Torsion of High-Strength Reinforced Concrete Beams and Minimum Reinforcement Requirement

To study the effect of high-strength concrete on the torsional behavior of reinforced concrete (RC) beams, nine full-size beams were tested under pure torsion. The main parameters in this study were concrete strength and amount of reinforcement. Concrete strength ranged from normal strength through all grades of high-strength concrete (defined as 50, 60, 80, and 95 MPa). The amount of reinforcement varied from less than the minimum to the so-called balanced condition (when expected crushing of concrete occurs at the same time as yield of steel). With the intent of keeping the inclination of the concrete struts approximately equal to 45 degrees, equal percentages of reinforcement were provided in the transverse and longitudinal directions. Results indicate that the minimum amount of reinforcement defined in ACI 318-99 is inadequate for equilibrium torsion of high-strength RC beams, and a new expression is proposed. It was found that the torsional capacity of under-reinforced beams is independent of concrete strength, and the amount of longitudinal reinforcement was more effective in controlling crack width than the amount of transverse reinforcement (stirrups).