EFFECTS OF FREQUENCY-DEPENDENT FOUNDATION-SOIL COMPLIANCE ON BUILDINGVERTICAL RESPONSES IDENTIFIED FROM EARTHQUAKE RECORDS

Significant soil-structure interaction effects were shown by the analy sis of the vertical response of Gisborne Post Office building in New Z ealand in the 1993 Ormond earthquake. The first vertical mode frequenc ies of the fixed-base building and the soil-structure (SS) system were estimated as 10.3 and 1.8 Hz respectively. The apparent second mode f requency of the SS system was estimated as 9.6 Hz which is less than t hat for the first mode of the fixed-base structure. The amplification of the second SS system mode with respect to the free field is much la rger than that of the first SS system mode. If the compliance of the f oundation-soil system is assumed frequency independent it is not possi ble for the second mode to have the estimated frequency and amplificat ion. Analytical solutions of a simple two-mass structure on a frequenc y-dependent spring and damper show that a large increase in the compli ance of the foundation-soil system is required for the second SS syste m mode to have the estimated frequency and amplification. This is conf irmed by numerical modelling of the soil site and the foundation. The results of this paper show that the frequency dependence of foundation compliance can have a large effect on the high frequency vertical res ponse of a SS system and that an appropriate water table level can lea d to a large acceleration amplification. (C) 1997 Elsevier Science Lim ited.