Development of Torsional Strength Model for Steel Fiber Reinforced Concrete Beams with Transverse Reinforcement

In this study, a torsional strength model for SFRC beams with transverse reinforcement was developed based on a simplified softened truss model. The proposed model had two unique features. One feature was the introduction of multi-potential capacity criteria by considering two failure modes owing to aggregate interlock and concrete crushing. The stresses in the model were calculated straightforwardly using simple equations for the torsional effective thickness and principal stress angle. Another feature was the introduction of the effective longitudinal and transverse cross-sectional areas of the fibers to estimate their contributions to the torsional strength. Two possible failure types for the steel fibers, debonding and fracture, were considered. The proposed model was validated by comparing the calculated torsional strength with test data for SFRC beams obtained from the literature. This comparison showed that the proposed model could capture the torsional strengths of SFRC beams with wide ranges for the reinforcement ratios in both directions, properties and amount of steel fibers, concrete properties, and cross-sectional details. The average value of the ratio between the predicted ultimate torsional strength and experimental value was 1.11, with a COV of 0.267, and the proposed model was able to well estimate the failure mode of the specimens. As a practical application of the proposed model, the approach used to estimate the steel fiber contribution was used to extend the torsional strength design procedure in the SNiP code, a building design code used in the Commonwealth of Independent States countries, to cover SFRC beams.