Behaviour of Low-Rise Shear Walls with Hybrid GFRP-Steel Reinforcement and Steel Fibre-Reinforced Concrete

Abstract

Recent earthquakes have revealed that conventional steel-reinforced concrete (RC) shear walls can exhibit considerable damage and residual displacements even after moderate intensity earthquakes. These residual displacements can result in high post-earthquake repair costs. Recent advances in the science of composite materials have motivated the search for cost-efficient solutions to improve seismic behaviour of low-rise walls. This study investigates the potential of fibre-reinforced polymer (FRP) bars and fibre-reinforced concrete (FRC) to improve the behaviour of low-rise walls. Use of hybrid vertical reinforcement consist of steel and GFRP bars is proposed. The steel lends ductility to the system, while the elastic behaviour of GFRP material enhances the self-centering ability of the wall to reduce residual displacements. First, using a preliminary finite-element (FE) analysis model, a parametric study was conducted to determine the most suitable hybrid scheme in terms of ductility, stiffness, strength and self-centering. Performance of the hybrid system subjected to earthquake loading was studied using a simplified nonlinear dynamic analysis. The response of RC and hybrid FRP-steel walls were shown to be comparable when designed properly in terms of stiffness and serviceability. To verify the modeling results, two low-rise concrete shear walls with similar geometry were built and tested up to failure under pseudo-static lateral cyclic loading. First wall was a conventional steel-reinforced concrete (RC) shear wall with height-to-length ratio of 1, designed as per the seismic considerations of CSA A23.3-14 and ACI 318-14 codes with a relaxation in confinement reinforcement. The second was an innovative wall with hybrid GFRP-steel vertical reinforcement and steel fibre-reinforced concrete (SFRC). The SFRC was used to mitigate the damage experienced by the concrete. This design also provides the opportunity to evaluate potential of SFRC for confinement relaxation in shear walls suggested in the literature. Then the FE model was improved to capture the test results by including bond-slip mechanism and buckling of the reinforcing bars in the analyses. The model was shown to be able to predict system performance variables with satisfactory accuracy for both walls, such as strength, stiffness, and self-centering. The effect of several reinforcement arrangement in low-rise walls was investigated using the analysis model and the advantages of hybrid system over exclusively FRP-reinforced and steel-reinforced walls were discussed. It is shown that in hybrid FRP-steel low-rise walls, an arrangement of FRP bars at the middle region of the wall together with steel bars at the wall boundaries is able to achieve comparable strength, stiffness and ductility with conventional RC walls. The hybrid system has improved self-centering behaviour in comparison to its RC counterpart, while maintaining a significant energy dissipation capacity. Backbone curve of force-displacement response of a hybrid wall shows a characteristic tri-linear behaviour, which is not associated with capacity deterioration. The developed model can be used to provide a better understanding of the performance of the hybrid system at the material and system levels. Addition of fibres increases post-cracking damage tolerance of the wall, but do not delay buckling of longitudinal bars. Some potential of steel fibre-reinforced concrete to increase ductility of low-rise walls are discussed using the developed model.