Modified softened membrane model for ultra-high-performance fiber-reinforced concrete solid and hollow beams under pure torsion

Limited research has focused on the development of rational theoretical models for evaluating the pure-torsional behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams. To fill this research gap, this study aimed to develop a modified softened membrane model for torsion (SMMT). Based on the direct tension force transfer model, a UHPFRC tensile constitutive model was derived by considering the fiber pull-out behavior. The proposed constitutive model was implemented in the SMMT, and the modified SMMT was found to be useful in predicting the pure-torsional behavior of UHPFRC beams with solid and hollow cross-sections. Moreover, the theoretical torsional responses were compared with the experimental results, and the experimental data reported in previous studies were used to verify the applicability of the proposed model. The results indicate that the estimated analytical responses matched well with the experimental outcomes, and the modified SMMT considered the influences of both the wall thickness and excellent UHPFRC material properties. The proposed model is expected to be effective for further investigation of the torsional mechanism and design methodology of UHPFRC members under pure torsion.