What kind of storm would it take to blow over Auckland's Sky Tower?

Saturday, 18 February 2023

Muizz Shah is a doctoral candidate at Auckland University, where he has been studying the effects of cyclones on inner-city structures.

As Auckland’s infrastructure is put to the test by extreme weather events, scientists are modelling what a worst-case scenario might look like and how the city’s buildings would stand up.

For example, take Auckland’s iconic Sky Tower. It’s rated to go “essentially undamaged” in winds of up to 200kph, according to SkyCity’s online fact file.

In winds of that speed, the tower would sway by a metre at the top of the concrete shaft, but would remain anchored by 2000 tonnes of reinforcing steel and foundations that go 15 metres beneath the earth.

That would be considered a “one-in-1000-year storm” according to SkyCity. It's the highest standard that can be required in the Auckland region.

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Auckland’s Sky Tower is designed to withstand winds of 200kph.

The “wind loading” standards are set by national building codes based on the location of a building, how tall it is and what type of material it’s made of.

The highest wind speed ever recorded by NIWA was 204kph at a weather station in Manukau Heads, while the next highest was 147kph at Auckland Airport.

During Cyclone Gabrielle, MetService recorded 152kph - also at Manukau Heads - while Auckland’s Harbour Bridge was blown by 115kph gusts.

On Friday, Finance Minister Grant Robertson told the Auckland Business Chamber that there was no point talking about “one-in-100-year events” any more, because the country would be dealing with them regularly.

MetService meteorologist Ashlee Parkes said cyclones were not expected to get more frequent as a result of climate change, but they were expected to get worse.

When high-rise buildings are close to each other they can increase and channel wind.

“With climate change and an increase of water vapour in the atmosphere, their intensity will be greater and therefore their impacts larger.”

She said meteorologists avoided talking about “one-in-100-year storms”, despite this being the terminology used in building regulations, because it’s “misleading”.

“Cyclone Gabrielle was a once-in-a-lifetime event given the area and intensity of impact,” Parkes said.

Nick Locke, principal research engineer at consultancy WSP, said if the 152kph winds recorded at Manukau blew through the CBD, it would be “pretty gnarly”.

The typical office building is built for a one-in-500-year storm.

“It would be enough to start blowing bits off buildings and shattering glass.”

When high-rise buildings are close to each other they can increase and channel wind causing more powerful gusts which could suck windows out of their frames.

While there is a theoretical limit to any building, Locke said the level of storm that could blow down a high-rise building in Auckland was “pretty hard to imagine”, even if not impossible.

“The typical office building would be built for a one-in-500-year storm, which is very unlikely to be experienced by any person.

“While its easy to conceive of thanks to Hollywood movies, the reality is that they probably wouldn’t fall down. But they might not be operational afterwards.”

Dr Ahmad Zaki in the University of Auckland “boundary layer” wind tunnel. Blocks of wood are arranged to create friction and simulate topography surrounding a given city.

The Harbour Bridge, he added, was built to withstand wind, although “it has been closed due to wind damage before.”

It’s patio furniture that keeps Locke up at night. He said in his view the scariest potential for damage in Auckland was things left outside of high-rise penthouse apartments.

“Even at ground levels trampolines can fly. Up there the wind speed would be much higher, and it's not well understood how a pot plant or a table on an apartment deck would interact with those winds and what damage that might cause.”

Experts look at historical wind data from New Zealand to predict what a worst-case scenario might look like in a 500 or 1000-year timeframe.

Smoke is blown around a scale model of the PwC Tower in the wind tunnel at Auckland University.

But meteorologists haven't been recording data for 500 years, and Locke admits these predictions are based on computer modelling, and experts never had 100% certainty.

“We can’t say what’s going to happen with the weather tomorrow, and there’s no doubt that wind speeds can increase very rapidly.

“We work to a low level of risk, but it's not no level of risk. It’s up to society to decide what the acceptable level is.”

Dr Ahmad Zaki is a senior technician from the aerodynamics lab at the University of Auckland’s Newmarket campus.

Doctoral candidate Muizz Shah has been looking into whether Auckland’s buildings create wind tunnels that could impact pedestrians during a serious storm.

Along with a team of researchers, he uses a 20-metre-long wind tunnel to measure how much pressure wind creates on scale models of cities.

The buildings made from foam blocks are cut to resemble the real thing – most recently Auckland and London. The model is around 3.5 metres wide, representing several kilometres, Zaki said.

The target building for testing sits in the centre. Small holes in the model are connected to tubes which run beneath the model through to a computer sensor which measures pressure.

The researchers use a fog machine aptly named “The Hurricane” to help visualise how the wind interacts with the models.

“We see if the building will hold or not, and may suggest a modification of the design.”

Sometimes the polystyrene buildings get sent flying, but Dr Zaki emphasises that is a failure of craft glue and not an indication that skyscrapers are about to crumble.

These models are also used to test how buildings themselves change wind speeds and what street-level pedestrians might experience in the event of a storm.

Doctoral candidate Muizz Shah has been doing just that, using 2017’s Cyclone Cook as a reference. He’s been blasting wind at a model of the PwC Tower to determine whether “wind tunnels” would form.

The research is part of a major collaboration between MBIE, NIWA, the University of Auckland and WSP.

High-rise buildings can deflect wind to the ground and then channel it around the base. These effects can be mitigated with concrete buffers or by planting trees.

Sure enough, Shah found that if 165kph winds hit Auckland CBD, wind tunnels would form on streets that intersect with Queen Street.

This would be bad news for anyone that happened to be off to the pub during the cyclone.

“It would definitely be advisable to stay inside, because it's not going to be comfortable. Signage and poles will be bending, and Coca-Cola cans will be flying.”