Gilberto Mosqueda

Nonstructural systems constitute a significant portion of the total property of a typical building. Recent earthquakes have repeatedly demonstrated that nonstructural damage results in significant loss of property and function with major catastrophic impact on communities. In a collaborative project between the NSF NEES program and the National Research Institute for Earth Science and Disaster Prevention of Japan, a full-scale, five-story steel moment frame building was subjected to a number of 2D and 3D ground motions using the E-Defense shake table. The building was tested under three different configurations: 1) base isolated with triple pendulum bearings (TPB), 2) base isolated with a combination of lead-rubber bearings and cross linear bearings (LRB/CLB), and 3) base fixed. As part of a NEESR-GC project on the "Simulation of the Seismic Performance of Nonstructural Systems," more than 75 m² (800 sq-ft) of suspended ceiling with lay-in tiles and 3 sprinkler branch lines were installed on the 4th and 5th floors of the building. The ceiling was enclosed by light gauge stud gypsum partition walls. This paper presents some of the preliminary observations on the response of nonstructural systems from these experiments.

Improved numerical integration procedures are essential for the extension of hybrid numerical and experimental simulation to large and complex structural systems. While implicit integration algorithms are widely used in pure numerical simulations for their superior stability and accuracy, their direct application to hybrid simulation has been partially limited by difficulties in estimating the tangent stiffness matrix of multi-degree-of-freedom experimental substructures. Current applications of hybrid simulation using integrators with improved stability have mostly resorted to methods that are noniterative or utilize the initial stiffness matrix for iterative corrections. To improve the accuracy of integration procedures for hybrid simulation, a new method for online estimation of experimental tangent stiffness is proposed. The stiffness estimation procedure is tailored for fast online applications by transforming the measurements into a coordinate system, which reduces the number of unknown stiffness coefficients that need to be updated during the simulation. The updated experimental stiffness matrix is used in a modified operator-splitting integration scheme to improve the accuracy of hybrid simulations with highly nonlinear experimental substructures. The application and effectiveness of the proposed approach is demonstrated through hybrid simulations with multi-degree-of-freedom experimental substructures.

Recent earthquakes have shown that even moderate ground shaking can produce large economic losses and major societal disruptions due to the widespread structural and nonstructural damage, and it is increasingly recognized that the traditional goal to protect against loss of life, may be insufficient. Seismic isolation offers a simple and direct opportunity to control or even eliminate damage by simultaneously reducing deformations and accelerations. While applications of seismic isolation in the U.S. have been limited to specialized facilities requiring high performance, the pace of innovation and application of isolation technology outside of the U.S. is growing at an exponential rate. A new NEES project, Tools to Facilitate Widespread Use of Isolation and Protective Systems, or TIPS, will explore innovative ideas to economically and reliably apply seismic isolation and supplemental damping to a much broader range of structures. The NEESTIPS project will fill critical knowledge gaps concerning device and system response, develop strategies and tools to reduce unnecessary costs of design and construction, and develop analysis and design tools to achieve targeted performance goals. This paper highlights the results of our strategic assessment of the knowledge gaps, economic barriers, and procedural barriers that limit the use of isolation systems in the United States. A research plan that addresses the above areas will be summarized. Through the NEESTIPS project, hybrid and shake table tests of seismically isolated building systems will be conducted at UC Berkeley and Univ. at Buffalo NEES facilities and also the E-Defense shaking table. Among the goals of the analytical and experimental program are: (1) obtain a better understanding of the behavior of full scale devices at realistic rates, (2) develop strategies for reducing design and construction costs such as relocating the level of the isolation plane or allowing the superstructure to yield, (3) identify performance limit state behavior of various isolated systems, (4) develop emerging isolation systems that can effectively achieve specific multi-level performance goals, and (5) extend performance-based design tools so that conventional solutions and isolation systems can be evaluated consistently.

The seismic vulnerability of non‐structural components and equipment with their expensive recovery and/or replacement costs has been demonstrated during past earthquakes. With the exception of the nuclear industry, the limited data collected from past earthquakes are not sufficient to completely characterize the seismic behaviour of non‐structural components and to develop effective mitigation measures. To address these limitations, the University at Buffalo has commissioned a dedicated non‐structural component simulator (University at Buffalo non‐structural component simulator (UB‐NCS)) composed of a two‐level testing frame capable of simultaneously subjecting both displacement and acceleration‐sensitive non‐structural components to realistic full‐scale floor motions expected within multi‐storey buildings. Current codes now require seismic qualification of important non‐structural components, and protocols for experimental qualification have been developed considering existing laboratory limitations. The new testing capabilities provided by the UB‐NCS are described to demonstrate the improved and more realistic qualification procedures that can be achieved. Results are presented from a test on two parallel partition walls aimed to verify the capabilities of the UB‐NCS. Copyright © 2008 John Wiley & Sons, Ltd.

