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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]

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.