Development of EFT for nonlinear SDOF systems

Effective force testing (EFT) is a test procedure that can be used to apply real-time seismic simulations to large-scale structures that can be represented by lumped-mass systems. In an EFT test, hydraulic actuators are used to applied dynamic forces through the mass center of the test structure anchored to a stationary base. The force to he imposed (effective force) is the product of the structure mass and the ground acceleration record, thus a known priori. Motions measured relative to the ground are equivalent to those of the structure in a shake table test or an earthquake event. This research extended the development and implementation of the EFT method to fully utilizing test equipment and testing nonlinear SDOF structural systems undergoing large deformation in real time.; Velocity feedback compensation is required for the actuator control to ensure that forces are applied to the test structure accurately. Nonlinearities in the servo-system could have a significant impact on the implementation of velocity feedback compensation when an EFT test requires large flow demands, which could be caused by large structural velocity responses and/or large forces applied to the structure. Detailed mathematical models were proposed for a test system at the University of Minnesota, which accurately described the servo-system behavior over a wide range of frequencies and across the major operation range of the servovalve.; Based on the nonlinear servo-system model, a nonlinear velocity feedback compensation scheme was proposed and verified through computer simulation and experimental tests on a mass-spring-dashpot structural system. Results showed that with the nonlinear velocity feedback compensation, dynamic forces could be applied to the nonlinear structure at all frequencies of interest. In addition, the effects of system uncertainties on the performance of the test system including stability were investigated through linear system analyses, computer simulation, and experiments.; The viability of the EFT method was further validated by a proof-of-concept test, in which a one-story steel structure was tested using a shake table and the EFT method. The comparison of the test results indicated that the EFT method can be used to apply real-time seismic simulation to structures undergoing large nonlinear deformation.