Torsion, Bending, and Shear in Prestressed Concrete

An ultimate strength interaction criterion for web reinforced, prestressed concrete beams under torsion, bending moment and flexural shear is presented. Principal variables considered are the level of prestress and its eccentricity, torque: bending moment ratio, and flexural shear. Test results for 54 rectangular beams include beam properties, ultimate strength, angle of twist at failure, torque-twist curves and typical crack patterns. Bending moment in moderate amounts increases the torsional strength, but the effect of flexural shear is always detrimental. Within limits, the web reinforcement provides a ductile type behavior with ample rotation capacity at failure. The initial torsional stiffness is not affected by the variables considered. Higher levels of prestress and web reinforcement increase the pure torsional strength. Torsion-bending moment interaction is represented by a second degree parabola, and a three-dimensional curved interaction surface is envisaged when flexural shear is also present. Proposed interaction equations indicate good correlation with test data.