Book excerpt: The 2010-11 Canterbury and 2016 Kaikoura seismic events led to the identification of several vulnerabilities in existing older-type and modern reinforced concrete (RC) frame structures. By mid-2017, the New Zealand parliament approved the Building Amendment Act 2016 which set new requirements for managing earthquake-prone structures. However, owing to a lack of sufficient knowledge on the seismic response of reinforced concrete (RC) frame structures on the global and component level, codified seismic assessment provisions in New Zealand and overseas are inherently conservative or not suited for structures with specific vulnerabilities; leading to unwarranted retrofits or building closures. Aside from the over-exaggerated risks to community seismic resilience, the high costs of these unwarranted retrofits pose a significant financial burden on New Zealand's economy. To address this, the main objective of this research is to develop refined seismic assessment provisions that can be adopted for the identification and intervention prioritisation of the most earthquake-vulnerable structures. A survey of the NZ assessment guidelines and overseas standards highlighted specific provisions that need to be improved. These include provisions for: (a) evaluating the yield rotation of beam-column RC components; (b) accounting for the influence of biaxial seismic demands on RC columns; (c) assessment of beam-column RC components susceptible to single-crack plastic hinge response; and (d) accounting for uncertainty in component failure mode in seismic assessment of frame structures. This thesis has been developed to address each of the aforementioned issues. Using a collated experimental database of columns, beams and coupling beams with normal- and high-strength longitudinal steel, simple mechanistic formulations were developed for estimating the yield rotation and effective stiffness of beam-column components. The proposed formulations explicitly account for the contribution of bar slip and shear deformations, as a function of aspect ratio, using a proposed coefficient. To address the absence of specific provisions for accounting for the influence of biaxial seismic demands on RC columns, a dataset of past tests on columns subjected to biaxial lateral demands was collated. Analysis of the dataset was used to identify the characteristics and effects of biaxial lateral displacement paths on the seismic response of RC columns. Using the available ii dataset, recommendations were provided on how the influence of biaxial lateral load on lateral strength, deformation capacity, and stiffness degradation can be accounted for. A failure mode-based approach was proposed to improve provisions for seismic assessment of components susceptible to single-crack plastic hinge response. Firstly, a database of past tests on columns with smooth reinforcement was collated. This database was then used to validate a proposed mechanistic formulation for estimating the rocking capacity of columns with smooth reinforcement. Furthermore, an experimental program was carried out on six full-scale beams which are replicas of beams observed to have exhibited a single-crack plastic hinge response following the 2016 Kaikoura earthquake. This experimental program provides data on seismic performance, repairability, and residual capacity of beams with curtailed bar detailing. Using these test data, formulations were proposed for seismic assessment of beams with curtailed bar detailing. The final part of this thesis focuses on a probabilistic failure mode-based approach. Experimental and analytical studies were carried out to demonstrate the existence and influence of component failure mode variability on the collapse response of RC components and frame systems. This source of uncertainty was previously neither recognised nor incorporated into probabilistic seismic assessment frameworks. It was shown that nominally identical components may exhibit different failure modes due to inherent material uncertainty, strain rate and displacement history effects. To account for this, a failure mode transition zone was defined to identify components susceptible to failure mode variability. Furthermore, a probabilistic framework was proposed to account for the influence of failure mode variability in component collapse fragility estimate. The study was concluded by developing a framework for incorporating uncertainty in the global collapse mechanism into the performance-based earthquake engineering (PBEE) framework.