Performance of C-Shaped structural concrete walls subjected to bi-directional loading

Reinforced concrete walls are commonly used as the lateral force resisting system for mid-rise buildings in regions of low and high seismicity. Wall geometries in buildings are generally complex configurations to accommodate architectural constraints during new construction or existing conditions in seismic retrofit applications. A typical configuration for seismic regions is the concrete core-wall system in which coupling beams link a pair of C-shaped walls. While a prevalent structural system, few experimental research programs have examined this wall type and codes of practice have focused on design provisions for planar walls which do not fully account for the effects of non-planar geometry and multi-directional loading. To improve the understanding of the three-dimensional and asymmetric response of coupled core walls, an experimental testing program of C-shaped walls subjected to uni-directional and bi-directional cyclic loading was completed. Three C-shaped walls representative of a ten-story core wall building were tested at the University of Illinois Newmark Structural Engineering Laboratory. Each wall test was subjected to progressively complex loading conditions, and a new stiffness-based loading algorithm was developed to conduct the experiment. Analysis of the experimental data studied the energy dissipation, progression of yielding, components of deformation to total wall drift, base deformations, strain fields generated from displacement field data, and overall displacement profiles of the prototype ten-story building. Subsequent evaluations using prior experimental tests of planar, coupled and non-planar walls identified the aspects of behavior unique to C-shaped walls. The experimental tests exhibited a ductile failure resulting from loss of boundary element confinement, bar buckling, and rupture of the longitudinal bars. However, the ductile failure mechanism was precipitated by increased shear deformation and undesirable shear related damage of base sliding and web crushing. The onset of damage mechanisms, propagation of damage, and drift capacity at failure was identified to be path dependent, and bi-directional loading decreased drift capacity. Effective flexural and shear stiffness values for the elastic analysis of non-planar walls were recommended for design. Design variables and demand to capacity ratios were parametrically studied for non-planar walls as a means to correlate drift capacity and ductility. To supplement the experimental data, a series of non-linear finite element analyses were conducted using a layered shell element model with comprehensive constitutive models capturing the cracked response of reinforced concrete in cyclic biaxial loading conditions. Model validation is conducted using reinforced concrete panel tests, and the impact of crack spacing on prediction is quantified. The resulting analytical models of the C-shaped walls provide a validation of the experimental results and a characterization of shear stress distribution as a function of drift level for strong axis and weak axis loading