It has emerged the seismic assessment that Hutt Hospital relied on for 10 years used a non-standard method that reduced the earthquake effects on a tower block since found to be quake-prone.
Photo: RNZ / Samuel Rillstone
The method gave the building a better score.
The Hutt Valley District Health Board accepted the method, a US one called FEMA 440, to model the Hutt's soft soils in an earthquake, without getting the work checked by another engineer.
The council did the same last year when it OKed the building, on the basis of the old 2011 assessment, without a peer review.
The building regulator, the Ministry of Business, Innovation and Employment, said the FEMA method did not provide a means to reduce seismic loads.
But the 2011 assessment said: "The use of the FEMA 440 analysis results in a further 7 percent reduction of the design seismic loads" on part of the tower.
Reducing the load had the effect of giving the building a better score.
It was rated 43 percent of New Buildings Standard (NBS) in 2011, and approved on this basis last year by the council, which had asked the DHB in 2019 if it was an earthquake-prone structure (under 34 percent NBS). If it had been, a deadline to strengthen it would have kicked in earlier.
The block rates just 15 percent now. The stairs, floors, columns and beams are particular weak points, a new DSA by the same engineer, Aurecon, as did the 2011 DSA, said.
As a building's NBS drops, the chances of harm to people in a significant quake rise.
Sometimes minor elements can drag down an NBS - but in Heretaunga's case the weaknesses are major and structural.
The ministry would not comment on the Heretaunga specifically. Nonetheless, it told RNZ that methods such as FEMA 440 are "more complex and typically … would be subject to peer review".
Neither the DHB nor council got a peer review done last year of the 2011 assessment.
This was despite the 2011 DSA not reflecting quake research which had advanced markedly, especially since 2016; and though the tower block was designed prior to modern seismic designs being introduced in 1976. WorkSafe instructs building owners they must stay on top of new and emerging earthquake information.
It was also despite the block's importance - accounting for a quarter of all physical capacity across the Wellington-Hutt-Kapiti area; and on the cusp of being an Importance Level 4 building, and not a level three (IL3). Had it been treated as an IL4, the seismic load that had to be accounted for in any assessment would have jumped by 40 percent.
It was also despite the word "collapse" occurring in the DSA. It warned the seven-storey tower had weak columns and strong beams - the opposite of what modern seismic design demands.
"This is an extremely undesirable failure mechanism as it can lead to hinges forming at the top and bottom of the columns which can lead to collapse of the floors," it said. "The nature of the failure mechanisms should be considered."
"Collapse" is a red-flag word in a seismic assessment.
The columns-beams weakness features in the new 2022 DSA.
Questioned by RNZ on whether they had been careful enough, both the DHB and council defended not getting a peer review done.
The council said legally, it did not have to get a peer review done - that this was up to the building owner. Ironically, prior to law changes in 2017, the council had a policy to get a peer review done itself.
The DHB said its management of buildings was "planned and systematic" throughout.
It says the 2017 changes were to guide how assessments were done, not when they were done.
MBIE told RNZ that FEMA 440 "explores different analysis procedures for existing buildings but is not an assessment methodology like the Seismic Assessment of Existing Buildings 2017".
"It does not provide a means to 'reduce loads'," the ministry said.
RNZ asked Aurecon if it was usual for it to use FEMA 440, or similar international methods, or to do so without seeking peer review of them. Aurecon declined to comment, citing client confidentiality.
Its 2011 assessment, explaining the use of FEMA 440, says: "Where this assessment differs from a 'standard' seismic assessment has been in the modelling of the foundation system ... This is particularly useful for this building due to its location on soft soils."
The 2022 assessment rates the foundations at just 20 percent NBS and at risk from liquefaction. The 2011 DSA does not mention Heretaunga facing a liquefaction risk.
Society of Structural Engineers vice president Nicholas Brooke would not comment on the Heretaunga block specifically but said a seismic assessment done in 2011 would not be as good as one done now.
The process was "more thorough and comprehensive" now, though many old assessments were good in their time, he said.
It was up to engineers if they wanted to use international methods, he said, though he added FEMA 440 was "a reasonably old document", having been developed in 2005 and "wouldn't be the document I would go to these days".
Using such a method on soils was less of a worry than applying one to structures, Brooke said.
Ministry guidelines say "the work has not been done" to correlate the likes of FEMA 440 to local seismic assessment guidelines.