In seismic design of structures, excitations are usually applied separately in two perpendicular directions of structures. In fact, the two component of ground motions occurs simultaneously. This thesis clarifies the effects of bi-directional excitations on structures and proposes the response spectra called "response spectra with consideration of bi-directional excitation effects". A simplified analytical model of a two-degree-of-freedom system is employed. In the development of the spectra, 30 horizontal ground motion records from rock, stiff soil and soft soil sites are considered. The effect of directivity of ground motions is taken into account by applying strong motion records in all directions. The analytical results are presented in the form of the displacement ratio response spectrum defined as the radial displacement response spectrum normalized by a conventional displacement response spectrum. In addition, the direction response spectrum is also presented to identify the direction of the maximumradial displacement. Finally, regression analysis was carried out to formulate the response spectra for design purposes. The result shows that maximum radial displacement occurs in the angle with the highest spectrum intensity of a ground motion acting in the longer period axis. It is found that a natural periods in two horizontal axes of the structure and the angle of a ground motion significantly affect the radial displacement spectrum and the direction response spectrum. The maximum radial displacement tends to occur in the axis of a longer period. The average displacement ratio response spectra have values about 1.1-1.7 for rock sites, 1.1-1.6 or stiff soil sites and 1.1-1.4 for soft soil sites. The effects of earthquake magnitude and the epicentral distance are not significant.
