Inelastic seismic torsional behaviour of elevated tanks

Torsional failure of elevated tanks has occurred in past earthquakes. The o verall axisymmetric structural geometry and mass distribution of such struc tures may leave only a small accidental eccentricity between centre of stif fness and centre of mass. Such a small accidental eccentricity is not expec ted to cause a torsional failure. This paper studies the possibility of amp lified torsional behaviour of elevated water tanks due to such small accide ntal eccentricity in inelastic range through detailed case studies; using t wo simple idealized systems with two coupled lateral-torsional degrees of f reedom and, strength-deteriorating and elasto-plastic hysteresis models. Th e systems are capable of retaining the characteristics of two extreme categ ories of water tanks namely, (a) tanks on staging with less number of colum ns and panels and (b) tanks on staging with large number of columns and pan els. The study shows that the presence of al small eccentricity may lead to localized unsymmetrical yielding in some of the reinforced concrete stagin g elements. This may lead to progressive strength deterioration through suc cessive yieldings in same elements under cyclic loading during earthquakes. Such localized strength drop may increasingly develop large strength eccen tricity resulting in large localized inelastic displacement and ductility d emand, leading to failure. These observations are also verified for a real- life example elevated tank. The tanks supported on staging with fewer colum ns and panels are found to have greater torsional vulnerability. The tanks located near a fault are found to he vulnerable under near-fault pulses wit h a large duration compared to the lateral period of tank. (C) 2001 Academi c Press.