Investigation of failure mechanisms and development of design recommendations for flexural reinforced concrete walls

Abstract

For many mid- and high-rise concrete buildings in seismic regions, earthquake loads are resisted by reinforced concrete walls that respond primarily in flexure (i.e. yielding in flexure without exhibiting significant shear damage or shear failure). Under lateral loading, flexural walls may sustain compressive damage to concrete and reinforcing bars as well as tensile rupture of reinforcement, with the onset of lateral strength loss resulting from one or more of these damage mechanisms. Given the prevalence of concrete walls, there is interest in advancing numerical modeling to enable improved prediction of wall response as well as in advancing wall design and detailing to achieve enhanced performance. The research presented here does both. First, modeling recommendation for nonlinear finite element analyses were developed and validated using data from prior laboratory tests of large-scale structural walls. Second, the validated modeling approach was used to conduct a parametric study to investigate the impact of two critical design parameters, normalized shear stress demand and cross-sectional aspect ratio, on the deformation capacity corresponding to onset of lateral strength loss. Few tests have considered the larger cross-sectional aspect ratios and shear stress demands that are common in design of modern walls. Results of this study show that walls with large cross-sectional aspect ratios and moderate to high shear demands develop high minimum principal stresses within the web region of the wall at the web-boundary element interface; these high stresses can lead to lateral strength loss at relatively low drift levels due to concrete crushing and bar buckling within the web. This failure mode is termed a compression-shear failure. The third phase of the study used the validated modeling approach to conduct a parametric study to develop a mitigation strategy for walls susceptible to compression-shear failure, namely extension of the boundary element confinement into the web of the wall. The results of this study were combined with test data to determine the confined length required to suppress the compression-shear mode. This required confined length is a function of the shear stress demand and is conservatively estimated as c, where c is the depth of the compression region when the wall achieves nominal flexural strength.