PRACTICAL NONLINEAR MODELING OF U-SHAPED REINFORCED CONCRETE WALLS UNDER BI-DIRECTIONAL LOADING

Reinforced concrete (RC) structural walls are the most commonly used structural elements in buildings to resist lateral load imposed by earthquakes. Therefore, analytical models capable of capturing important nonlinear response characteristics of RC walls at global and local response levels are essential for engineering design and evaluation, particularly for practicing performance-based methodologies. Although various analytical models for nonlinear analysis of RC walls are available in the literature, the majority of computer models have been validated for planar walls only, as experimental data on RC flanged walls are sparse. Furthermore, very limited number of nonlinear modeling approaches is implemented in commercial simulation platforms to be used in practice. Currently, Perform 3D (Computers and Structures Inc.) is the most commonly used in the US, and one of the only commercial structural analysis software for nonlinear analysis of RC structural walls. While being widely used, analytical models for walls available in this software have not been validated in details, particularly for flanged wall specimens subjected to multi-directional loading. This paper provides detailed information about sensitivity of predicted wall responses to modeling parameters, calibration, and validation of analytical models available in Perform 3D against experimental results obtained for U-shaped wall specimen tested under bi-directional quasi-static cyclic loading regime. The U-shaped wall was tested at ETH Zurich and represented the lower two levels of a six-story building. The wall was subjected to a constant axial force and cycles of horizontal displacements. These cycles were applied along the two principal horizontal axes, along one diagonal axis and using a sweep pattern. The cycles were applied with increasing displacement amplitudes. During the test, the forces applied by the actuators and global and local deformations were recorded. The wall failed due to crushing of the compression diagonal in the web. At the point of failure, two bars of one flange had buckled but not fractured. The behavior of the U-shaped wall was simulated using two conceptually different analytical models available in Perform 3D: shear wall element and general wall element. Material models for steel and concrete were calibrated to match as tested material properties. Lateral displacement history at the top of the wall (bi-directional loading) was applied in the analysis to simulate experimental loading conditions. Sensitivity of analytical results to model geometry discretization and material modeling parameters was investigated. In addition, detailed comparison between experimentally measured and analytically predicted wall responses is conducted at both global (force-deformation) and local (strain) levels. It has been observed that general wall element captures experimentally measured load-deformation behavior of the wall specimen more accurately than the shear wall element, as well as that predicted wall responses are less accurate in diagonal loading direction for both models. Plastic hinge length of the wall was predicted reasonably well regardless of geometry discretization, whereas magnitudes of vertical axial strains within the plastic hinge region are considerably sensitive to mesh size. Based on results of analytical studies presented, capabilities of analytical models available in Perform 3D are assessed and recommendations for practical applications are provided.