SHEAR-FLOW ZONE IN TORSION OF REINFORCED-CONCRETE

Rausch's classical formula overestimates the torsional strength of reinforced concrete members to an unacceptable degree. The error is traced to the incorrect determination of the centerline of the circulating shear flow. The position of the centerline of shear flow is directly related to the thickness of the shear flow zone (td). The determination of td in torsion is analogous to the determination of the depth of the compression zone in bending. This paper presents a simple theoretical method to calculate td based on the softened truss model theory. The method utilizes the equilibrium and compatibility conditions, as well as a softened stressstrain relationship for concrete struts. Since td is calculated by a rigorous procedure, an accurate torsional strength can be predicted. The prediction of the torsional strengths of 61 beams found in the literature compares extremely well with the test values. In addition, a very simple formula for td is also proposed for the practical design of members subjected to torsion. © ASCE.