Seismic Behavior of RC Columns with Interlocking Spirals under Combined Loadings Including Torsion

Abstract

Reinforced concrete (RC) columns of skewed and curved bridges with unequal spans or column heights can be subjected to combined loading including axial, flexure, shear, and torsion loads during earthquakes. This combination of seismic loading can induce the complex flexural and shear failure of bridge columns. Seismic performance of bridge RC columns is largely controlled by the level of confinement provided by transverse reinforcement. Interlocking spirals are commonly used in noncircular RC bridge columns because they can provide more effective confinement than rectangular hoops and simplify the column fabrication. However, the seismic behavior of columns with interlocking spirals has been studied only to a limited extent with respect to the hysteretic behavior of pure torsion and the interaction between flexure, shear, and torsion. The presence of torsion significantly affects the inelastic flexural response of RC members under seismic loadings and results in brittle failure modes. This paper presents an experimental study on the inelastic behavior of the RC columns with double interlocking spirals under combined action of cyclic flexural and torsional moments. The columns were designed with aspect ratio (H/B=5.5) and tested under various loading conditions: cyclic pure torsion, and combined cyclic flexure and torsion. The hysteretic torsional and flexural response, damage distribution, stiffness degradation and ductility characteristics with respect to various torsion-to-bending moment (T/M) ratios are discussed. The significant confinement of interlocking spirals to core concrete and its effect on the torsional resistance under combined loadings is also highlighted. Finally, interaction diagrams were established based on experimental results.