SYSTEM IDENTIFICATION STUDIES RR KOYNA DAM

Abstract

The dams are very important structures and assessment of their safety is done at regular intervals. This is a particulary rigorous exercise in case of a major dam in active seismic zone. One of the tools used In such assessment is the dynamic analysis using updated data base. In the last two decades strong motion accelerographs are installed in a number of major dams and a few of them have triggered during medium/strong earthquakes. Koyna is one of such dams. Information obtained from these records can be used for the identification of the system of the dam. The strong motion accelerograms are analysed to find the natural frequencies of vibrations of the dam-foundation system. Natural frwquencles of vibrations of the dam-foundation system are computed for known parametric values viz, the stiffeness modulus of dam concrete and foundation rock, the Poisson's ratio, the mass densities etc. The values of the computed frequencies are compared with the observed values.The elgen value analysis Is carried out by modifying the parametric values within the reasonable range of variation till attaining a fair match between the computed results and the observed values. The study shows that the values of the modulus of elasticity of materials under dynamic loding conditions are distinctly Niger than those under static loading conditions. It is also seen that the presence of the reservoir water is accounted for by the virtual mass concept as suggested by Zanger. The Koyna dam has been subjected to continuing seismic activity for last thirty years. However there are no signs of deterioration of concrete as adjuged from consistency in the natural frequencies over last two decades. It is concluded that the strong motion recording instruments can be considered as a tools for collection of data that can be used as the reference for the designs.