Building & Construction Minister Nick Smith said on Friday design standards & building laws would be reviewed in response to an investigation into structural damage to Statistics House in Wellington in the Kaikoura earthquake on 14 November.
He released an independent panel’s findings into the performance of the building during the quake, which focused on its design & construction and the land influences on it.
The panel found a combination of 4 factors contributed to the partial failure of lower floor segments.
2 of the factors – the flexible frames & style of floor construction – combined with significant shaking for up to 120 seconds, and localised amplification of the shaking, to compromise the support of the lower precast concrete floor units.
The panel also noted that the combination of factors that led to the partial collapse of floor units in Statistics House wasn’t anticipated by the design standards in place when it was built in 2005.
The Ministry of Business, Innovation & Employment, the Institution of Professional Engineers (IPENZ) & the institution’s technical societies have produced information for owners & building professionals responsible for assessing & designing multi-storey concrete moment-resisting frame buildings with precast concrete floor systems that may be vulnerable to loss of floor support during an earthquake.
Dr Smith said: “The performance of Statistics House in the Kaikoura earthquake was unacceptable and could have caused fatalities. This quake was large & unusually long, but a modern building like Statistics House should not have had life-threatening structural damage. The building was designed to the industry practice of the time, but this did not fully account for the effects of beam elongation during an earthquake, an issue that was deficient in the concrete structures standard at the time of the design.
“The design flaw is quite specific to highly ductile framed concrete buildings with precast floor slabs, and particularly those with multi-bay frames. We need to follow up on similarly designed buildings through councils & engineering companies so that where it is a problem, it can be rectified. This has already been done in respect of Wellington as a consequence of the preliminary findings in Statistics House, but now needs to be followed up elsewhere.
“We also need to amend the concrete structures standard to ensure newly designed buildings are adequately designed to cope with beam elongation during long-duration earthquakes. This will be done this year.
“A compounding factor was geological basin effects that are not well understood but which have also been observed in other earthquakes internationally. This is not to do with reclaimed land but the amplification of ground shaking in a basin. This phenomenon is similar to the way sea waves respond to a wall in an enclosed bay. This is an area of seismic science that needs further research, particularly in respect of Wellington, and to be considered as part of a review of the earthquake actions standard.
Building law issue
“There is a building law issue that arises from this report on which I have asked officials to report. The Ministry of Business, Innovation & Employment has limited powers to follow up on design deficiencies like those identified in this report, beyond those specifically provided for following civil emergencies. This means the ministry cannot require building owners to follow up on these sorts of potentially serious technical problems. I have asked the ministry to report on whether additional powers are needed in the Building Act.”
Dr Smith said New Zealand was at the cutting edge of international seismic design standards, but hadn’t yet solved all of the potential ways a building can fail: “Most buildings in Wellington performed well despite the ferocity of the Kaikoura earthquake. We need to take the opportunity following such earthquakes to learn as much as we can and to further strengthen our standards & systems to improve building safety for the future.
“These detailed issues over the performance of modern buildings are important for improving design standards, but they should not divert attention away from the far more significant risk to life of older buildings. The Kaikoura earthquake was sufficiently distant from Wellington that the city did not get the dangerous high frequency shaking that poses the greatest risk to life.
“The largest safety gains for Wellington are to be made in the initiatives requiring unreinforced masonry facades & parapets to be tied back over the next year, and all earthquake-prone buildings under 34% of Building Code to be upgraded under the new law coming into effect on 1 July.”
Attribution: Ministry website & ministerial release.