Assessing the seismic torsional vulnerability of elevated tanks with RC frame-type staging

Recurrence of torsional failure of elevated water tanks in past earthquakes (including 1952 Kern County and recent 1993 Killari earthquakes) has highl ighted the importance of this problem. It is established that these structu res may have amplified torsion-induced rotation if their torsional-to-later al natural period ratio tau is close to 1 and amplified displacement of str uctural elements due to the coupled lateral-torsional vibration if tau is w ithin the critical range 0.7 < tau < 1.25. The present study aims to estima te the range of variation of tau for usually constructed reinforced concret e elevated water tanks with frame-type stagings for assessing their torsion al vulnerability. Closed-form expressions for torsional and lateral stiffne ss of tank stagings are derived and verified by standard finite element sof tware. These expressions are used for studying the variation of tau for fea sible ranges of influencing parameters. It is seen that a very large number of such tanks may have tau within the said critical range. Closed-form exp ressions for moments and shear forces of columns and beams under torsion an d that under lateral force are also derived. It is also seen with the help of these expressions that the frame stagings of these tanks normally design ed for seismic lateral force, may yield through formation of plastic hinges simultaneously in all columns instead of in beams if they are subjected to large rotational response for having tau possibly very close to 1. Such a pattern of yielding generally converts the whole system suddenly into a mec hanism causing immediate collapse. Therefore, torsional coupling seems to b e a potential cause of failure for these structures. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.