Prediction of nonlinear seismic demand of inter-story isolated systems using improved multi-modal pushover analysis procedures

The engineering community requires a simple and engineering-friendly procedure to evaluate the seismic performance of inter-story isolated structures (IIS), considering the widespread application of IIS and the limitations of current static nonlinear analysis procedures. This paper proposes an improved multi-modal pushover analysis procedure (IMMPA) that is compatible with the code design spectrum and can account for the effects of higher mode responses and the strongly nonlinear nature of IIS. Nonlinear response history analysis (NRHA) is unnecessary for a reliable assessment of IIS seismic performance when IMMPA is utilized. The IMMPA involves converting the IIS into a linear non-proportional damping structure to satisfy performance requirements, decoupling it into modal oscillators with the equivalent of the nonlinear structure in the modal coordinates using complex-mode-superposition approaches, adjusting the input response spectrum excitation with its equivalent parameters, and converting the response spectrum excitation into the demand spectrum. The main modal oscillators are identified by the complex modal participation mass ratio; the respective acceleration is used as the weight factor of the lateral pushing force; and the capacity spectrum procedure is employed to determine the performance point. Combining the main modal responses using a complex-mode superposition complete square combination rule (CCQC) produces an effective nonlinear peak response. Finally, the IMMPA procedure is demonstrated using an IIS example and evaluated with NRHA data. The evaluation demonstrated that the IMMPA procedure can overcome conventional pushover limitations and efficiently evaluate IIS seismic requirements.