Seismic stability of cracked concrete dams using rigid block models

Several gravity dams subjected to severe ground motions are likely to experience cracking and sliding in the upper section where dynamic amplification is important. A high acceleration spike realistically applies large inertia forces computed from mass times the acceleration. However, these impulsive inertia forces might not be applied in the same direction for a sufficient long period of time to induce significant rotational or sliding displacements detrimental to the seismic or post-seismic structural stability of the cracked components. When it is of interest to estimate residual sliding displacements, a convenient and simple tool is to perform transient rigid body “sliding block” analysis of the “cracked” component. However, this requires the definition of proper seismic input motions at the base of the block with due consideration of dynamic amplification. The possibility to compute in-structure response spectra (ISRS) at the base of the block to define suitable spectra compatible accelerograms is presented in this paper. An important conclusion is that it is not conservative to use accelerograms compatible with the linear (uncracked) dam ISRS to perform transient rigid body sliding response analyses. Dam base and upper joint cracking affects its dynamic properties such that there are modifications of the intensities and frequency content of seismic motions as they propagate over the dam’s height. An envelope of nonlinear ISRS computed from cracked beam models of the dam is recommended to obtain compatible accelerograms and to provide a conservative estimate of upper block residual sliding displacements.