Thermomechanical performance of reinforced concrete beams with normal and high strength steel reinforcements under fire exposure: a FEA-based comparative study

Purpose A thermomechanical finite element analysis (FEA) was conducted on reinforced concrete (RC) beams reinforced with normal-strength steel (NSS, fy = 500 MPa) and high-strength steel (HSS, fy = 700 MPa) subjected to fire exposure per ISO 834. Design/methodology/approach The study examines the influence of rebar quantity and arrangement on structural behavior at elevated temperatures. Findings HSS beams demonstrate higher initial flexural strength due to greater yield strength and reduced steel content but experience faster performance degradation after 90 min, attributed to lower thermal mass and reduced redundancy. Conversely, NSS beams, though initially less strong, maintain structural integrity longer under fire due to higher reinforcement density, improved stress redistribution and greater thermal inertia. The findings highlight the critical role of reinforcement detailing and thermomechanical interactions in designing fire-resistant RC beams with HSS. Originality/value This study provides a detailed evaluation of the fire resistance of RC beams reinforced with NSS (500 MPa) and HSS (700 MPa). The methodology combines nonlinear mechanical FEA, thermal analysis based on the ISO 834 standard fire curve and fully coupled thermomechanical simulations to assess the beams’ load-bearing capacity at various fire exposure durations. By integrating these modeling phases, the research enables a direct comparison of the structural behavior and mechanical equivalence between NSS- and HSS-reinforced beams under elevated temperatures. The findings offer critical insights into the thermal sensitivity of different reinforcement grades, supporting more informed, performance-based fire design approaches.