Seismic lessons

Derek and Lynn Anderson still remember the swaying. Not a familiar restful motion but a relentless oscillation as Christchurch’s September 4 earthquake grasped their modern eighth-floor city apartment.

For 30 seconds, the couple huddled in a doorway as their solid and much-loved home in the eight-year-old Park Terrace apartments became a small lifeboat tossed by seismic waves.

“My first thought when I woke was how much the building was swaying and creaking. We’d just returned from Australia where we’d been in a boat in rough seas off the Great Barrier Reef. So we’d had some practice,” Lynn Anderson says.

“All we could do was ride it out before walking down 160 steps to the ground floor. That was the most difficult experience.”

A month later Derek Anderson views their experience of a 7.1 earthquake pragmatically.

“The swaying is what modern buildings are supposed to do in an earthquake. I’ve always said that no-one is going to be killed in a building designed to handle an earthquake. You’re more likely to be killed by a car around the corner. The only damage to the apartments was cracked gib-board, which is exactly what was supposed to happen.”

Civil engineer John Hare supports this perspective with a simple analogy.

“Modern buildings are designed to bend like a piece of wire rather than breaking like a stick,” the director of the city’s largest structural and civil engineering firm, Holmes Consulting Group, says.

“It was doing exactly what it was designed to.”

Hare and his colleagues are extremely busy people these days. Their work began at dawn on September 4 and shows few signs of easing. These are challenging and, as some engineers quietly admit, exciting days for their profession. While attention has focused on the damage to the city’s older buildings and private homes, the effect on the city’s modern commercial buildings was less dramatic but nevertheless holds important lessons for the future.

Hare says there have been several different stages of earthquake design over the years.

“Essentially, earthquake design in New Zealand began in 1965. In 1976, it was brought into the modern era with research at the University of Canterbury with an approach which concentrated on saving lives. Buildings would fail but in a very controlled way,” Hare says.

“There was an acceptance and growing realisation that you couldn’t make buildings indefinitely stronger because there would always be a stronger earthquake. It would be impractical to simply build extremely strong buildings – they must also have the ability to yield, flex and move in an earthquake. Many, but not all, of Christchurch’s taller buildings have been designed in this way and most handled the earthquake well.”

There were still exceptions to the rule – the incidences where buildings sustained more damage than was expected by the architects and engineers, according to Christchurch branch of the New Zealand Institute of Architects chairman Jasper van der Lingen.

“I’m aware of a high-rise building where there were cracks on the seventh floor shear wall – not fatal but certainly more than was expected. What the engineers suspect is that this was possibly caused by the resonance occurring throughout the building during the tremor; an effect which affected some floors more than others.”

The most visible damage to a contemporary building occurred in the Westpac Canterbury Centre at the intersection of Cashel and High streets. Designed by Warren and Mahoney in 1985, the decorative external columns and beams at each side of the six-sided building were connected by what is, in lay terms, an oversized mortise and tenon joint.

While the building’s internal structure behaved perfectly during the earthquake, these strong joints moved up and down, cracking and eventually spalling the concrete.

“It did what it was designed to do 30 years ago when the tools to predict a building’s behaviour were not as sophisticated as today’s technology,” Hare says.

“The September 4 earthquake taught us lessons which are still emerging as the engineers provide material which will be studied for years. This is an interesting opportunity to see how what was an accepted standard two decades ago has coped. The size of the September 4 earthquake and the level of ground displacement were not up to the design-code levels of these buildings. All those buildings designed since 1976 have not yet been fully tested. We experienced a big earthquake, but not a huge one where shaking might last up to 45 to 60 seconds.”

The 1929 Murchison and the 1931 Hawke’s Bay earthquakes profoundly shaped New Zealand’s perception of the hazards of living in a seismically active environment.

Attention was focused on weaknesses in building construction, especially poor building standards and the lack of any provision for earthquake-resistant design.

This led to a draft by-law in 1931, which was incorporated into a building code in 1935. Building codes in 1965, 1976, 1984 and 1992 have added requirements to accommodate changes in building materials and design. Rather than prescribing specific materials, designs or construction methods, the 1992 New Zealand code outlines how a building must perform to withstand the forces expected during an earthquake.

This allows builders to use innovative design and construction methods to create earthquake-resistant buildings. For a moderate earthquake, the main aim is to protect a building from structural damage. For a major earthquake, however, the goal is to protect life by ensuring a building will not collapse and people can escape from it, even if the building itself is badly damaged.

Andy Buchanan, professor of civil engineering at the University of Canterbury, says Christchurch’s contemporary buildings still face the possible test of an even larger earthquake at some time.

“September 4 was a major earthquake but the level of shaking which occurred was comparatively moderate. Its duration was also less than we expected. Buildings were subjected to high forces but most only shook once or twice, which is what they were designed to do. If the tremor had been twice as long, there would have certainly been more damage but they would not have fallen down around our ears.

The full text and other local earthquake news can be found at The Press.