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Progressive Building (Oct / Nov 11) Scene Set for Damage Avoidance Design Systems

01 Nov 2011

Rob Gaimster
Cement & Concrete Association (CCANZ)

Chief Executive of the Cement & Concrete Association of New Zealand (CCANZ) Rob Gaimster outlines how new approaches to seismic structural design for multi-storey concrete buildings are assured options to rebuild a resilient Christchurch city centre.

Before the events of 22 February 2011 the heart of Christchurch stood proud as one of this country’s most unique cities. Public and commercial buildings that embodied the architecture of their time combined with the distinctive grid street system to form an urban landscape seen nowhere else in New Zealand.

Through the adoption of innovative structural design systems and sympathetic urban design principles there is no reason why the city cannot surpass its former standing.
 
Recent advances in “damage avoidance design” using concrete systems, specifically PREcast Seismic Structural Systems (PRESSS) and base isolation, will play an important role in making possible strong and resilient buildings that exceed current New Zealand Building Code seismic requirements, and in turn help produce a high-quality built environment for the central city of Christchurch.

The vast majority of concrete buildings performed well during the 22 February 2011 earthquake. The recently released results from the Department of Building and Housing technical investigation into the Pyne Gould Corporation (PGC), Forsyth Barr and Hotel Grand Chancellor buildings concluded that these buildings failed principally because 22 February was an extremely violent earthquake and that they were built to design requirements not as rigorous as today.

While the Department’s investigation into the failure of the Canterbury Television Building (CTV) continues, and will feed into the work of the Royal Commission, it is anticipated that a likely outcome will be amendments to the best-practice requirements for the design, construction and maintenance of buildings in central business districts throughout New Zealand.

It is into this space that PRESSS and base isolation, as examples of damage avoidance design using concrete systems, will emerge to meet what will inevitably become more demanding seismic code specifications.

Both these systems have been proven in practice during the Canterbury earthquakes, with the Southern Cross Hospital Endoscopy Building, an example of PRESSS, and Christchurch Women’s Hospital, an example of base isolation, both unscathed and immediately available for use as medical facilities following the 22 February quake.

PRESSS and base isolation represent an assured way forward for multi-storey construction in seismic prone regions such as Canterbury. By enabling rapid and cost-effective construction, while also delivering optimal seismic performance that minimises business interruption in the event of an earthquake, these systems will play a crucial role in allowing the Christchurch central city to fulfil its social and environmental objectives. Economic goals will also become more achievable through the commercial viability of multi-storey construction.

PRESSS uses un-bonded post-tensioning cables and rocking joints within a precast wall, beam and column structure to ensure the building returns to upright without significant structural damage, even after a major seismic event. 

Specifically, the PRESSS system reduces movement under small earthquakes through its high, pre-rocking stiffness. For larger seismic events potential damage is concentrated towards replaceable energy dissipaters at the column/beam joint or where shear walls meet the ground, as the post-tensioned cables act to clamp these joints closed and spring the building back to vertical.
 
Along with the Southern Cross Hospital Endoscopy Building in Christchurch, the Alan MacDiarmid building at Victoria University of Wellington is New Zealand’s other concrete PRESSS example.

Developed in New Zealand during the 1970s, base isolation has proved successful elsewhere around the world. Designed to minimise damage during earthquakes, the systems utilises large lead-rubber bearings, or base isolators, which separate the building from the ground, offering controlled rocking that absorbs displacement.

Although not suitable for all types of structures and designed for hard soil, base isolation already protects many significant buildings in Wellington, such as Te Papa, the new Wellington Hospital, as well as the retrofitted Parliament Buildings.

It has recently been reported that the New Zealand Society for Earthquake Engineering endorses both PRESSS and base isolation as effective in mitigating the damage caused by earthquakes. The Society believes they should be used, where appropriate, in all new buildings in New Zealand, and note they are quick and cost-effective systems.

Such support is complemented by damage avoidance design development work, in the area of non-tearing joints, at the University of Canterbury.

Having already been put to the test in Christchurch, damage avoidance design using concrete systems must be given serious consideration in central city rebuilding efforts as offering enhanced levels of protection to buildings and their occupants.

Article appeared in Progressive Building (Oct / Nov 2011).