Quake-risk buildings could reopen

04:19, Dec 03 2012

A raft of community facilities closed because they are considered earthquake-prone could be reopened if Christchurch City councillors agree to plans to rethink the danger threshold.

Since the quakes any council-owned building assessed as being below 34 per cent of the New Building Standard (NBS) has been immediately closed to the public because of the risk it could pose in another big shake.

But with the frequency and size of aftershocks diminishing, council staff are advocating a change in policy so that buildings below the 34 per cent threshold can remain in use if engineers can satisfy the council they pose no danger to the public.

Christchurch City Council community services general manager Michael Aitken said this morning the existing policy was put in place when the city was still being rocked by strong, frequent aftershocks so the council had taken a very conservative approach, opting to shut down any building that might potentially pose a risk. 

Now there was less seismic activity, staff felt the policy could be revised and some of the facilities lost to the community potentially reopened.

Information from GNS Science shows the probability of a large aftershock in the central Canterbury earthquake zone is dropping.


The frequency of aftershocks is now less than one third of that a year ago and the average maximum magnitude has dropped from about magnitude 5.5 to about magnitude 4.

"We think we can loosen up a little bit on things,'' Aitken said. 

The Department of Building and Housing, now part of the Ministry of Business Innovation and Employment (MoBIE) had revised its occupancy guidelines earlier in the year and the council's policy, even if the proposed revision was accepted, would still be taking a more conservative approach than it was recommending.

The council has been getting detailed engineering evaluations (DEEs) of its buildings as part of its Facilities Rebuild Plan.

The DEEs help the council to determine the level of damage a building has sustained in the earthquakes and its capacity to withstand future quakes.  

Based on the findings of the DEEs the council then makes a decison on whether a building should continue to be occupied. 

So far it has closed 161 buildings that have been identified as having a capacity of less than 34 per cent of NBS.

Under the changes being proposed by staff buildings that are between 17 per cent and 34 per cent of NBS but have not suffered any significant damage during the quakes and have no ''identifiable brittle collapse mechanism'' (or major structural weakness) could continue to be used if engineers attest to their safety.

''If council agrees to this we think we should be able to reopen some of the facilities that currently are closed. We're working on that the moment.

"We're asking all the engineers who have done the assessment of the buildings so far ... to go back and look at whether under the new criteria they could be safely occupied.

''We think there will be a few around the place that we will quickly will be able to get back into use,'' Aitken said.

The Press