An improved hysteresis model for circular reinforced and square reinforced concrete columns under cyclic torsional loading

It has been observed in the past that, reinforced concrete (RC) bridge columns are, very often, subjected to torsional moment in addition to flexure and shear during seismic vibration. However, the torsional moment is generally ignored in typical design pr actices. Previous studies have show n that, ignoring torsional moment may lead to brittle shear failure of the columns triggering collapse of the entire or part of the bridge structure. Therefore, rational models n e ed to be developed to consider the effect of torsion in the design of RC bridge columns. Performance based seis mic design is an emerging design concept which calls for accurate prediction of the hysteresis behavior of structural elements to ensure safe and sustainable design under earthquake loadi ng. However, very few investigations in the past focused on the development of analytical model to accurately predict the response of RC members under cyclic torsion. Though quite a good number of models are available for prediction of shear and flexural h ysteresis, they are not readily applicable for torsion owing to significant pinching and stiffness degradation associated with torsional loading. Hysteresis models taking into account pinching and stiffness degradation effect under cyclic torsional loading are scarce. The present study aims at filling this knowledge gap by proposing a n improved polygonal hysteresis model which can accurately predict the hysteretic behavior of RC circular and square columns under torsion. The proposed empirical model is vali dated through experimental data o f two circular columns and two square columns tested under pure torsion . Close correlation is observed between the predicted and measured torque - twist curves .