A piece-wise linear modal analysis method is developed for analyzing seismic uplifting responses of multistory buildings modeled as uniform shear-beam on rigid ground. Classical modal analysis is applied to calculate responses for two phases: contact phase and uplift phase. Two sets of modes corresponding to the two phases are converted to each other at the incipient of uplift and the instant of landing. The assumption of complete plastic collision is adopted at landing instants. After showing the validity of the method by comparison with experimental results, examples show the effect of damping and vertical seismic motion.
