Reinforcement for Torque in Spandrel L-Beams

The torsional stiffness of reinforced concrete beams drops drastically when diagonal cracking occurs, to 1/10 or 1/30 of the initial stiffness. Elexural stiffness drops only to 1/2 or 1/3 when flexural cracking occurs. Thus the elastic analysis appropriate at small loads cannot be projected to the ultimate or collapse state. The strength of web reinforcement is not fully developed until the rotation angle is 10 to 15 times that first causing diagonal cracking. Such large twist angles are not usable, even at an ultimate condition. A stiffness limit rather than a strength limit probably controls the proper use of web reinforcement. Longitudinal reinforcement for torsion carries stresses dependent more on twist angle than on the magnitude of torque. With properly limited twist angles (at ultimate) the extra longitudinal reinforcement for torsion can be reduced below that now recommended.