Estimation of Fundamental Time Periods of Elevated Reservoirs

Abstract

Elevated water reservoirs commonly known as overhead water tanks are important structures and need to remain functional after major seismic events. Failure of these structures adds to the misery of the affected population and adversely affects the post-earthquake relief operations. Therefore these structures are designed for higher seismic loads. Fundamental time period, which is defined as first mode time period of vibration of a structure, has great influence on expected seismic load on the structure. Precise estimation of fundamental time period of a structure, thus, is very important for its earthquake resistant design. Ambient vibration testing have been carried out across the world to find fundamental time period of structures like buildings, bridges etc. and may become useful tool for precise estimation of fundamental time period of elevated water tanks also. The work summarizes the results of experimental study on ambient vibration testing of reinforced concrete elevated water tanks. During this study, a total of five reinforced concrete elevated water tanks are considered for determining their fundamental time periods. The tanks are situated in Gwalior in State of Madhya Pradesh, India. The tanks considered for testing are also modeled as per prevailing design practices to estimate their fundamental time period analytically. By comparing the real values obtained by ambient vibration testing and the analytical values, it is found that the analytical method prescribed by IS code, in general, results in good estimation of fundamental time period of elevated water tanks. However, the water depth to plan size ratio, flexibility of braces and stiffness of staircase also influences the value and should also be considered for analytical estimation.