Out-of-plane Instability of Rectangular RC Structural Walls Subject to In-plane Loading

PI: Prof Rajesh Dhakal

PhD Student: Farhad Dashti

Co-Supervisor: Prof Stefano Pampanin

Out-of-plane instability is identified as one of the likely failure modes of slender RC walls under earthquake excitations. This mode of failure was previously observed in experimental studies of rectangular walls, and has attracted more attention following the observed damage of several walls in the recent earthquakes in Chile and Christchurch.

In this study, out-of-plane instability of slender rectangular walls subject to in-plane loading is investigated. For this purpose, the ability and robustness of a finite element model in predicting nonlinear cyclic response and failure patterns, particularly out-of-plane instability, of rectangular walls has been investigated.

Experimental results of walls with different failure modes are used for model verification. The model is found to be able to reasonably capture the lateral load versus top displacement response of the specimens and predict most of the experimentally observed failure mechanisms of rectangular walls. The simulated failure patterns include shear, flexure, flexure-shear and flexural out-of-plane failure modes. The model is currently unable to predict bar buckling due to inherent deficiencies of embedded bar elements and limitations of material models available in the program. The causes and consequences of bar buckling in RC shear walls is under investigation separately in another PhD project.

Using the validated model, some of the parameters known to be influential on out-of-plane instability of walls have been scrutinized, and the effects of these parameters on this mode of failure have been evaluated by testing three rectangular wall specimens subject to cyclic quasi-static loading. The specimens were half-scale, representing first story of four story prototype walls. The test setup was thus designed to apply the lateral load as well as the bending moment coming from the upper stories. The specimens were designed according to NZS3101:2006 and were different in thickness and length to investigate the effects of these parameters on the onset and extent of out-of-plane displacement of rectangular walls. The experimental results have been scrutinized in terms of significant stages of wall response observed during the test and the effects of the above-mentioned parameters on the sequence of these stages. Based on numerical investigations as well as experimental findings carried out in this research, design recommendations will be proposed to reduce/prevent out-of-plane displacements of rectangular walls.

ACKNOWLEDGEMENTS:

Thank you to the Quake Centre and MBIE - their financial assistance helps to make this research possible.