3:06 PM, 29 June 2011

Rebuilding Christchurch: The Role of the Steel Construction Industry

Commentary from the Manager of Steel Construction New Zealand, Alistair Fussell

In just over a decade, the capacity of New Zealand’s steel construction sector has doubled thanks to significant investment in new fabrication technology and workshops. This technology has increased not only productivity, but also the quality and precision of the fabricated product.

These features, combined with the material’s many other attributes, mean the steel construction industry is well-placed to play a significant role in the rebuild of Christchurch. In fact, the work has already begun: In the immediate aftermath of the Canterbury earthquakes, the local steel construction industry was busy helping to stabilise unsafe buildings.

Steel structures have, on the whole, performed very well in both earthquakes. A surprising aspect has been the resilience of eccentrically braced frame construction (EBF) when cast integrally with concrete slabs. The two tallest examples in Christchurch, the 22-storey Pacific Residential Tower and the 12-storey HSBC Tower, have suffered only minor seismic damage and have both been passed fit for reoccupation without requiring structural repair to the seismic load resisting systems. In fact, the HSBC Tower, according to newspaper and television news items, is the first central city high rise building to be reoccupied after the February 22nd Lyttleton earthquake with tenants JBWere returning to work in the building on Monday 30th May 2011.

So unexpected was this high performance that the University of Auckland has proposed a research project into the beneficial effect of the concrete slab on the performance of eccentrically braced frames, and the overall post-earthquake building displacements. The good seismic performance of the steel structures in Christchurch is a credit to the expertise of the structural engineers involved and the quality workmanship of local fabricators.

That said, one of the unfortunate outcomes of the recent earthquakes has been the loss of confidence in multi-level construction with many calls, particularly from the non-engineering community, to limit the rebuild to four storeys. While this approach may allay psychological concerns over multi-level construction, it does not address the economic damage caused by earthquakes. What is required is a paradigm shift in terms of building design philosophy.

The traditional approach to seismic design, known as ductile design, has been to engineer buildings for controlled damage during a major earthquake. Ductile design’s sole aim is to protect lives and, admirably, it has contributed to saving many. Its inability to minimise structural damage, however, has resulted in significant economic loss.

If there is to be any silver lining in a very dark cloud, it is that the high cost of the earthquake-induced building damage will drive widespread uptake of new low damage seismic-resisting technology. These systems can withstand major earthquakes and require no major post-earthquake repair. Several good examples of smart low damage structural steel systems such as sliding hinge joints for steel moment resisting frames and rocking steel frames have been developed in New Zealand by researchers such as Dr Charles Clifton and Wellington based Structural Engineers Aurecon.

Over $2.5 billon of new steel structures built in the last few years in New Zealand use this technology. Notably, low damage seismic-resisting technology does not come at a significant cost premium. Take the award-winning Te Puni Village, Wellington, for example, which won a prestigious Institution of Structural Engineers award in 2009. The additional cost of applying low damage systems in lieu of a conventional approach was just over 0.5% of the total building cost.

As a result of this paltry premium the Victoria University has a building that can be used as a post-earthquake emergency site in the event of a nearby fault rupturing. Multiple research programmes into low damage steel-framed seismic-resisting systems are currently underway, or in the pipeline, at Auckland and Canterbury universities.

Consequently, new systems will emerge to complement the existing sliding hinge joint and rocking frame systems used on Te Puni Village. On that note, we join with the chorus of voices advocating that the choice of building materials for the rebuild of Christchurch be left to the technical advisors to the Canterbury Earthquake Recovery Authority (CERA) and the building owners.

There will clearly be opportunities for all building materials; crucially, however, the choice of construction material for any given project should be based on its technical and economic merits.

It is predicted that the Christchurch rebuild will be a very slow process, and our thoughts are with the city’s people who have suffered such devastating human and economic loss. Yet, while this will plainly be frustrating for many, the upside is that a slow and considered rebuild will allow decisions to be made based on solid facts, not emotions.

For a more detailed description of the performance of steel structures in the recent Christchurch earthquake, visit the New Zealand Society for Earthquake Engineering web site. SCNZ will, in due course, disseminate some of the design lessons learnt from these recent Canterbury events.

Source: Steel Construction NZ ( SCNZ)