The Statistics House investigation has revealed that buildings with flexible frames in combination with precast floors may be vulnerable in an earthquake that’s both large and long – Photo courtesy of MBIE
Statistics House investigation findings released
Design standards and building laws will be reviewed in response to an investigation into structural damage to Wellington’s Statistics House following the Kaikoura earthquake.
Statistics House is a six-storey reinforced concrete office building in the CentrePort Harbour Quays business park and was built in 2005. As a result of the magnitude 7.8 Kaikoura earthquake on 14 November 2016, three precast concrete floor units came off their seating and fell onto the floors below.
The Ministry of Business, Innovation and Employment (MBIE) commissioned an independent expert investigation into the performance of Statistics House in the earthquake in order to understand implications for the building regulatory system, and the report was released on 31 March by Building and Construction Minister Dr Nick Smith.
The report focuses on the design and construction of Statistics House and the land influences on it. It found a combination of four factors contributed to the partial failure of lower floor segments. Two of the factors – the building’s flexible frames and style of floor construction – combined with significant shaking for up to 120 seconds, and the amplification of the shaking by the geological basin beneath Wellington (known as ‘basin-edge effects’) compromised the precast floor units.
“The performance of Statistics House in the Kaikoura earthquake was unacceptable and could have caused fatalities. This quake was large and unusually long, but a modern building like Statistics House should not have had life-threatening structural damage,” says Dr Smith.
The Kaikoura earthquake was a highly complex earthquake and is challenging existing seismic hazard models. More than twelve separate faults broke during the quake, including some that had not previously been mapped, and the frequency of shaking that arrived in Wellington 240 km from the epicentre mostly affected mid-rise Wellington buildings.
Dr Smith says Statistics House 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.
“This design flaw is quite specific to highly ductile concrete frame buildings with precast floor slabs and particularly those with multi-bay frames. We need to follow up on similarly designed buildings through councils and 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,” he adds.
Since the report’s release, MBIE has contacted the Hutt City, Upper Hutt and Porirua City Councils advising that they contact owners of buildings with similar structural characteristics to Statistics House and requesting that they carry out damage assessments like what has been undertaken in Wellington City.
“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,” Dr Smith says.
Structural engineers say results from the Statistics House investigation will help build more resilient buildings. “Investigations like this are crucial to making our buildings safer,” says New Zealand Society for Earthquake Engineering (NZSEE) president Peter Smith.
“Statistics House illustrates how different factors can combine in unexpected and unprecedented ways. Because the earthquake was so far away, only low-frequency waves made it to the capital. These waves resonated with mid-height buildings which, combined with the earthquake’s long duration, meant mid-height buildings experienced severe shaking,” he explains.
“Basically, these flexible, modern buildings experienced a design-level earthquake – an earthquake that met or exceeded what they were designed to withstand. At the same time, shorter and stiffer buildings experienced an earthquake that wasn’t even one-third of Building Code.”
Structural Engineering Society (SESOC) spokesperson Paul Campbell says the Statistics House investigation has revealed that buildings with key characteristics – flexible frames in combination with precast floors – may be vulnerable in an earthquake that’s both large and long.
“Flexible frames are designed to bend so that the ends of the beams experience controlled damage. But the Kaikoura earthquake has confirmed that if an earthquake is strong enough and long enough, the damage can make the beams grow in length. This means the supports for the precast floor system can move too far apart, potentially causing parts of the floor to lose their support and collapse,” Mr Campbell says.
“When Statistics House was built, the design standards did not allow for this combination of factors – but it does now. The Canterbury earthquakes led to changes – and now more changes to the design standards are likely given what we have learned from the Kaikoura earthquake.”
Improving building safety
Dr Smith says New Zealand is at the cutting edge of international seismic design standards, but has not yet solved all 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 and systems to improve building safety for the future,” he adds.
“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.”
Dr Smith says the largest safety gains for Wellington are to be made in the initiatives requiring unreinforced masonry facades and 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.
The Statistics House investigation report can be downloaded from the MBIE website.