Impact of cross section, web reinforcement and load history on the seismic performance of slender concrete walls

Many reinforced concrete buildings in seismic regions employ reinforced concrete shear walls as part of the lateral force resisting system and these walls often have non-planar cross-sectional geometries. To date, the majority of experimental tests on slender concrete walls have been conducted on planar walls which have been subject to low shear stress demands. An experimental program was developed to examine the response of flanged C-shaped walls with respect of load history and a computational parametric study was conducted to focus more specifically on the impact of web reinforcement for walls subject to a range of shear stress demands. The experimental program investigated the impact of bi-directional loading on flanged C-shaped walls that were designed to meet the minimum ACI 318-08 special structural wall requirements. The results indicate that irrespective of load history the C-shaped walls have a similar damage progression leading to a buckling-rupture failure and a nearly identical strong-axis load-deformation response up to the peak flexural strength. However, bi-directionally loaded walls exhibit earlier onset of critical damage limit states and reduced strong-axis drift capacity. Compared to experimentally-tested planar walls that tend to fail via crushing-buckling, the flanged C-shaped wall geometry has a more ductile failure mode despite being subject to higher shear stress demands. The improved response can be attributed to the ability to redistribute forces to the boundary elements and flanges after considerable web damage. Damage to the unconfined web of the flanged C-shaped walls was substantial. Though walls developed distributed cracking, there was a single wide crack plane that developed near the wall base. Widening of this crack led to high tensile strains in web reinforcement and ultimately the widespread fracture of vertical web bars, limited fracture of horizontal web bars, as well as severe concrete degradation in the surrounding region. This performance suggests that the minimum web steel content required by ACI 318 may be insufficient. As such, the current minimum web reinforcement requirements were studied using an experimentally-validated, high-resolution finite element modelling approach. The computational parametric study examined the impact of the shear stress demand and web reinforcement ratio on wall deformation and ductility. The study results indicate that increased shear stress demand can significantly reduce wall deformation and ductility; however, designs with excess horizontal reinforcement, beyond what is required by ACI 318-14 to meet shear demand, can improve ductility. The data suggest there are similar performance benefits of reducing the design shear demand-to-capacity ratio. A second stage of the parametric study explored the combined effect of modifying the horizontal reinforcement ratio and increasing boundary element length from the ACI 318-14 minimum to the full neutral axis depth. For walls with low-moderate shear stress demands, this combination results in even greater wall ductility than providing excess horizontal reinforcement alone. The experimental tests provide critical data to developing performance-based design criteria for non-planar walls, since most prior efforts have been related to planar walls. The computational parametric study results are of value in developing new code recommendations for the minimum horizontal web reinforcement ratio which have essentially remained unchanged throughout the history of the ACI 318 building code