Advanced Implementation of Hybrid Simulation, PEER Report 2009-104


Experimental testing of structures under simulated seismic loading is one of the best approaches to gain knowledge about the response and behavior of structures during earthquakes and to confirm the effectiveness of design methods for new earthquake-resistant structures and the retrofit of existing structures. Hybrid simulation, where a test is executed based on a step-by-step numerical solution of the governing equations of motion for a hybrid model formulated considering both the numerical and physical portions of a structural system, is one of the well-established experimental testing techniques.

In order for the earthquake engineering community to take full advantage of this technique, it is essential to conduct vigorous research to standardize the deployment of the method and extend its capabilitie s to applications where advanced numerical techniques are utilized; boundary conditions are imposed in real time; and dynamic loading conditions caused by wind, blast, impact, waves, fire, traffic, and, in particular, seismic events are considered. Accordingly, the objectives of this research are to investigate, develop, and validate advanced techniques to facilitate the broader use of local and geographically distributed hybrid simulation by the earthquake engineering community and to permit hybrid simulation to address a range of complex problems of current interest to researchers.

Utilizing a rigorous systems analysis of the operations performed during hybrid simulations, an existing object-oriented soft ware framework for experimental testing (OpenFresco) is evaluated, and extensions are devised and im plemented to facilitate the standardized execution of hybrid simulations. The OpenFresco middleware is environment independent, robust, flexible, and easily extensible, and supports a large variety of computational drivers, structural testing methods, specimen t pes, testing configurations, control and data-acquisition systems and communication protocols. One of the key contributions presented herein, is the application of a multi-tier client/server software architecture to the existing OpenFresco core components to support any finite element analysis software and facilitate an array of local and geographically distributed testing configurations.

Time-stepping integration methods that act as the computational drivers during a hybrid simulation and that need to be provided by or implemented in the finite element analysis software are thoroughly investigated. Integrat ion schemes from both the class of direct integration methods and the class of Runge-Kutta methods are examined and their applicability to hybrid simulation is evaluated. It is shown that the proposed operator-splitting methods and Rosenbrock methods, which are unconditionally stable, relatively easy to implement, and computationally nearly as efficient as explicit methods, are excellent techniques for solving the equations of motion during hybrid simulations.

The next part of the research focuses on the predictor-corrector algorithms that provide the synchronization of the integration and transfer system processes and that need to be implemented in a real-time environment on the control and data-acquisition system side of a hybrid simulation. Improved and new algorithms are proposed, which provide means for performing more accurate, continuous hybrid simulations, especially in situations with excessive delays. Event-driven solution strategies that employ the predictor-corrector algorithms are investigated and several new algorithms are introduced that are based on adaptive control concepts that automatically adjust parameters of the event-driven controller, such as time steps and actuator velocities, according to some suitable performance criteria.

In order to confirm the robustness, flexibility and usability of the OpenFresco software framework and to validate the integration methods and the event-driven synchronization strategies in actual tests, two novel and challenging hybrid simulation examples are carried out. The first case study performs a continuous, geographically distributed hybrid simulation in soft real-time for the first time, and the second case de monstrates how to utilize hybrid simulation to conduct safe and economical tests of structural collapse.

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Andreas H. Schellenberg
Stephen A. Mahin
Gregory L. Fenves
Publication date: 
September 1, 2009
Publication type: 
Technical Report
Schellenberg, A. H., Mahin, S. A., & Fenves, G. L. (2009). Advanced Implementation of Hybrid Simulation, PEER Report 2009-104. Pacific Earthquake Engineering Research Center, University of California, Berkeley, CA.