A new lateral force distribution formula for base-isolated structures

It has been suggested that the static lateral response procedure for a base-isolated structure presented in the International Building Code (IBC) somewhat overestimates the seismic storey force. In this paper an IBC-equivalent static method for base-shear distribution of seismic isolated structures is evaluated. The results of the equivalent static procedure in the vertical distribution of base shear are compared with results obtained from dynamic linear and nonlinear analysis and the efficiency of its application is studied. In general, the results of the equivalent lateral response procedure in the proposed triangular distribution of base shear are somewhat conservative. Based on linear and nonlinear analysis results of isolated structures, therefore, two new formulae are proposed for the vertical distribution of the base shear. The proposed linear formula is based on the first mode shape of the isolated structure and the nonlinear formula follows three first mode shapes, which imply contribution of higher modes in a more realistic nonlinear analysis. The accuracy of the new formulae is examined by comparing the results obtained from this study with those calculated from linear and nonlinear time history analysis.