Performance-based Assessment and Seismic Strengthening of the Christ Church Cathedral

Abstract

The Christ Church Cathedral is a significant landmark for New Zealand. It was severely damaged in the 2011 Canterbury earthquakes – the Tower and Rose Window/western façade of the Cathedral suffered partial collapses while the remainder of the building sustained extensive cracking and displacement to its unreinforced masonry superstructure. As a result, the Cathedral was declared a Dangerous Building and fenced off from the public. Following a 2017 decision by the Anglican synod to proceed with stabilisation and reinstatement of the Cathedral, Christ Church Cathedral Reinstatement Ltd. (CCRL) was formed and charged with delivering the project. Holmes have provided structural engineering services to CCRL, including design of a strengthening/retrofit scheme to bring the building’s seismic rating up to 100% of new building standard (NBS) for an Importance Level 3 building. To achieve this, the Cathedral will be retrofitted with base isolation to reduce seismic demands on the building and therefore, the amount of strengthening required to the superstructure. This paper focuses on Holmes’ use of non-linear time history analysis (NLTHA) to inform and validate the design of the earthquake repair and strengthening scheme. The use of an NLTHA model during the project’s concept phase enabled the scope of seismic strengthening to be refined, thereby minimising disruption to the building’s heritage fabric. The NLTHA procedure that was adopted is described, including element types, post-elastic degradation characteristics, acceptance criteria and their application, and modelling of soil-structure interaction effects.