Experimental study on steel fibers reinforced concrete beams subjected to pure torsion and models assessment

In this study, two truss models are developed for the inclusion of steel fibers: a simplified rotating angle softened truss model and a space truss model. These models incorporate fiber contributions through a rational approach, calculating the compression strut angle using a space truss analogy and using the variable engagement model for determining the fiber component's contribution to the web's tensile strength. The models account for crack rotation post-yielding and predict the torsional strength of steel fiber reinforced concrete (SFRC) beams. Additionally, the torsional behavior of SFRC beams with and without conventional transverse reinforcement is critically examined through testing of six large-scale rectangular beams with varying fiber dosages (0, 30, and 60 kg/m³) and different transverse reinforcement configurations (no hoop reinforcement, 0.5 code minimum, and 1.0 code minimum). The study highlights significant differences in crack patterns and failure mechanisms between reinforced concrete and reinforced steel fiber reinforced concrete (R-SFRC) beams. R-SFRC beams without conventional hoop reinforcement exhibited reduced ductility, while those with at least 50% of the minimum hoop reinforcement maintained similar ductility to the control specimen. The findings recommend maintaining at least 50% of the minimum torsional hoop reinforcement with stirrup spacing not exceeding 75% of the effective depth of the section. Lastly, the proposed models are tested against 63 specimens reported in the literature (including tests conducted in this study), demonstrating a strong correlation with the test data.