Rectangular Prestressed Beams in Torsion and Bending

An interaction criterion for prestressed concrete beams under torsion and bending is presented. Test results for 42 rectangular specimens include properties of beams, their ultimate strength, crack patterns, and torque-twist and bending-deflection curves. Substantial reduction in torsional stiffness due to torsional cracks was observed; however, the effect on bending stiffness is relatively small. Beams with stirrups and longitudinal steel have ample rotation capacity at failure. Having established three modes of failure by experimental observations for varying bending to torque ratios, a generalized skew bending theory is developed to evaluate cracking and ultimate torques. Although 150 tests by various investigators including the writers have been evaluated to check the validity of the theory, only 90 tests are given in this paper. A comparison of these values with the test results is found to be satisfactory. For prestressed beams supplemented with longitudinal steel of a minimum amount of about 1%, the nondimensional interaction curve can be conservatively taken as a square.