Page - 1222 - in Book of Full Papers - Symposium Hydro Engineering

NEAR-FAULT SEISMIC VULNERABILITY OF GRAVITY DAMS Y. Yazdani, M. Alembagheri* *Assistant Professor of Hydraulic Structures, TARBIAT MODARES UNIVERSITY I.R.IRAN 1. INTRODUCTION Near-fault ground motions recorded close to a ruptured fault can be significantly different than those observed further away from the seismic source [1]. The near-fault zone is dependent on the earthquake magnitude. In the near- fault area, depending on the rupture mechanism and slip direction relative to the site, the ground motion may exhibit a unique characteristic known as directivity effects. There is also another property resulted by the permanent ground displacement at the site which is called fling-step. These special aspects of near- fault ground motions should be accounted for in the analysis and design of structures in near-fault areas [2]. The directivity effects, in particular, would lead to high specific pulses in the velocity time-history of the record in the direction perpendicular to the fault rupture. It has been shown that the amplitude and period of the pulse are parameters that control the performance of structures; it cannot be easily predicted by typical measures such as response spectra [1-11]. It has been observed that pulse-like motions have larger elastic spectral acceleration values at moderate to long periods. For the concrete dams, Bayraktar and co-workers [10,11] determined near-fault ground motion effects on the nonlinear response of dams including dam-reservoir-foundation interaction. They selected four different types of dam, which are gravity, arch, concrete faced rockfill and clay core rockfill dams, to investigate the near-fault ground motion effects on dam responses. They showed that near-fault ground motions have different impacts on the dam types. Wang and co-workers [12,13] presented results of a study aimed at evaluating the effects of near-fault and far-fault ground motions on seismic performance of concrete gravity dams. They selected Koyna gravity dam as a numerical application. Four different near-fault ground motion records with an apparent velocity pulse were used in the analyses. The results obtained from the analyses showed the effects of near-fault ground motions on seismic performance of concrete gravity dams and demonstrated the importance of considering the near- fault ground excitations. 1222