Nonlinear finite element analysis for the torsional and bending behavior of heat-damaged RC beams strengthened with CFRP composites

Purpose High stresses are generated within the reinforced concrete (RC) beams upon exposure to combined bending and torsional loading and the situation worsens when high temperatures exist, where material properties are degraded. In addition, there might be a problem in ensuring enough reinforcement spacing within the structural member, especially under high torsional and bending stresses, that could be solved using external carbon fiber reinforced polymers (CFRPs). This study fully examined the interaction between the three stresses and introduced a new expression for predicting the cracking and torsional strength under the effect of the investigated parameters with high accuracy. Design/methodology/approach The nonlinear finite element analysis (NLFEA) approach was used in this study, using the ANSYS software package to study the behavior of RC beams under combined torsion and bending loading, where the CFRP strengthening efficiency and the effect of high-temperature levels were examined. The simulation process was first calibrated using a control specimen without strengthening or heat-damaging effects from a reputable published experimental work from the literature. Then, a parametric study was carried out on the effect of different variables, such as the elevated temperatures (23, 200, 400, 600 and 800)°C and the center-to-center spacing of the U-shaped CFRP sheets (0 (without), 225, 175, 125, 75 and 50 (full wrapping)) mm. The structural performance of each simulated RC beam was evaluated using the following structural behavior characteristics: torsion-twisting behavior, failure mode, torsion and twisting angle capacities at ultimate, pre- and post-cracking stiffness, toughness, strain values in the longitudinal steel, transverse stirrup and CFRP sheets. Findings It was found that decreasing the spacing between the provided CFRP sheets increases the resulting enhancement in the structural behavior. In contrast, the improvement percentages decrease when increasing the temperature value. Originality/value The combined action of torsional and bending behavior was not previously addressed in detail, especially under high temperatures.