Analysis of implicit HHT-a integration algorithm for real-time hybrid simulation

Real-time hybrid simulation is a viable experiment technique to evaluate the performance of structures equipped with rate-dependent seismic devices when subject to dynamic loading. The integration algorithm used to solve the equations of motion has to be stable and accurate to achieve a successful real-time hybrid simulation. The implicit HHT a-algorithm is a popular integration algorithm for conducting structural dynamic time history analysis because of its desirable properties of unconditional stability for linear elastic structures and controllable numerical damping for high frequencies. The implicit form of the algorithm, however, requires iterations for nonlinear structures, which is undesirable for real-time hybrid simulation. Consequently, the HHT a-algorithm has been implemented for real-time hybrid simulation using a fixed number of substep iterations. The resulting HHT a-algorithm with a fixed number of substep iterations is believed to be unconditionally stable for linear elastic structures, but research on its stability and accuracy for nonlinear structures is quite limited. In this paper, a discrete transfer function approach is utilized to analyze the HHT a-algorithm with a fixed number of substep iterations. The algorithm is shown to be unconditionally stable for linear elastic structures, but only conditionally stable for nonlinear softening or hardening structures. The equivalent damping of the algorithm is shown to be almost the same as that of the original HHT a-algorithm, while the period elongation varies depending on the structural nonlinearity and the size of the integration time-step. A modified form of the algorithm is proposed to improve its stability for use in nonlinear structures. The stability of the modified algorithm is demonstrated to be enhanced and have an accuracy that is comparable to that of the existing HHT a-algorithm with a fixed number of substep iterations. Both numerical and real-time hybrid simulations are conducted to verify the modified algorithm. The experimental results demonstrate the effectiveness of the modified algorithm for real-time testing. Copyright (C) 2011 John Wiley & Sons, Ltd.