Seismic Assessment of Singly Reinforced Concrete Walls in Pre-1970s Multi-Storey Buildings

Abstract

During the 2010/2011 Canterbury earthquakes in New Zealand, undesirable failure behavior was observed of the reinforced concrete walls in multi-storey buildings constructed before mid-1970s, which highlighted the structural vulnerabilities of older reinforced concrete walls. In this thesis, the structural characteristics of older reinforced concrete walls in Wellington New Zealand were summarised. Full-scale experimental testing and finite element modelling analysis were conducted to investigate the drift capacity and failure modes (i.e. the critical indicators considered in guidelines on evaluating seismic behaviour) of reinforced concrete walls representative of typical older construction practice in New Zealand. The testing results highlighted the limited ductility of older reinforced concrete walls without ductile detailing and identified their critical failure modes. Premature reinforcement fracture, concrete crushing and brittle axial failure were typical failure modes in older reinforced concrete walls controlled by flexure. To supplement the experimental testing and study the behaviour of older reinforced concrete walls further with a wider range of structural parameters, finite element model was developed in VecTor2 and it was able to accurately capture the global and local behaviour of both flexural- and shear- dominated reinforced concrete walls with non-ductile detailing. The modelling results indicated that the wall response and drift capacity of reinforced concrete walls representing typical older construction practice in New Zealand before mid-1970s would be significantly influenced by shear span ratio, axial load and concrete strength. In addition, the walls with shear span ratio less than 1.0 were prone to shear failure characterised by sliding along a diagonal crack. The drift capacity model in NZ assessment guidelines was reviewed and further verified by the experimental and numerical modelling results to justify its capabilities of evaluating the response of older reinforced concrete walls. The current provisions were able to conservatively estimate the drift capacity at lateral failure for older reinforced concrete walls with shear span ratio larger than 1.0. However, the drift capacity of walls with shear span ratio less than 1.0 was overestimated and recommendations were proposed to address this limitation. In addition, a simplified drift capacity model and parameter checklist were provided for engineers as a practical tool to quickly tell the drift limits of older reinforced concrete walls in initial seismic assessment. Lastly, the mechanism of axial failure in older reinforced concrete walls without ductile detailing was investigated. The axial failure was characterised by the development of a diagonal failure plane through the wall thickness and shifting of the upper wall panel in out-of-plane direction. The magnitude of axial load and boundary confinement were identified to be critical for axial failure. Both mechanics-based model and simplified model were proposed to identify the walls vulnerable to axial failure from compression-critical walls exhibiting gradual concrete crushing, and an empirical model was derived to estimate the drift capacity at axial failure.