Parametric Seismic Analysis of FRP-Strengthening Length at Beam and Column Ends of Existing RC Frames

In view of the seismic performance deficiencies commonly observed in existing reinforced concrete (RC) frame structures, the application of fiber-reinforced polymer (FRP) composites for seismic strengthening has become a key technical approach to enhance the safety reserve of such structures. However, most existing studies primarily focus on the overall effectiveness of strengthening schemes, while systematic investigations on key geometric parameters, such as strengthening location and length, remain insufficient. In this paper, based on the ABAQUS finite element software and calibrated against a quasi-static test of a 1/2-scale two-story two-bay RC plane frame, refined numerical models of both unstrengthened and FRP-strengthened frames were established. Adopting a “control-variable parametric analysis” strategy and using cross-sectional dimensions as the reference, nine strengthening cases were designed with column-end FRP lengths of 0.5b, 1.5b, and 2.5b and beam-end FRP lengths of 1h, 2h, and 3h. The differential effects of FRP-strengthening lengths at beam and column ends on the seismic performance of the frame structure were revealed. The results indicate that FRP strengthening can effectively enhance the load-bearing capacity and improve the hysteretic performance of members. Nevertheless, the underlying mechanisms by which beam-end and column-end strengthening lengths affect mechanical behavior are fundamentally different: column-end FRP strengthening primarily governs the initial stiffness and load-bearing capacity, with a maximum increase in peak load of 40.7%; beam-end FRP strengthening mainly controls the post-yield stiffness degradation rate and energy dissipation capacity, achieving a peak load increase of up to 53.2%, although the marginal benefit tends to diminish with increasing length. Beam-end strengthening is the critical factor governing the transition of failure mode (from brittle joint shear failure to ductile beam-end flexural failure), while column-end strengthening plays a supplementary role in restraining the formation of column hinges and maintaining the desirable “strong column-weak beam” failure hierarchy. This study reveals the influence of FRP strengthening length on the seismic performance of structures, and to some extent addresses the deficiency of existing studies in systematic parametric analysis.