24 Sep 2015

Bridge plunge blamed on manufacturing fault

8:00 pm on 24 September 2015

A manufacturing fault is being blamed for causing a suspension bridge to break, which sent four French tourists plunging 8m into a river on one of the country's Great Walks.

Hopuruahine swing bridge.

Photo: SUPPLIED / kiwiontheat

A cable on the Hopuruahine Bridge on the Lake Waikaremoana track in northern Hawke's Bay, snapped earlier this month.

Department of Conservation operations director Mike Slater said it broke due to a manufacturing fault in one of the links.

Its engineering report, by the University of Canterbury's Department of Mechanical Engineering, revealed there was a defect in a section of the high-tensile steel chain used in the construction of the 65m long bridge in 1996.

The tourists escaped without serious injury.

Mr Slater said all other bridges had been checked.

"We've got no evidence to suggest that any faulty chain has been used in other bridges and we have checked all of the 111 bridges that we have which also use high-tensile chain," he said.

"We've found no bridges where the lengths of that same chain size have been used."

Mr Slater said only electro-microscopic investigations by a laboratory could have picked up the fault.

The report said the ultimate design load for the chain on the bridge's 10-person load restriction was about 8 tonnes.

Report engineering manager Jon Calder said he believed the "particular loading at the time resulted in catastrophic failure".

However, Mr Slater said the bridge was designed for a loading of that weight.

"We established that a 10-person loading was well within those design paramaters and our design was designed to ensure there was a high safety margin."

Mr Slater said the chain was sourced locally, and DOC had confidence that particular batch of chain was not used anywhere else in the network.

"We have gone back and advised the supplier of our findings, the supplier will be talking to the manufacturer. We have also advised the relevant authorities of our findings, including WorkSafe."

It is up to those organisations which now had the information to consider if it was appropriate to check other bridges beyond those built for DOC.