Confinement Behavior of Rectangular Reinforced Concrete Prisms Simulating Wall Boundary Elements

Observations following recent earthquakes, and from structural testing, indicate numerous brittle compression failures in reinforced concrete seismic-resisting walls. This is unexpected, as most seismic-resisting walls are designed to be tension-controlled. The problematic compressive response led to two independent studies, each individually aimed at identifying design and loading parameters that affect the seismic deformability of the compression regions (or boundary elements) of seismic-resisting walls. These experimental studies are combined here for a more complete understanding. Both studies used axially loaded, rectangular reinforced concrete specimens that simulate seismic-resisting wall boundary elements. The rectangular prisms were tested under cyclic axial loading or monotonic compression, with a focus on the following parameters: boundary element detailing classification, detailing of transverse reinforcement, maximum tensile strain preceding compressive demand, and cross-sectional aspect ratio. Test results indicate that expected strength and deformation capacity can be overestimated unless a rectangular hoop restrains every longitudinal reinforcing bar; use of crossties does not guarantee stability of the longitudinal reinforcement. Tensile strains of 2 and 5%, imposed prior to reaching the compressive capacity, resulted in compression strength reductions of 20 and 50%, respectively, indicating that load-history can also be important.