Seismic Resurrection of 13-storey Apartment Building at 66 Oxford Terrace, Christchurch

Abstract

This precast concrete apartment building at 66 Oxford Terrace was completed in 2005 and has considerable architectural merit. The Engineers employed by the Body Corporate post the CES of 2010 / 2011 discovered that the building was not up to Code when designed in the early 2000’s. Structural shortcomings are listed below. The negotiations between the Body Corporate and the insurers became more complex when the original designers were also joined in the legal action. The building stood empty until finally purchased “as is, where is” in 2020.

The main structural shortcomings were:

a. Detailing which fell far short of providing the assumed ductility factor of 5. b. An unintended nutcracker effect between the ground floor and basement c. The building has generous walls to resist N/S earthquakes but the detailing of the east and west walls was inadequate in terms of ductility. d. Under E/W earthquakes, the building has far less going for it, essentially a 6m deep box of stairwell walls cantilevering 40m, which were compromised by door openings at lower levels. e. In the N/S direction there are a number of “non-structural” walls. These are 180mm thick and have a single layer of reinforcing. This type of wall lacks confinement and ductility.

Essentially our intention was to base isolate this building. New shear walls have been poured against the existing walls from basement to Level 3. Energy absorbing devices have been built into these walls at ground floor level. The existing walls will be saw cut at just above ground floor so that the building can rock. The base isolation has been tuned firstly so that the building is above 100% NBS, and secondly so that the walls above Level 3 will only be loaded to their current capacity and will require little repair. During construction several grout sleeves were extracted for testing. The results were underwhelming to the point we concluded they could only be regarded as 45% reliable. To address this late emerging shortcoming vertical post tensioning was introduced to the four “corner” columns and the stairwell wall. We are introducing a new shear wall to bolster the E/W capacity in the lower floors. The non-structural walls have been rebuilt at ground floor 250mm thick with adequate confinement, and hinges will enable the walls to rotate whilst carrying virtually no seismic load. This system results in some high uplift forces. A composite raft has been formed by pouring a new raft between existing foundations.