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Engineering issues arising from earthquakes

By: Pam Graham, NZPA
23 March, 2011

American forensic engineer David Biggs knew the magnitude 6.3 earthquake in Christchurch on February 22 had changed public perceptions of engineering issues when a New York taxi driver asked his wife to quiz him about liquefaction.

Biggs had just finished presenting a seminar on seismic assessment and improvement of building foundations in a room on the third floor of the Holiday Inn Hotel in Christchurch when the earthquake struck.

The engineer, who helped assess the damage caused by the World Trade Centre tower collapses considered it a "luck of scheduling" that he was able to observe the impact of the earthquake and help assess buildings in its aftermath.

Buildings sustained damage from liquefaction and from shaking, or design overload, and an issue for investigators who will be separating the two.

"The question is why didn't more collapse?" Biggs said.

The shaking was twice the design level for new buildings. Heritage buildings were commonly strengthened to 33 percent of code. It had been decided after the magnitude 7.1 earthquake in September to strengthen them to 67 percent, though this had not happened by February.

His observations on the website of the American Society of Civil Engineers were both personal and with the eye of a masonry expert who lectures on building restoration.

He saw an unreinforced masonry building already damaged and braced after the September earthquake sway about a metre side to side and was amazed that the bracing broke but the roof did not collapse. The streets and parks filled with sand volcanoes.

Biggs said in the US he had spent hours explaining to clients why they must spend more on building foundations to counter liquefaction, which tends to occur in pockets in US cities, rather than across wide areas as in Christchurch.

He was astonished when he was talking to his wife in New York when her taxi driver said "ask him about the liquefaction".

"Imagine a New York taxi driver asking about liquefaction. These are terms engineers use in conversation but Christchurch has made it a worldwide term," he said.

He believes that liquefaction may have been a major contributor to the collapse of the Pyne Gould Corp building where many people died.

Biggs was staying on the 20th floor of the Hotel Grand Chancellor the night before the earthquake. He said a letter from management in his room stated that the building performed as expected in the September earthquake. That earthquake produced accelerations of just 20 percent of design.

"Every earthquake has a signature," he said.

He did not endorse calls for more and higher wooden buildings. He said people prefered non-combustible buildings.

You could not earthquake proof any building, you could only make it earthquake resistant, he said.

After the Christchurch earthquake people may ask more questions about the engineering of buildings and clients may be more willing to invest in stronger buildings.

"Do you want to go to a doctor on the fifth floor of an old building," he said. He believed cities would publish more information about the strength of buildings and people will seek it out.

Technology was getting better and it was easier to require higher engineering standards if the playing field was level for everyone, he said. Liquefaction could be countered by deeper foundations and better drainage systems but it may not be economic for all buildings.

"I take the position that technically anything is possible but it has to be done as a community," he said.

There are no easy answers. One heritage building Biggs had consulted in in the US needed strengthening that would cost many times the building's value.

"It is an iconic building -- how can you let it go? Is there a next best thing?"

If you can't bring a building up to standard you can try to replicate it but replica buildings are a "bit like Disneyland".

That means being selective about which buildings are saved. He said the issue of whether people will move back into some buildings, particularly high rise buildings, was a personal one.

"I expect the public will be wary of multi-storey buildings and unreinfroced masonry," he said.

Source: AAP

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