Reinforced Concrete Rectangular Columns in Torsion

Twenty-five 5.5-in. × 7. 5-in. × 38.5-in. rectangular under-reinforced concrete columns were tested to failure in a vertical position under different combinations of axial load and torsional moments. Both axial load P and torque T were applied simultaneously in a fixed P:T ratio till rupture. The torque-unit twist curves are almost linear till approximately 60% of the ultimate capacity which was attained either at the appearance of continuous diagonal cracks followed by concrete crushing in compression for specimens of low P:T ratio, or by concrete crushing in a spalling abrupt manner for high P:T ratios. The rupture criteria are analyzed, rupture surface defined, and equations to predict the ultimate capacity presented. These equations define a load-torque interaction diagram. An expression is also given to evaluate the unit twist at ultimate capacity.