The hybrid simulation test method is a versatile technique for evaluating the seismic performance of structures by seamlessly integrating both physical and numerical simulations of substructures into a single test model. Using hybrid simulation, the seismic response of complex structural systems partitioned into multiple large-scale experimental and numerical substructures at networked geographically distributed experimental and computational facilities can be evaluated. A scalable framework with a fault-tolerant distributed controller is presented to support the implementation of advanced hybrid testing methods with distributed substructures. The control strategy is based on a multithreaded simulation coordinator for parallel communication with remote sites and an event-driven controller at each remote experimental site to implement continuous loading. The distributed controller provides faster rates of testing and improved accuracy in the simulation results. The effectiveness of the proposed framework is demonstrated by computing the earthquake response of a six-span bridge model with multiple remote experimental and numerical column substructures distributed within NEES laboratories across the United States. Further, the distributed tests were implemented using a secure network link between the testing sites that was developed for the NEES cyber infrastructure.

Explicit integration procedures have been widely adapted and applied to hybrid simulations of the seismic response of structures due to their simplicity. However, these procedures are conditionally stable and require small time steps for stiff systems, or systems involving high-frequency modes. In this study, a combined implicit or explicit integrator is proposed to solve the equation of motion for hybrid simulations. In this method, command displacement predictions are determined to load the experimental substructures beyond the current step using explicit kinematics expressions. The displacement and force measurements are then used to iteratively correct the explicit displacement prediction within the numerical model until an implicit formulation is satisfied. An explicit solution is accepted only in time steps for which the iterative correction of displacement fails to convergence after maximum number of iterations. This procedure limits communication between the experimental substructures and the numerical integrator, thus it is well suited for networked applications of hybrid simulation. Numerical and experimental simulations demonstrate the effectiveness of this numerical integration scheme, especially in utilization of longer time steps, prevention of excitation of higher modes, and testing of stiff systems.

Compensation of delay and dynamic response of servo‐hydraulic actuators is critical for stability and accuracy of hybrid experimental and numerical simulations of seismic response of structures. In this study, current procedures for compensation of actuator delay are examined and improved procedures are proposed to minimize experimental errors. The new procedures require little or no a priori information about the behavior of the test specimen or the input excitation. First, a simple approach is introduced for rapid online estimation of system delay and actuator command gain, thus capturing the variability of system response through a simulation. Second, an extrapolation procedure for delay compensation, based on the same kinematics equations used in numerical integration procedures is examined. Simulations using the proposed procedures indicate a reduction in high‐frequency noise in force measurements that can minimize the excitation of high‐frequency modes. To further verify the effectiveness of the compensation procedures, the artificial energy added to a hybrid simulation as a result of actuator tracking errors is measured and used for demonstrating the improved accuracy in the simulations. Copyright © 2007 John Wiley & Sons, Ltd.

Explicit integration procedures have been widely adapted and applied to hybrid simulations of the seismic response of structures due to their simplicity. However, these procedures are only conditionally stable and have limited recent applications of hybrid simulations to simple structural models with few degrees of freedom. A novel integration procedure is proposed herein, in which a fully implicit formulation is applied to solve the equation of motion for the hybrid model, but defaults to an explicit or noniterative formulation in steps that fail to converge. The advantages to this approach are the ensured continuity of the simulation and the reduced accumulation of errors that occur during consecutive explicit steps that may lead to instability. The implicit procedure is applied by loading the experimental substructures beyond the expected displacement for the current step, then using the displacements and forces measured through the load path in the iterative implicit scheme. This approach captures the instantaneous behaviour of experimental substructures without physically imposing iterations. Numerical and experimental simulations demonstrate the effectiveness of the proposed integration scheme for multi‐degree‐of‐freedom models, especially in utilization of longer time steps that exceed stability limits of explicit methods, prevention of excitation of higher modes, and testing of stiff systems. Copyright © 2007 John Wiley & Sons, Ltd.