Inelastic seismic torsional behaviour of elevated tanks

Abstract Torsional failure of elevated tanks has occurred in past earthquakes. The overall axisymmetric structural geometry and mass distribution of such structures may leave only a small accidental eccentricity between centre of stiffness and centre of mass. Such a small accidental eccentricity is not expected to cause a torsional failure. This paper studies the possibility of amplified torsional behaviour of elevated water tanks due to such small accidental eccentricity in inelastic range through detailed case studies; using two simple idealized systems with two coupled lateral–torsional degrees of freedom and, strength-deteriorating and elasto-plastic hysteresis models. The systems are capable of retaining the characteristics of two extreme categories of water tanks namely, (a) tanks on staging with less number of columns and panels and (b) tanks on staging with large number of columns and panels. The study shows that the presence of a small eccentricity may lead to localized unsymmetrical yielding in some of the reinforced concrete staging elements. This may lead to progressive strength deterioration through successive yieldings in same elements under cyclic loading during earthquakes. Such localized strength drop may increasingly develop large strength eccentricity resulting in large localized inelastic displacement and ductility demand, leading to failure. These observations are also verified for a real-life example elevated tank. The tanks supported on staging with fewer columns and panels are found to have greater torsional vulnerability. The tanks located near a fault are found to be vulnerable under near-fault pulses with a large duration compared to the lateral period of tank.