How safe are the capital's office buildings?
Problems tipped for 1980s-era city buildingsHANK SCHOUTEN
The low earthquake rating of Greater Wellington Regional Council's office block raises serious questions about the safety of many other relatively modern Wellington high rise buildings, says structural engineer Adam Thornton.
The 10-storey block, built during the mid 1980s building boom, has been assessed as earthquake prone – at only 30 per cent of new building standard and liable to fail in a moderate to severe earthquake.
Thornton said this was a typical "brittle" 1980s block.
A lot of attention had been given to older unreinforced masonry buildings "but we really need to address these modern brittle buildings", he said.
Many were now rated at 50-60 per cent of code and tended to be worse when built on the softer Te Aro flat soils and better on The Terrace.
The fact this building now rated at only 30 per cent suggested it was not built to the codes that applied at the time.
Wellington City Council earthquake resilience manager Neville Brown also believed many other buildings of that era would have similar faults.
"But to be honest we just don't know because the target of our assessments across the city have been pre-1975 buildings and we're not at the end of that.
"Without doubt the 1980s vintage ara is one we are going to have to investigate and we'll be on to it when we finish the current series of work," Brown said.
Engineering consultants Spencer Holmes, who were commissioned to assess Greater Wellington's Wakefield St office block, described it as a high risk D grade building.
It was built on land classified as having a high potential for liquefaction and bore tests found the top three to four metres of soil was loose, sandy and likely to liquefy in a large earthquake, which the building's foundations were not designed for.
"The implications on the building structure are serious and could lead to a potential failure of the piles, gravity structure, severe damage of the shear wall foundations and unpredictable behaviour of the seismic resisting system."
The piles were described as very lightly reinforced and did not appear to have the load capacity to comply with current codes. Other design flaws were also identified.
There was less reinforcement and stiffness above the fifth level than on lower levels and there was an uneven mass distribution through the building.
The 10-storey tower and its five-storey annex were too close to each other – just 80 millimetres – and in the event of a major earthquake there was potential for severe pounding damage as they crashed against each other.
Shear walls – designed to brace the buildings against lateral forces – were brittle and liable to crack and lose their strength because they lacked sufficient reinforcing.
Hollow core concrete flooring, used in a lot of commercial buildings in the 1980s, were fragile and liable to fail as the building flexed in a major quake.
Ceiling tiles were liable to fall and bring down fire sprinklers.
The pre-cast concrete cladding panels were also identified as a hazard.
The way they were fixed to the building exterior did not make enough allowance for the building to flex in a moderate quake.
The way the pre-cast concrete stairs were fixed meant they would be subject to very high forces as the building moved in a quake.
Concerns about the building go back more than 20 years when it was damaged by vibration from groundwork during construction of Te Papa, more than 200m away. And in 1990 council staff found cracks in columns on level three.
"An investigation of the columns established a poor standard of workmanship in general for the building construction with low concrete strength in the columns. Strengthening of selected columns was undertaken as was repair of the seismic joint.
"Significant inadequacies in the fixing of the pre-cast concrete panels were also identified soon after the column strengthening was undertaken."
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Spencer Holmes director Philip McConchie said he was unable to compare this building to the CTV or PGG buildings which killed so many in the Canterbury quakes because he was not familiar with them.
However, "it is not a building we identified as having critical structural weaknesses".
"Seismic performance is extremely complex. Some buildings of that era with similar features are good, some are bad and some are in between."
He would not be reluctant to work in the Greater Wellington offices, saying "the risk of being in that building is a fraction of the risk I take everyday driving to work".
"If we felt the building was unsafe we would make that recommendation [to vacate it] but the there are lots of buildings in the city like that."
The consultants estimated that strengthening the Greater Wellington building to 40 per cent of new building standard would cost $5.2 million and a full upgrade to 100 per cent would cost $32.5m. Both options have been dismissed by council officers.
The cheaper option was deemed imprudent because it was unlikely to meet new minimum standards that could result from the Royal Commission on the Canterbury earthquakes.
And Greater Wellington chief executive David Benham said the $32.5m needed for a full upgrade was totally uneconomic because it would cost more than the building was worth.
Council development manager Murray Kennedy said the building's earthquake status was not the only problem. In the event of a major quake damage to other buildings in the area would probably mean it was red zoned and inaccessible, and it was important for council to be able to function if there was a major disaster.
Options for moving were now being considered and a report would be prepared for council by September.
Finding alternative office space could be hard. Buildings meeting at least 67 per cent of the new building standard in Wellington were in short supply, he said.
- © Fairfax NZ News
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