant structures; Seismic design; Flanges.

An analytical study was conducted to investigate the seismic behavior of reduced beam section (RBS) moment connections to a deep wide flange column. Calibrated three-dimensional finite element models of RBS connections in perimeter special moment resisting frames with deep columns were used to perform parametric studies under inelastic monotonic and cyclic loading. The parameters in the study included: beam-to-column connection type, column section, composite floor slab, panel zone strength, and beam web slenderness. The results from the parametric study show that a composite floor slab provides restraint to the top flange of the beams, reducing the magnitude of beam top and bottom flange lateral movement in the RBS, column twist, and strength degradation due to beam instability in the RBS. The effect of beam web slenderness contributing to an increase in the lateral movement of the beam flange in the RBS and column flange is significantly reduced when a composite floor slab is present. A weaker panel zone results in higher potential for ductile fracture in the connection region. An RBS connection to a deep column is shown to have less potential for ductile fracture in the connection region than a welded unreinforced flange connection. The potential for fracture is increased in RBS connections when the column section properties are reduced, leading to larger stress and strain. DOI: 10.1061/(ASCE)0733-9445(2006) 132:3(358) CE Database subject headings: Columns; Beams; Finite element method; Slabs; Seismic effects; Connections.

An experimental study was conducted to investigate the seismic behavior of reduced beam section (RBS) moment connections to a deep wide-flange column. The test matrix for the experimental program consisted of six full-scale interior RBS connections, where the column for the specimens ranged in depth from a W24 to a W36 wide flange section. Five of the specimens had a composite floor slab, with the remaining specimen having no floor slab and a supplemental lateral brace at the end of the RBS. The results from the study show that a composite floor slab provides restraint to the beams, reducing the magnitude of beam top and bottom flange lateral movement in the RBS, column twist, and strength degradation due to beam instability in the RBS. The performance of each of the test specimens was found to meet the seismic connection qualification criteria in Appendix S of the AISC seismic provisions. The results of the experimental study were used to develop a procedure for predicting the torque applied to a column from an RBS connection. DOI: 10.1061/(ASCE)0733-9445(2006) 132:3(346) CE Database subject headings: Beams; Columns; Connections; Bracing; Slabs; Experimentation.

Six full-scale interior connection subassemblies of posttensioned wide flange beam-to-column moment connections were subjected to inelastic cyclic loading up to 4% story drift to simulate earthquake loading effects. Bolted top and seat angles are used in the connection, along with posttensioned high strength strands that run parallel to the beam. These strands compress the beam flanges against the column flange to develop the resisting moment to service loading and to provide a restoring force that returns the structure to its initial position following an earthquake. The parameters studied in these experiments were the initial posttensioning force, the number of posttensioning strands, and the length of the reinforcing plates. The experimental results demonstrate that the posttensioned connection possesses good energy dissipation and ductility. Under drift levels of 4%, the beams and columns remain elastic, while only the top and seat angles are damaged and dissipate energy. The lack of damage to the beams, columns, and the posttensioning enable the system to return to its plumb position (i.e., it self-centers). Closed-form expressions are presented to predict the connection response and the results from these expressions compare well with the experimental results. CE Database subject headings: Steel frames; Steel structures; Steel construction; Seismic design; Connections; Post tensioning; Energy dissipation.

The behavior of high strength square concrete filled steel tube (CFT) beam columns subjected to constant axial load and cyclically varying flexural loading was investigated experimentally. The effects of the width-to-thickness (b/t) ratio, the yield stress ([[sigma].sub.y]) of the steel tube, and the level of axial load on the behavior (stiffness, strength, ductility, and energy dissipation) of high strength square CFT beam columns were studied. Eight cyclic beam-column specimens were tested. Crushing of concrete and cyclic local buckling in the steel tube affected the cyclic strength and stiffness of the CFT specimens. The elastic section flexural stiffness under cyclic loading decreases rapidly due to tension cracking of the concrete infill and local buckling of the steel tube. The total energy dissipated by the cyclic specimens is dominated by the flexural energy dissipated in the failure segment (plastic hinge region). Both the current American Concrete Institute code provisions and the modified Architectural Institute of Japan method predict the moment capacities of the high strength square CFT specimens accurately. The cyclic curvature ductility ([[mu].sub.[PHI]-c]) decreases significantly with an increase in the axial load level. At higher axial load levels, the steel tube b/t ratio and [[sigma].sub.y] have a small influence on [[mu].sub.[PHI]-c]. However, at lower axial load levels, increasing the steel tube b/t ratio or [[sigma].sub.y] reduces [[mu].sub.[PHI]-c]. CE Database subject headings: Seismic tests; Seismic design; Beam columns; Composite columns; Column strength; Earthquake resistance structures; Experimental data.

The behavior of angles in a bolted angle beam-to-column connection is experimentally investigated. The purpose of this investigation is to determine how the angle size and bolt gage length, defined as the distance between the bolt line passing through the column and the heel of the angle, affect the connection stiffness, strength, energy dissipation capacity, and resistance to low-cycle fatigue. The test results show that the angles are capable of considerable strength beyond the point at which a yield mechanism occurs. The associated stiffness, i.e., postyield stiffness, is approximately linear and comprised of both geometric and material hardening. The bolt gage length greatly influences all connection characteristics studied. A larger amount of energy can be dissipated for connections with smaller bolt gage lengths. However, connections with smaller bolt gage lengths generally have a lower fatigue life. An analytical model is presented that predicts the behavior of bolted beam-to-column angle connections. The correlation with test results is good. CE Database subject headings: Cyclic loads; Connections, bolted; Stiffness; Energy dissipation; Fatigue life.

The results of an experimental study of the seismic performance of improved, welded unreinforced beam-to-column moment connections are presented. The study involved the inelastic cyclic testing of 11 full-scale connection specimens to evaluate the effects of weld access hole geometry, beam web attachment detail, panel zone strength, continuity plates, and composite slab on connection performance. With a high toughness weld metal and modified detailing, it is demonstrated that a welded unreinforced flange moment connection can reliably achieve an inelastic rotation of 0.03 rad or more prior to failure. The modified details include the use of a weld access hole with a modified geometry and a welded beam web. The test results indicate that a strong panel zone enhances inelastic connection performance. Based on the results of the study recommendations are given for the seismic-resistant design of improved welded unreinforced connections for steel moment-resisting frames. CE Database keywords: Cyclic tests; Connections, welded; Web beams; Plates; Ductility; Inelastic action.

A comprehensive study has been conducted to investigate the behavior and strength of a new type of partially encased composite column made with thin-walled, welded I-section, stiffened with transverse links. Concrete is poured between the flanges of the steel section. The paper describes and presents the results of the testing of five large-size 600x600 mm short column specimens. Failure of all specimens was due to local buckling of the flange plates along with concrete crushing. Transverse stresses measured in the steel section were found to be small and did not impair the axial compressive strength of the steel section. High stresses, however, developed in the transverse links as a result of the lateral expansion of the concrete. The study also shows that closer link spacing and the use of additional reinforcements can improve the post-ultimate load behavior. A comparison is made with test results obtained in previous studies on smaller specimens and an improved formula for predicting the column axial load capacity accounting for local flange buckling and the variation of the concrete strength with column size is proposed for design application. DOI: 10.1061/(ASCE)0733-9445(2002)128:3(279) CE Database keywords: Composite columns; Strength; Axial loads.