Current seismic design codes of practice specify the earthquake loadings independently in each direction of the structure’s principal axis. However, the lateral earthquake excitations could be simultaneous in both directions or in a skewed direction to the principal axis. Therefore, if the seismic design or performance is based on the inelastic behaviour of the structure up to the ultimate limit state, then the deformation capacity of the structure in the skewed direction must be ensured from another point of view.

Columns as elements which are under flexural and axial actions were recognized by researchers as members that are under the influence of bi-directional loading. Several experimental and analytical studies were carried out on bi-axially loaded columns and magnification factors for the column actions were also introduced in recent seismic design of reinforced concrete (RC) structures. On the other hand, due to the good performance of RC structural walls in the past earthquakes, the need for relatively advanced investigations and considerations such as the effects of bi-directional loading were not justifiable. Over the past several decades, a large number of studies (experimental and numerical) were conducted on the seismic behaviour of RC walls. Most of the walls were subjected to in-plane cyclic lateral loading and gravity loads simulating the calculated response of actions in a proto-type structure during an earthquake, while a small number of these walls were tested under bi- directional loading. Therefore, the effect of bi-directional loading on rectangular RC shear walls is not yet fully understood by researchers. However, unexpected failure modes observed in RC walls in the 2010 Chile and 2011 New Zealand earthquakes raised a global concern on the contribution of bi-directional loading to these failure modes.

This thesis aims to provide a better understanding on the seismic performance of rectangular RC walls subjected to a more realistic loading regime (bi-directional loading). Due to lack of experimental data on rectangular RC walls under bi-directional loading especially for slender walls, a series of experiments were conducted to cover some of the key research questions on the seismic performance of rectangular RC walls subjected to bi-directional loading. To complement the experimental investigation, a comprehensive numerical study was carried out targeting the key research gaps that were not investigated in the laboratory due to the limitations with the number of specimens.

The experimental programme included two main phases. In the first phase, three identical walls subjected to different lateral loading patterns were tested to investigate the effects of various lateral loading patterns on the seismic behaviour of rectangular RC walls which was not investigated before. In the second phase of experiments, three rectangular walls designed for different section detailing ductility based on NZS3103:2006-A3 (2017) were investigated under high axial load ratio and bi-directional loading targeting a failure mode suspected to be a product of bi-directional loading (out-of-plane shear failure). Out-of-plane shear failure was captured in the laboratory which significantly helped with understanding the mechanism of this failure mode. One of the main challenges with the experimental phase was the test setup which had seven hydraulic actuators. Second phase of the experimental study was even more challenging with the combination of high axial load and bi-directional loading.

There were challenges with the numerical phase as well, as there was limited number of numerical studies on rectangular RC walls subjected to bi-directional loading. Therefore, a finite element (FE) model had to be developed and validated for RC walls subjected to bi- directional loading. New experimental data provided by the current study significantly helped with the development of the FE model. The FE software DIANA was used for the numerical phase of the study. The FE model was validated extensively against 7 rectangular RC walls with 4 different lateral loading patterns and complex failure modes such as lateral instability and out-of-plane shear failures. Using the validated FE model, failure mode of a rectangular RC wall collapsed in the 2011 NZ earthquake in out-of-plane shear was successfully captured. The last part of the numerical phase was to identify the key parameters contributing to the development of out-of-plane shear failure. Each parameter was investigated comprehensively. Using the numerical parametric results, a matrix of walls was formed based on the two most influential parameters in causing vulnerability in rectangular RC walls to out-of-plane shear failure. An analytical method was proposed using the matrix results which can be used for identifying RC walls prone to out-of-plane shear failure in practice for both design and assessment purposes.