Evolution of out-of-plane deformation and subsequent instability in rectangular RC walls under in-plane cyclic loading. Experimental observation

In this study, to understand the causes and consequences of out-of-plane instability in rectangular RC walls, the sequence of events observed during a rectangular wall experiment campaign where out-of-plane instability was the primary failure pattern is discussed in detail. Large tensile strains developed in the boundary zone longitudinal bars at large in-plane curvature demands and caused subsequent yielding in compression during load reversal before crack closure could activate contribution of concrete to the load-carrying capacity of the wall. The specimen deformed in the out-of-plane direction when this phenomenon progressed along a sufficient height and length of the wall and led to significant reduction of its stiffness in the out-of-plane direction. One of the major sources of inherent eccentricity that could potentially affect development of this failure pattern is identified to be the postyield response of the longitudinal bars across the wall thickness, generating different values of residual strains and consequently inducing their asynchronous yielding under compressive actions. Analytical models proposed in literature for prediction of out-of-plane instability failure as well as their relevant assumptions are compared with the test measurements. While the observations presented in past research on development of out-of-plane instability in concrete columns representing the boundary zones of rectangular walls are in line with the sequence of events observed in this study, the assumptions made in the analytical models regarding the height of the wall effectively involved in formation of out-of-plane deformations (effective buckling height) were found to be different.