The engineering firm whose report spurred workers to leave a Wellington multi-storey building warns that designing for earthquakes may get harder.
The Ministry of Education building in central Wellington was emptied out of staff a month ago due to a much lower New Building Standard rating. Photo: Ministry of Education
The Ministry of Education moved out of its head office in Bowen Street a month ago, saying its engineers Aurecon rated it at 25 percent of New Building Standard (NBS) due to the concrete floors.
Aurecon said in its targeted detailed seismic assessment (DSA) that the latest science, including on how the harbour basin amplified shaking for mid-rise structures during the 2016 Kaikōura quake, may end up raising the bar for many buildings.
"A future increase in the hazard factor may lead to an increase in the design level for new buildings in Wellington and potentially may increase the standard required for existing buildings," it said.
It comes on top of shifts in the seismic goalposts in recent years.
Just seven years ago, engineers rated the 12-storey Mātauranga House - where about 1000 ministry staff used to work pre-Covid-19 - as 90-100 percent NBS following strengthening work.
The new "targeted" DSA, done in March for the ministry, focused on the floors because of weaknesses in some designs exposed by the Kaikōura shake.
The "methodology of assessing and strengthening existing buildings has changed significantly", the new assessment said.
The Aurecon assessment recommended the owners get a full DSA done, investigate the soil conditions under the building and then strengthen it to at least 67 percent NBS.
The building owner, Wellington Investment Group, said it expected to have a full assessment completed by engineers Beca by August.
Aurecon's reference to the "hazard factor" possibly going up, related to the new National Seismic Hazard Model due out later this year from GNS Science, and the subduction zone around the Wellington region.
The model is expected to raise the hazard level in some areas.
The earthquake actions design code - called NZS1170.5, and put out in 2004 then revised after the Kaikōura quake - does not factor in the incoming hazard model.
The code also might need updating to take into account how the "basin edge effects" on soft soils in the sedimentary basin of Wellington harbour accelerated ground movement in the 2016 quake, Aurecon said.
Both the hazard factor and basin effects were still being looked into "with no fixed timeframe".
The assessment used an updated methodology that "provides clear and concise methodology for the assessment of precast floor systems" to assess the floors at Bowen St.
However, the previous methodology was the one that counted, legally, in determining if a building was at risk of earthquakes.
Engineers have protested to the government that having two assessment methods in play was confusing and risky.