A SIMPLIFIED PROCEDURE FOR THE A 'EISMIC DESIGN OF CONCRETE GRAVITY DAMS

Abstract

In Water Resourees- Development Projects' dams are very vital structures whose safety against seismic forces has to be fully ena.ured. by the designers. The be✓iaviour of some dams during --strong motion earthquakes has revealed the inadequacy of pseudostatic methods so far adopted. The dynamic analysis of gravity dams, ' . either by Beam Analysis or inite Element Technique is however much involved and can be carried out only with the help of a digital computer. Such techniques and facilities are not readily available to most of design offices in our country, and hence specialist Institutes are approached for detailed analysis of the dam thus likely to involve delay in processing of the designs. On the other hand, the assumptions of design earthquake characteristics, damping factor, dynamic strength of material etc. contain many uncertainties which do not therefore warrant at the present state of knowledge too great a precision for the dynamic analysis. The necessity of a simpler procedure giving reasonably accurate results and - amenable for easy computations even in the field offices is keenly felt. The proposed simplified procedure of aseismic design of concrete and masonry gravity dams is presented to fulfil this need. This procedure is based on the well known Rayleigh method in which an approximate assumption is made of the dynamic configuration of a vibrating system by static deflection consideration. The mode shape and the natural period of vibration in the first mode are derived therefrom from which the evaluation of dynamic displacements, forces, shears and moments is made using response spectra of the earth-quake. The Dynamic Analysis by the proposed procedure is carried out on four existing dams whose heights range from 64 m to 129 m which practically covers most of the gravity dams in our country. These dams also differ in cross sectional slopes and material properties as well . The accuracy of this procedure is found by comparing with Beam Analysis results obtained with the help of computer. The proposed procedure is found to give reasonably accurate results both in respect' , of dynamic characteristics as well as dynamic responses life dynamic moments and shears, as compared to Beam Analysis values. The proposed procedure requires only manual calculations and is quite simple to use. Thus the proposed procedure can be readily adopted in the design offices as a reasonably accurate and simple method of aseismic design of concrete and masonry gravity dams.