Prediction of torsional crack angle in reinforced concrete beams

The shear and torsional strengths of reinforced concrete (RC) members are affected by the diagonal crack angle. According to the space truss model used for torsional design in international codes (ACI 318-19, EN 1992-1-1:2004, CSA A23.3 and JSCE-17), the torsional crack angle is dependent upon the ratio of transverse to longitudinal reinforcement. However, owing to differences in the crack initiation mechanism, the actual crack angle often differs from the value predicted by these models, which may lead to over- or underestimation of the torsional strength. In this study, experimental tests were performed on ten RC beams under torsion to evaluate the effect of the torsional reinforcement ratio on the crack angle. Additionally, 207 test results were collected from the literature to examine the relationship between crack angle and torsional strength. The results showed that the difference between the predicted and measured crack angles became more pronounced as the reinforcement ratio deviated from 1.0. Unlike shear, the crack angles caused by torsion remained nearly constant throughout the loading process. A simplified equation incorporating the tensile strength of concrete was developed to improve crack angle prediction. The proposed equation provided better agreement with test results compared with current design equations and thus offers a more reliable approach for evaluating the torsional behaviour of RC members.