Effects of weakly bonded ultra-high-strength steel bars details on the seismic performance and resilience of shear walls

The use of ultra-high-strength steel bars (UHSB) has attracted increasing attention because it can improve the resilience and sustainability of structural components. However, UHSB may increase the risk of shear failure in structural walls. This study proposes a concealed bracing system incorporating UHSB to mitigate this adverse effect. Five wall specimens were tested to examine the effects of shear span ratio, steel fibers, and recycled aggregate concrete (RAC) strength on hysteretic behavior and resilience. The braced walls exhibited satisfactory hysteretic and resilient performance up to a 2% drift ratio, with a stable increase in lateral resistance. The UHSB concealed bracing system reduced residual drift ratios by 8.7%–31.3%, and all specimens maintained residual drift ratios below the 0.5% limit up to 2% drift. Steel fibers effectively restrained crack propagation and mitigated stiffness degradation. Numerical models were developed using OpenSees and validated against the test results. Parametric analyses were then conducted to investigate key resilience-related parameters, including bracing reinforcement ratio, bracing height, longitudinal reinforcement ratio, axial load ratio, and UHSB replacement ratio. Based on the results, design recommendations are proposed.