Steel structures performed well in the earthquakes, writes ALISTAIR FUSSELL.
The recent reoccupation of the HSBC Tower by JBWere is one of many important milestones that Christchurch will achieve on its reinvention journey.
The move is a real vote of confidence - not just in psychological terms for central-city workers, but also in terms of the technology behind high-rise buildings.
On the whole, steel structures performed very well in both the September 2010 and February 2011 earthquakes.
The two tallest examples in Christchurch, the HSBC Tower and the Pacific Residential Tower, suffered only minor seismic damage and both have been passed fit for reoccupation without requiring structural repair.
This pleasing performance is a credit to the expertise of the structural engineers involved, and to the quality workmanship of New Zealand steel fabricators.
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.
But despite the good performance of steel, and the fabricating industry behind it, one of the outcomes of the recent earthquakes has been the loss of confidence in multi-level construction, with many calls to limit replacement buildings to a height of four storeys.
While this approach might allay psychological concerns over multi-level construction, it does not address the economic damage caused by earthquakes.
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 as evidenced by the number of badly damaged buildings in Canterbury that now need to be demolished.
What is required is a paradigm shift in terms of building-design philosophy. The high cost of 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.
Good examples of smart low-damage structural steel systems have been developed in New Zealand by researchers such as Dr Charles Clifton, of the University of Auckland. More than $2.5 billion of steel structures built in the past few years in New Zealand use this technology.
Low-damage seismic-resisting technology does not come at a significant cost premium. In a recent project the additional cost of applying low-damage systems in lieu of a conventional approach was just over 0.5 per cent of the total building cost.
Research programmes into low-damage, steel-framed, seismic-resisting systems are under way, 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.
Sustainability has been a common thread throughout the rebuild conversation thus far and, in our view, steel is the ultimate sustainable building material.
It is manufactured in factories under safe conditions, made and tested to precise specifications to minimise waste, and is quick to erect. Made from the most abundant element on earth, iron, steel can be recycled or reused endlessly without detriment to its properties.
The potential for steel to be reused and adapted was evident when the stadium used at the Aquatic Centre for the Sydney Olympic Games was moved to Wollongong. The grandstand was disassembled and taken to Win Stadium, where it was refabricated, rewelded and reassembled.
Steel's superior strength-to-weight ratio means a little steel goes a long way, giving architects complete flexibility to achieve their most ambitious designs. Pre-engineered steel components are delivered to sites ready for rapid assembly by relatively few workers, with no waste.
It is likely the Christchurch rebuild will be a very slow and considered process, and it needs to be.
The choice of building materials for the rebuild of Christchurch should be left to the technical advisers to the Canterbury Earthquake Recovery Authority (Cera) and to building owners.
It's important to note that we in the steel industry are not advocating for a steel-only rebuild. There will 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, and not on emotions.
We in the steel industry, like all Kiwis, look forward to seeing the vision for a new Christchurch emerge from this disaster, and to playing our part in bringing it to life.
* Alistair Fussell is a structural engineer and manager of Steel Construction New Zealand.
- © Fairfax NZ News
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