DYNAMIC ANALYSIS OF SPILLWAY PIERS

Abstract

Spillway piers supporting radial gates are the highly stressed members in the water control system in dams. The overflow depth and spans of spillways have of late increased enormously to accommodate probable maximum floods (PMF). Spillway bays of size 20m x 20m may soon become common occurrence. Dams, if located in seismically active zones, will be subjected to dynamic forces caused by earthquakes. The stresses induced at the base of the pier due to earthquake forces are significant and therefore for dams located in seismically active areas the dynamic analysis of spillway pier is essential. The analysis of spillway piers is a complex problem due to variety of factors. High speed electronic digital computers have enabled engineers to employ various, numerical discretization techniques for obtaining solutions of such complex problems. Existing practice in the design of spillway piers is based on conventional methods, treating the pier as a one dimensional cantilever fixed at its base and accounting for the earthquake force by pseudostatic approaches. This dissertation presents the dynamic analysis of spillway piers using Finite Element Method. Spillway pier of Omkareshwar dam has been analysed. The pier is idealized as a two dimensional plate, fixed along the base and free along the other three sides. The analysis simulates a plate bending problem. First three natural frequencies and the corresponding modal vectors are obtained. Deflected shapes at six different vertical sections of the spillway pier are plotted. Dynamic responses for the occurrence of an earthquake of magnitude 6.5 at a distance of 10 km from the dam site is obtained. Variation of dynamic responses along the height of the pier as well as along the spillway crest are also plotted. It is found that rigorous method of dynamic analysis should be used for the safe and economic design of spillway piers.