Vertical Acceleration Effects on the Soil-Structure Interaction of Multistory Buildings

In this paper, a method is proposed for the soil-structure interaction analysis which includes both horizontal and vertical earthquake effects. A single-degree-of-freedom system is adopted to model multistory buildings with a soft first story [I]. The model is analyzed as a linear elastic system with a rigid footing, which rests on the surface of a viscoelastic half space. The proposed procedure also include the second order P-A effecs on the system. A numerical example is presented, in which the seismic response of the superstructure together with its foundation system is analyzed, and based on the earthquake data recorded at JMA Kobe Observatory on Jan. 17, 1995. It is found that the vertical acceleration aggravates the P-A effects. This leads to significant increases in the lateral deflections and story shears. The increases are a function of soil condition, the structural flexibiliy and the ratio of axial force to the critical load of the columns.