Stability of Reinforced Concrete Wall Boundaries

Structural (shear) walls are used worldwide to resist gravity and earthquake loads. In many countries, structural walls commonly are constructed with a rectangular cross section, or a cross section made up of interconnected rectangles, without an enlarged boundary element. In some countries, design practice has resulted in walls that are more slender than those used in the past. For example, in Chile and elsewhere it is not unusual to find rectangular wall edges having thickness of 6 to 8 in. (150 to 200 mm), resulting in floor-to-floor slenderness ratios reaching hu/b = 16 or greater. Such walls can be susceptible to overall wall buckling in which a portion of the wall buckles out of the plane. The main objective of this research is to develop a methodology for evaluation of the onset of lateral instability in reinforced concrete slender walls. First, a simplified buckling mechanics solution for prismatic columns under inelastic tension/compression cycles is presented and later evaluated using the results of column tests. Later, three numerical models for buckling in columns are evaluated: nonlinear beam-column elements with fibers, and two-dimensional and three-dimensional nonlinear finite element models. Wall boundaries have strain gradient along the wall length, which would tend to brace the edge of the wall with the result that the simplified mechanics solution may give an over-conservative estimation of the onset of buckling. The theory may also be conservative for walls where the axial force in the boundary elements is not constant along the unsupported height, as may occur where moment gradients occur. To study these effects, analytical models of columns and walls are implemented. A simple approach is proposed to reduce the over conservatism of the simplified mechanics in cases where strain gradients cannot be neglected. Later, two-dimensional nonlinear finite element models are used to analytically reproduce the experimental response of wall tests. These models are used to estimate strain profiles for evaluation of the onset of lateral buckling in slender boundaries. Strain profiles are also estimated from a plastic hinge model. Finally, three damaged buildings in Chile are analyzed using linear models for the buildings and nonlinear models for isolated walls. Both buildings had some buckled walls after the 2010 Maule earthquake. Based on these studies, it is concluded that buckling in Chilean buildings most likely was a secondary failure that occurred after initial crushing of the wall boundaries.