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Dynamic behavioir of base cracked gravity dam by the way of the hybrid frequency time domain procedure Hui, Pak K.

Abstract

Due to deterioration, past loading conditions, and shrinkage, concrete tensile cracks may form along the dam-foundation interface within a typical concrete gravity dam monolith. These cracks render the assumption that a dam remains linear elastic during dynamic response invalid and requires a nonlinear analysis procedure. In this research, the response of the cracked dam system with the effects of a flexible foundation and hydrodynamic effects is studied using the Hybrid Frequency Time Domain (HFTD) analysis procedure. This nonlinear dynamic analysis procedure preserves the frequency dependent stiffness and damping in the foundation medium when modeled as a visco-elastic halfplane. Similarly, the HFTD procedure honours the frequency dependent hydrodynamic mass and damping of the reservoir medium when the hydrodynamic effects are modeled with a two dimensional wave equation. The nonlinear behavior of the crack opening and closing process is directly accounted for by applying the required restoring or contact force along the crack interface. The result of the study shows that dams with crack lengths less than 25% of the total base length have nominal differences in both magnitude and frequency of vibration when compared to the same uncracked system. For crack lengths in excess of 25%, larger response amplifications than those from the uncracked response were observed. The fundamental effective system frequency of vibration of the cracked dam system is found to be bounded by fundamental frequencies of the uncracked and cracked pseudo-linear systems of the dam. Large amplifications of the maximum dam top response obtained from the cracked system in comparison to the uncracked system are expected for systems near the dominant frequency of excitation. However, for systems with properties that are far removed from the dominant frequency of excitation, little, if any, amplification of the peak response is evident for all crack lengths. A simplified approach of representing the cracked dam system as a SDOF system with bi-linear stiffness proves to be an efficient and accurate method of estimating the cracked dam displacement and velocity response. The speed of the procedure allows for preliminary assessments to be made prior to dedicating valuable time to a full nonlinear analysis. In addition, this procedure can be used to produce cracked response spectra which can accurately predict the amplifications expected in the maximum relative displacement and velocity in a cracked dam system as compared to an uncracked dam system.

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