Friday, November 6, 2015
Mike Butler: Opera House could be used as-is
Mr Harrison, who is chair of Evidence Based Seismic Strengthening, addressed more than 70 people at the Havelock North Function Centre.
He said that any decision on the future of the Opera House must take into consideration the legal position, the current risk of the building, and the costs and benefits of strengthening.
Those attending were either friends of the Opera House or ratepayers concerned at a projected $20-million strengthening bill.
An assessment by earthquake engineers that the building is earthquake-prone because it is less than 34 percent of new building standard has nothing to do with what the law says, Mr Harrison said.
Legally, a building is earthquake-prone if (a) it would have its ultimate capacity exceeded in a moderate earthquake, and (b) would be likely to collapse causing injury or death or damage to other property, he said.
A moderate earthquake is defined as one that would generate shaking that is of the same duration but is one-third as strong as that used to design a new building.
The probability of collapse in a moderate earthquake is very low, or around 1:40,000, he said.
Mr Harrison explained how life safety risk was calculated and explained that an individual who attends a given number of performances over a year would be killed, on average, once every 10 million years.
Compared with the risk of driving, if you take one three-hour car journey each year you can expect to die every 1.5 million years.
“The riskiest part of attending a performance is the car journey,” he said.
He also explained how cost-benefit was calculated and concluded that spending around 6.8-million to strengthen the building to around 35 percent of new building standard would bring a benefit of $19,700.
“In other words, an investment of $100 has a return of 30 cents”, he said.
There is no need for a strengthening decision to be made soon because a test case going through court could make a difference and the practical outcomes of legislation are not locked in concrete, he said.
Because the New Zealand Society of Earthquake Engineers’ recommendation lacks a risk assessment, and because the recommendation brings a very limited reduction in risk, it is questionable whether the building should be strengthened as recommended, he said.
at 9:02 PM