Vertical distribution of equivalent static loads for base isolated building structures

It has been pointed out that the static lateral response procedure for a base isolated structure presented in UBC-97 somewhat overestimates the seismic story force. In this study the UBC-91 and UBC-97 static lateral load procedures for isolated structures are investigated, and a new formula is proposed for the vertical distribution of seismic load. The formula is derived by combining the fundamental mode shape of the isolated structure idealized as two degrees of freedom system and the fundamental mode shape of a fixed-based structure. The seismic story forces resulting from the proposed method are compared with those obtained from dynamic time history analysis and the code procedures. The results show that the proposed method provides conservative results compared with those from dynamic analysis and UBC-91 approach, and produces a more economic solution compared with the UBC-97 static lateral response procedure.