EFFECT OF SEISMIC PROVISIONS ON COST OF GRAVITY DAMS

Abstract

Dams form an important element of multipurpose projects and need a huge amount of investment. In India, most of the concrete or masonry storage dams are gravity dams. The Indian Standard Code I.S.:1893 (1X2,3) specifies the forces for the earthquake resistant design of such dams. These code provisions (1 ,2,3) have been revised from time to time to provide a more rational approach in the design. Several dams have been designed accor-ding to these code revisions. It would be,therefore,useful to study the quantitative difference in the code revisions and their adequacy in dam design. A comparative study of the seismic forces worked out as given in the various revisions of I.S. Codes (1X2,3) and the dynamic analysis (4) has been carried out for dams of heights varying from 30.0 in to 200.0 m. The study has shown that the distribution of seismic forces as given by the I.S.Code revision of 1975 (Latest third revision) (3) is most rational and compares well with those obtained by dynamic analysis. For evaluating hydrodynamic moments and shears, I.S.Code (1,2,3) gives simple expressions. It is noted that these are approximate and over estimate the values over the one obtained by derivation using the parabolic hydrodynamic pressure distribution given in the code (1,2,3 ). The maximum percentage of overestimation in case of hydrodynamic moments is 20 percent and in case of hydrodynamic shears it is 14 percent. The I .S .Code 1693(1,,2,3) recommends the distribution of hydrodynamic forces without reckoning the compressi-bility of water and dam-reservoir interaction. Simpli-fied procedure is also available (5,6) or use in design which accounts for these parameters. Suitability of this method has been checked by applying it to design the dams of heights varying from 50.001 to 150.0m. It is c.3ncluded that the method provides a more rational approach in the design but it would be uneconomical to use it in practice unless reasonable and realistic criteria for permissible tonsL-.a stresses in gravity dams are evolved. The gravity dams have to be adequately designed for seismic forces that would act on it in future. The seismic forces acting on a dam depend on the seismicity of site which is difficult to assess precisely before hand. To make a judicious estimate of design seismic coefficient, it would be useful, from economic consi-derations, to know the increase in cost of dam with an increase in seismic forces. An investigation has been carried out to study the influence of design seismic coefficient on the cost of concrete gravity dams. The study shows that the relationship between the design value of seismic coefficient and percentage increase in cost is not linear. For a value of design seismic coefficient equal to 0.04, the total percentage increase in concrete volume is about 9 to 10 percent over the concrete volume of same dam if it were located in non—seismic zone. Thus, careful decision is essential to determine the design value of seismic coefficient to avoid undue investments in the seismic resistance of dams...