Seismic performance of steel and BFRP bar reinforced concrete shear walls

Abstract

Reinforced concrete (RC) shear walls are widely utilized in modern buildings due to their superior lateral strength, stiffness, and seismic performance. Despite this, past catastrophic earthquakes have highlighted potential vulnerabilities, necessitating the investigation into their seismic behavior for the safety of existing buildings and future structures. This is especially significant in regions with low-to-moderate seismic risks and long-established development, like Hong Kong. Many of these existing structures were built according to older codes or standards that lacked seismic design detailing, raising concerns about their seismic safety. The need for sustainable, low-carbon construction further complicates the design of shear walls. This study, therefore, investigates the seismic performance of existing shear walls, explores the development of low-carbon shear walls, and assesses the seismic performance of existing shear wall buildings to address these multifaceted challenges. The study includes an experimental investigation of non-seismically designed shear walls with moderate shear span-to-length ratios (SLRs) and varying axial load ratios (ALRs). The results are analyzed for crack patterns, failure modes, hysteresis behavior, stiffness degradation, and reinforcing bar strains. In particular, the effective stiffness of shear walls, including effective shear stiffness and flexural stiffness, is calculated and discussed in this study and the recommended values are given. Subsequently, a backbone curve model that considers the effects of both flexure and shear is proposed and validated by the experimental results. Furthermore, a simple criterion for determining the occurrence of axial failure is obtained. This study investigates substituting steel reinforcement with BFRP bars in RC shear walls. To mitigate BFRP limitations, a novel composite shear wall, incorporating BFRP bars, UHPC boundary elements, and normal strength concrete, is proposed. Seismic performance of this innovative shear wall is validated via cyclic tests on four BFRP reinforced shear wall specimens. Comprehensive test results are provided, along with a methodology for computing the flexural strength of BFRP reinforced shear walls. Moreover, to aid in the design of UHPC-BFRP structures, a pull-out test was executed to examine the bond strength between the BFRP bar and the UHPC. Consequently, a model for determining the bond strength between FRP bars embedded in UHPC is proposed. Finally, a novel component damage-based approach to obtain seismic fragility curves for high-rise buildings is proposed. The proposed method is applied to a 34-story building with a conventional RC shear wall designed according to the old code BS8110 and a hypothetical building featuring a BFRP bar-reinforced shear wall. Component-level fragility curves and system-level fragility curves are obtained for each building, respectively.