Currently in NZ, there is a significant proportion of building stock designed prior to 1995 with inadequate transverse reinforcement in the reinforced concrete columns. Recent earthquakes (such as the 1994 Northridge and 1999 Kocaeli earthquakes) and research has highlighted the susceptibility of these columns to lose shear and axial load carrying capacity. In view of the current focus in New Zealand towards assessing and upgrading the performance of existing structures, a model to quantify the performance of these columns is required. Special focus in this research is towards the performance of columns designed to support gravity only as these ‘gravity’ columns did not have the detailing requirements associated with the lateral force resisting system. Previous research has typically focused on the performance of these columns under in-plane seismic actions only. To increase the understanding of these columns it is necessary to extend the current research to capture the full bi-directional performance. In this thesis, results of experimental investigations performed on representative Reinforced Concrete columns are discussed. The investigation consisted of uni- and bi-directional tests performed on six cantilever ‘gravity’ columns. Two detailing configurations were tested under three different loading protocols; uni-directional quasi-static, bi-directional quasi-static, and bi-directional quasi-earthquake loading. The test specimens are representative of columns designed to carry gravity only, constructed during the period from the introduction of deformed bars in the mid 1960’s through to the early 1990’s. An assessment of existing capacity models is carried out for the uni-directional tests and then extended for bi-directional loading. The adequacy of the proposed ‘Limit Surface Model’ is verified using the quasi-earthquake tests to ensure the appropriateness of the model under realistic displacement histories. Additionally, a conservative ‘Simplified Backbone Model’ is proposed and assessed.