The objective of this study is to grasp the relation between structural responses due to simultaneous horizontal and vertical input motions and the phase characteristics of input motions. First of all, we confirmed that responses due to simultaneous input motions are related to cross-correlation between structural response due to horizontal motion and that due to vertical motion, using observed earthquake motions. From these results, we formulated and confirmed analytically that the responses due to simultaneous input motions will be affected by phase differences between horizontal input motion and vertical one around structure's fundamental predominant frequency.
