An improved multi-support response spectrum (IMSRS) method is developed for the structural systems subjected to spatially varying seismic excitations. In the formulae, only pseudo-static terms and dynamic terms are included, and the dynamic cross-correlation coefficient is decoupled with each normal mode. Several spectral parameters, which are dependent only on the characteristics of each modal vibrator, are defined. With the assumption of ideal white noise excitations and the approximate squared transfer function, the approximate analytical formulae of these spectral parameters are founded. The analytic results show that it is important for the long-span cable-stayed bridges to consider the incoherent support motions and the local site effect. It can be seen from the results that the proposed method is reliable, feasible and timesaving.
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