The ground motion caused by an earthquake can be considered as the resuitant of three orthogonal translational componets along three cartesian coordinates, x, y, and z; and three rotational components about these coordinate axes as shown in Figure 1 (Rosenblueth, 1974). Consequently, the response of a multi-degree-of-freedom structure to such an input excitation will be the result of the corresponding three translational and three rotational responses. In this paper. response of a structure to simultaneoue saction of only the translational components of the earthquake is studied. The effects of rotational excitations will ba considered in other papers.