Seismic torsional vibration in elevated tanks

Some elevated water tanks have failed due to torsional vibrations in past e arthquakes. The overall axisymmetric structural geometry and mass distribut ion of such structures may leave only a small accidental eccentricity betwe en centre of stiffness and centre of mass. Such a small accidental eccentri city is not expected to cause a torsional failure. This paper studies the p ossibility of amplified torsional behaviour of elevated water tanks due to such small accidental eccentricity in the elastic as well as inelastic rang e; using two simple idealized systems with two coupled lateral-torsional de grees of freedom. The systems are capable of retaining the characteristics of two extreme categories of water tanks namely, a) tanks on staging with l ess 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 eccentr icity may lead to large displacement of the staging edge in the elastic ran ge, if the torsional-to-lateral time period ratio (tau) of the elevated tan ks lies within a critical range of 0.7 < tau < 1.25. Inelastic behaviour st udy reveals that such excessive displacement in some of the reinforced conc rete staging elements may cause unsymmetric yielding. This may lead to prog ressive strength deterioration through successive yielding in same elements under cyclic loading during earthquakes. Such localized strength drop prog ressively develop large strength eccentricity resulting in large localized inelastic displacement and ductility demand, leading to failure. So, elevat ed water tanks should have tau outside the said critical range to avoid amp lified torsional response. The tanks supported on staging with less number of columns and panels are found to have greater torsional vulnerability. Ta nks located near faults seem to have torsional vulnerability for large tau.