A structural engineer who helped trigger investigations into seismically flawed building designs has revealed dozens of common but serious mistakes he is finding.

Photo: 123rf

Auckland engineer Gordon Hughes recently decided to speak up, spurred on by what he found while investigating six defective buildings in Masterton for the government three years ago.

He cited consequences of design faults, including in a 60m transmission tower that snapped in half.

"This is a painful, costly - financially and emotionally, that is - but a very powerful learning. And this is the first time I've actually shared that in public," Mr Hughes said in an hour-long presentation to engineers in Auckland earlier this month.

Mr Hughes has compiled a long list of errors discovered in reviewing more than 50 buildings.

He has been aware for some time of what Engineering New Zealand has only recently found out from its investigation - that basic design mistakes are being made, and slipping through.

"In the design of most of these buildings, it surprised me that they already had some sort of a review." Mr Hughes told the engineers. He said some had internal reviews of their engineering design, others "had high-level reviews, and from some big-name firms, but they've all missed these issues".

Mr Hughes, who would not speak to RNZ directly, told the engineers he found multiple serious mistakes in more than half the buildings he'd looked at.

Most were in buildings a few storeys tall with steel portal frames and concrete panels, and built in the last decade. Mr Hughes would not identify the buildings, concerned he might be sued.

It was "very common" for him to find a mismatch between how flexible a building was meant to be (called ductility) and the actual design details; also, to find earthquake loads were underestimated, or their support paths down to the foundations were incomplete, or a building's "eccentricities" had not been allowed for.

Among the dozens of examples Mr Hughes gave were:

"The wall transom or the upper beam, not designed for restraints - it was coming in something like 15 to 55 percent of what it should have been."

"Steel portal frames - not enough fly braces or resistance to buckling. We figured that the strength of those frames varied from 26 to 60 percent. This is a building not even 10 years old."

"Selected ductility three but failed to follow through on the detailing."

Mr Hughes recalled being asked 18 months ago to look at damage to an almost 60m communication tower after a small storm. He does not specify the location.

"The top part was this mess on the ground. My first impression was, 'Look at those bolts on that flange - they look tiny, they look very widely spaced." They were, and the repair meant adding many more bolts and stiffeners to the tower, he said.

Mr Hughes said engineers had to start learning from each other's mistakes instead of trying to hide them for fear of liability, or they would keep repeating the errors.

"Many of the mistakes are the same similar, even though they've been designed by different engineers from different firms."

Engineering New Zealand's three-year-long reviewis about to make public.

From this December, the Ministry of Business Innovation and Employment will have new powers to investigate building failures to determine if there are systemic failures.

RNZ last week reported on seismically-flawed designs found in eight buildings in Palmerston North, related to the six buildings Mr Hughes investigated in Masterton. Building and Construction Minister Jenny Salesa has said there was no evidence from that case of a systemic problem as they were all designed by a single firm.