Detailing for compression in reinforced concrete wall boundary elements: experiments, simulations, and design recommendations
Reinforced concrete structural walls are one of the most common lateral force-resisting systems in buildings located in high seismicity regions. Walled structures are expected to provide significant strength and ductility under cyclic earthquake loading. In part, this expectation is based on the assumption that current design approaches suppress compression failure of the boundary element. However, compression failures observed following recent earthquakes in Chile (2010) and New Zealand (2011), suggest that the behavior of reinforced concrete walls may not meet expectation and further study is required. As part of a larger effort initiated by the National Earthquake Hazards Reduction Program (NEHRP) at the National Institute of Standards and Technology (NIST), a research program was developed to understand and improve the seismic behavior of concrete walls. This portion of that larger research program focuses on evaluation of, prediction of, and recommendations to improve boundary elements in structural walls. The research is divided into three phases. The first phase uses experimental research methods to evaluate the impact of salient study parameters on vertically loaded, large-scale rectangular reinforced concrete prism members that simulate the boundary element of a special concrete wall. This experimental program explored the effects of detailing of both transverse and longitudinal reinforcement and loading protocol on the compressive confined strength and strain capacity of boundary elements. In the second phase of research, these experimental results were combined with prior tests to evaluate commonly used confined concrete constitutive models and current detailing requirements in ACI 318-14. The test results indicate that rectangular reinforced concrete prism members designed to meet the minimum ACI 318-14 detailing requirements for special boundary elements exhibit little to no significant increase in compressive strength or deformation capacity as compared to boundary elements that do not meet those minimum requirements. By looking at the full data set, a hierarchy of iii detailing parameters was developed. In order of importance, the following detailing parameters are shown to increase both strength and strain capacity: (i) ratio of transverse reinforcement spacing to longitudinal bar diameter of at most 4.0, (ii) full development of transverse reinforcement legs used to restrain longitudinal reinforcement, (iii) buckling restraint for all longitudinal reinforcement, and (iv) increased transverse reinforcement ratio. In addition, the results indicate that commonly used confined concrete constitutive models over-predict the strength and deformation capacity of ACI 318-14 compliant boundary elements. A new formulation for this model is proposed and validated to more accurately predict compressive performance. In addition, the new formulation is also shown to provide accurate prediction of wall performance when it is implemented into modern nonlinear analysis techniques. The third phase of the research used the results to investigate possible improvements to ACI 318-14 boundary element detailing requirements. Three performance-based levels of detailing were studied, each providing an estimated expected peak stress and strain capacity; the applicability of commonly used confined concrete constitutive models was also investigated for each detail level. These detailing levels are intended to provide the design engineer with guidance in selecting proper boundary element detailing to meet specific performance needs.
reinforced concrete, special reinforced concrete structural walls, boundary elements, boundary element detailing, confined concrete modeling
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign