Cyclic Torsion Tests of Concrete Box Columns

Abstract

Stiffness reduction and material energy dissipation of reinforced concrete box‐members have been investigated experimentally under cyclic torsional loading. Six specimens were tested; four in pure torsion and two with axial force in addition to the alternating torque. The amount of reinforcement, the axial load and the load history were varied. The load histories were chosen to simulate the main features of a lightly damped structural response to random environmental loading, such as wind. The specimens were instrumented to trace time‐histories of torque, axial load, twisting angle, concrete and reinforcement strains and displacement across cracks. Hysteretic loops are presented in the form of torque‐twisting angle curves and strain‐torque curves. Developments of stiffness reduction, material energy dissipation, an equivalent viscous damping ratio as well as bond deterioration are shown as functions of time. Stiffness reductions by a factor of 8‐30 were found when compared to the uncracked stiffness. The post‐cracking damping ratio was 2‐3 times the corresponding uncracked values.