Numerical study on reinforced concrete beams strengthened with steel plate and <scp>ultra‐high‐performance concrete</scp> and optimization of prediction model

Abstract This study develops a sophisticated finite element model (FEM) to investigate the flexural behavior of reinforced concrete (RC) beams strengthened with combined steel plate and ultra‐high‐performance concrete (SP‐UHPC) in terms of flexure. The simulation covers the full strengthening process, including four‐point bending pre‐damage, unloading, SP‐UHPC application, and reloading to failure with four‐point bending. The FEM of the SP‐UHPC beam successfully reproduces both the interface debonding and key bending moments. To study flexural behavior, ideal bonding is assumed for parametric analysis. Results show that each 20 mm increase in UHPC thickness improves bearing capacity by 7.3%, stiffness by 16.2%, and cracking moment by 53.2%. Additionally, increasing steel plate thickness by 1 mm enhances the flexural performance parameters by 12.0%, 14.7%, and 18.0%, respectively. The optimal UHPC‐to‐RC depth ratio for strengthening efficiency is 1:10. Additionally, the analytical results become gradually larger than the numerical ones as the UHPC layer thickness increases and a correction factor is proposed for the analytical model. Based on a surrogate model with reasonable assumptions, the heights of RC beams corresponding with the UHPC layer and steel plate thicknesses are optimized to have low weight and cost, but high cracking moment, stiffness, and flexural capacity for the strengthened beams.