Seismic performance of reinforced concrete beam column joints strengthened with ECC shells

Reinforced concrete (RC) beam–column joints are vulnerable to brittle failure under seismic actions due to their limited deformation capacity, posing a threat to the overall safety of RC frames. To address this issue, this study proposes and evaluates a novel strengthening strategy for RC joints using an engineered cementitious composite (ECC) shell, forming a beam–column joint with an ECC shell (BCJES). The mechanical behavior of the BCJES under cyclic loading was investigated through a refined finite element (FE) model. The FE model, validated against test results, enabled a systematic parametric study to quantify the effects of ECC shell height and thickness, longitudinal reinforcement ratio (at the beam region), and axial compression ratio on seismic performance. Results demonstrate that the proposed ECC shell markedly enhances the seismic capacity of RC joints. Increasing the longitudinal reinforcement ratio from 0.05% to 0.2% improved peak load from 33.87 to 85.58 kN (152%), while increasing shell thickness from 30 to 90 mm enhanced peak load by 11.9%. However, a saturation effect was observed, as further thickening the shell to 150 mm resulted in only a 2.46% gain. Based on the parametric results, this study, for the first time, establishes a quantitative predictive model for the ultimate bearing capacity of BCJES using multiple linear regression (R²= 0.943). Furthermore, a new theoretical shear–capacity model incorporating both main and lateral diagonal bracing mechanisms is developed. Theoretical predictions agree well with FE simulations and experimental results, with maximum deviations of only 7.5% and 7.9%, respectively, confirming the reliability of the proposed approach. These findings highlight the potential of the ECC shell as an effective and practical seismic-strengthening solution for RC joints, offering new insights for performance-based design and retrofit strategies.