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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]

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.

The flexural force-deformation behavior of high strength square concrete-filled steel tube (CFT) beam-columns was experimentally investigated. The parameters in the study included the width-to-thickness (b/t) ratio, the yield stress ([[sigma].sub.y]) of the steel tube, and the axial load level. Eight CFT beam-column specimens were tested under constant axial load and monotonically increasing flexural loading. The CFT specimens were 305-mm square robes with nominal b/t ratios of 32 or 48, made from either conventional (A500 Grade-B) or high strength (A500 Grade-80) steel, and filled with high strength (110 MPa) concrete. Experimental results indicate that the moment capacity of CFT beam-columns is reached as inelastic behavior of the steel robe and concrete infill is combined with local buckling of the steel robe and crushing of the concrete. The moment capacity of high strength square CFT beam-columns can be predicted with reasonable accuracy using the current American Concrete Institute code provisions for composite columns. The curvature ductility of high strength square CFT beam-columns decreases significantly with an increase in either the axial load level or the b/t ratio of the steel tube. The yield stress of the steel tube does not seem to have a significant influence on curvature ductility. DOI: 10.1061/(ASCE)0733-9445(2002)128:3(309) CE Database keywords: Experimentation; Beam columns; Tubes; Steel; Concrete; Deformation.

A nondestructive methodology is presented for detecting structural damage in structural systems. The procedure is based on using experimentally measured modes and frequencies in conjunction with vibratory residual forces and a weighted sensitivity analysis to estimate the extent of mass and/or stiffness variations in a structural system. Determination of the residual forces and weighted sensitivity analysis involves the use of an analytical model that is correlated to the experimental baseline data from a reference state. This reference state defines the undamaged structural configuration. The method is demonstrated by using a ten-bay space truss as an experimental test bed for various damage scenarios. The experimental results show that the method can accurately predict the location and severity of stiffness change as well as any change in mass for different damage scenarios. The use of an analytical model that is correlated to the baseline test data is shown to improve the prediction; however, reasonable results are also obtained using an uncorrelated analytical model.

A stress resultant plasticity-based formulation with shear failure criteria and post-shear failure effects is presented for modeling the response of nonductile reinforced concrete bridge columns subjected to biaxial seismic loading. The formulation consists of a single component model, in which plastic hinges exist at each end of an elastic beam-column. Deterioration in column strength following a shear failure, kinematic flexural strain hardening, and degradation of elastic unloading stiffness under cyclic load reversal are accounted for in the hinges' force-deformation relationships. The degradation of the transverse shear strength in a reinforced concrete column's flexural plastic hinge zone is also accounted for. The formulation is shown to predict reasonably well the cyclic response of nonductile beam-column experimental test specimens that develop either ductile flexural yielding, a brittle shear failure, or a ductile shear failure in the flexural plastic hinge zone when subjected to uniaxial or biaxial nonproportional cyclic lateral loading. Inelastic seismic time-history analysis is also presented to illustrate the significance of including nonductile effects in a model of a bridge structure susceptible to column shear failure during an earthquake.

An experimental investigation was conducted to investigate the behavior of composite columns subjected to simulated seismic loading conditions. Eight two-thirds - scale specimens were tested, each consisting of a structural steel shape encased in reinforced concrete. The parameters studied in the test program included the degree of concrete confinement required to achieve adequate ductility under cyclic loading, effectiveness of flange shear studs for enhancing flexural stiffness and strength, concrete compressive strength, and the shear resistance mechanism of the composite column. The results of the test program indicate that encased composite columns possess exceptional cyclic strength and ductility if buckling of the longitudinal reinforcement is inhibited. The encased steel shape was found to provide the primary resistance to transverse shear during overloading, with the shear studs not effective in enhancing the flexural resistance to lateral loading. The specimen flexural capacity under combined axial and lateral loading was found to be accurately predicted by analytical models based on full composite action, with ACI and AISC LRFD criteria providing conservative results.

ABSTRACT: A large-scale experimental test program was conducted to assess the residual strength and repair of dent-damaged offshore tubular bracing. The program involved performing 13 tests that included nondamaged, dent-damaged, and repaired specimens. Damaged specimens had a dent depth of 10% of their diameter. The nominal diameter-to-thickness (D/t) ratios were 34.5, 46, and 64 for the specimens. The dent damage significantly reduced the specimen's strength, indicating the need for repair. The repair of the dented specimens involved using internal grouting and a grouted steel sleeve. Test results showed that by arresting the dentdepth growth, both repair techniques were able to reinstate a damaged specimen's strength. Existing and newly developed ultimate-strength formulations for damaged members, and existing strength formulations for repaired members were evaluated through a comparison with specimen behavior. Results of the evaluation show a reasonable agreement with test results.

A formulation for modeling links in eccentrically braced steel frames (EBFs) subjected to random inelastic cyclic loading is presented. The formulation is plasticity-based, and includes shear and flexural yielding. In addition, combined isotropic and kinematic shear strain-hardening effects are accounted for, as well as flexural strain hardening. The formulation is shown to accurately predict the behavior of test specimens involving short links. Inelastic seismic analysis of EBFs with short links was conducted to illustrate the effect of link-modeling assumptions on EBF response. The analysis results show that link strain hardening can have a pronounced effect on EBF response, leading to larger member forces in braces and columns attached to the links. Furthermore, combined flexural and shear yielding occurred in the links adjacent to columns, indicating the need to include these types of inelastic deformations in the link-modeling formulation. Finally, the use in the analysis of viscous damping in the link was found to cause excessive axial forces to develop in braces and columns adjacent to yielded links. To reduce this effect, a form of nonproportional viscous damping is illustrated.