Seismic Performance of Eight Elevated Water Tanks During Silakhor, Iran Earthquake of 31 March 2006

Abstract eng:
Many of elevated water tanks damaged during an earthquake occurred at 31 of March 2005 in Silakhor, Iran. In this study, the seismic performance of eight elevated water tanks have been evaluated qualitatively and .three of them have been studied analytically. Based on a detailed survey, the following modes of failure are recognized: (1) Buckling of the slender bracings due to compressional stresses, (2) Failure of the turnbuckles, (3)Brittle failure of the vertical bracings, (4) Rupture of the bottom plate of the tanks in vicinity of the joints of the supporting structure elements, (5) Formation of plastic hinge (hinges) in one of the top beams in the supporting structure, (6) Failure of joint wildings of the pipes, connected to the end of shaft, which cause the water in the tank to run off. A force-based method based on the Iranian Seismic Code (ISIR 2800) is employed to evaluate the components of three elevated tanks. A performance based design approach has been used to verify the results. A performance factor is calculated through nonlinear analyses which are related to performance based methods. The results of this study show that the force based method fail to estimate accurately the capacity of these structures including the nonlinear behavior of the elements. Nevertheless, a more convenient method for designing elevated tanks such as a performance-based seismic methodology is needed. Moreover, if the force methods are applied for designing new tanks the response modification factor (R) must be reduced from 3, according to the Iranian Seismic Code (ISIR 2800) to 2.

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