TORSIONAL EFFECTS ON SEISMIC PERFORMANCE OF SQUARE vs. CIRCULAR RC BRIDGE COLUMNS A. Belarbi 1 , Q. Li 2 , and S. Suriya Prakash 3 Abstract Reinforced concrete (RC) bridge columns could be subjected to combined flexural, axial, shear, and torsional loading during earthquake excitations. This combination of seismic loading can result in complex flexural and shear failure of bridge columns. Several researchers have investigated and proposed various models for predicting seismic performance; however, knowledge of the interaction between flexure, shear, and torsion in RC bridge columns is still limited. An experimental study is being conducted at Missouri S&T to understand the behavior of circular and square RC columns under combined loading including torsion. The main variable being considered is the ratio of torsion-to-bending moment (T/M). The differences in behavior between RC columns of square and circular cross section under combined loading are discussed in this paper. The main difference between the behavior of circular and square sections under combined loadings including torsion is related to confinement characteristics due to transverse reinforcement arrangement as well as warping effect in square cross sections due to torsion. In particular, the effect of cross- sectional shape on hysteretic torsional and flexural response, damage distribution, and ductility characteristics under combined flexure and torsional moments are discussed. Introduction RC bridge columns can be subjected to multi-directional ground motions which result in the combination of axial force, shearing force, flexural and torsional moments. The addition of significant torsion is more likely in skewed or horizontally curved bridges, bridges with unequal spans or column heights, and bridges with outrigger bents. In addition, structural constraints due to a rigid decking, movement of joints, abutment restraints, and soil conditions also lead to combined loading effects. This combination of seismic loading can result in complex flexural and shear failure of bridge columns. Moreover, the cross-sectional details also affect the seismic behavior of RC bridge columns, such as damage distribution and ductility characteristics. The effect of cross section on the behavior of RC columns under combined loading including torsion is investigated. Test results of four square and four circular columns under cyclic flexure and shear, pure cyclic torsion, and combined cyclic flexure and shear and torsion are presented and discussed. 1 Distinguished Professor, 2,3 PhD Candidate, Missouri University of Science & Technology, USA.