When the earthquake status of Wellington’s Freyberg Pool was on a knife edge, an engineer told the city council he would see if they could “eke” a bit more out to put the building in the clear.

It turns out they could, reports and correspondence released under the Local Government Official Information and Meetings Act have revealed.

Despite an initial assessment finding the building was earthquake-prone, the pool has been rated at 40 per cent of the New Building Standard (NBS) after engineering consultancy firm Beca was brought in for a second opinion.

A building is considered earthquake-prone if it is rated less than 34 per cent NBS.

If a building is deemed earthquake-prone it needs to be strengthened for safety reasons, which can be a costly, time-consuming, and stressful process. So, the opinion of engineers can have significant consequences for building owners.

In this case, the owner of Freyberg Pool is Wellington City Council.

Liquefaction worries

In 2016 detailed seismic assessments were undertaken for several of the council’s pools to check on their earthquake status.

Freyberg Pool was given an indicative NBS rating of 45 per cent, but there was some uncertainty about liquefaction and what that would mean for the building’s structural performance.

The key point at stake was how the building, located on Wellington’s waterfront, would interact with the ground.

Liquefaction is the process which causes soil to behave more like a liquid than a solid, usually during a moderate or strong earthquake.

So, a geotechnical investigation of the land underneath the pool was undertaken, which included taking readings and soil samples through boreholes.

These findings were pulled together in a draft report by Spencer Holmes, with the geotechnical assessment by Tonkin & Taylor, and presented to council officials in June 2021.

The report deemed Freyberg Pool to be between 25 to 30 per pent NBS, meaning it was earthquake-prone.

It found in a moderate level of shaking the ground underneath the pool was expected to liquefy and the building’s foundations would no longer be properly supported.

A summary of the report said the connection between the building structure and piles would likely fail, causing movement at the base of the bleachers.

“As the bleachers are directly connected to the roof structure there is a possibility of the roof being pulled down with the bleachers.”

One remediation option included installing new piles around the perimeter of the pool and ground beams to tie the structure together, at a cost of $46.7 million.

The NBS rating was so much lower because something called a “step change” had to be applied, effectively halving the rating. It’s applied when engineers find the ground cannot support the building, resulting in catastrophic failure.

Due to the significance of these findings, council officials considered it was prudent to obtain a second opinion.

A second opinion

Beca’s building structures chief engineer Rob Jury told the Herald their job was to facilitate a discussion between the geotechnical experts and the structural experts.

“It was really trying to encourage them to look at different things in terms of what they had already found, but putting the two things together.”

During this process, in November 2021, an engineer from Beca emailed a council official to advise where the review was headed.

The engineer said the building would likely still be considered earthquake-prone.

“We’re not fully there just yet though, and we are doing a few more checks to see if we can eek [sic] a bit more out, and get the building over 33 per cent. There’s still a chance.”

Jury said he didn’t consider that email to be inappropriate and he was comfortable with what was said.

“We try to get these scores and ratings of these buildings up as high as you can reasonably justify…. you must go down every avenue I think because as I say, you’ve got to give these buildings the best chance basically without over-egging it.”

Tonkin & Taylor earthquake foundation engineering technical director Stuart Palmer acknowledged the initial assessment was on the conservative side.

“It’s relatively easy to be conservative on these things, it’s really difficult to be realistic.”

“It was good to have Beca to be challenging the simplified approach that we’d been taking to date and allowed us to think ‘okay, maybe if we look at it this way, or this way, will that change our view?'”

In the end all three consultants were happy with the final opinion they reached, Palmer said.

Further assessment found the pool’s piles actually could resist ground flowing around them, meaning they could still support the roof.

This meant there was no longer justification for applying the geological step change point because there would in fact not be a catastrophic failure of the building.

Freyberg Pool was therefore given a 40 per cent NBS rating, which meant it was not earthquake-prone.

Easy for engineers to be too conservative

Palmer said it was not unusual for an NBS rating to change between an initial assessment and a final assessment. In this instance, the difference between applying the step change or not was the difference between the building being deemed earthquake-prone.

“Rob and I have been involved in a number of these battling through. Do we apply the step change in this situation, or don’t we? And it’s not easy, it’s not easy at all.”

Jury said just because there were initially different opinions on the pool didn’t mean any of them were entirely wrong.

“It just means you’ve got to objectively take the results and put them together.”

Jury said it was easy for engineers to be very conservative because they had nothing to gain from being more liberal.

“The reality of it is we’ve got to be more realistic in the way we rate these buildings. We can’t be conservative, it just sends all the wrong mixed messages out there. If society really was worried about going into low rating buildings, they wouldn’t be able to move.”

Both Jury and Palmer agreed there was an over-emphasis on NBS ratings.

Palmer said it was concerning that some corporates were chasing ratings of 80 per cent and 67 per cent, which were well above the legal requirement of 34 per cent.

There was still a lot of uncertainty and NBS ratings were not an absolute thing, Jury said.

Earthquake-prone buildings can still survive earthquakes while others don’t. For example, Statistics House, a relatively new building on Wellington’s waterfront, partially collapsed in the 2016 Kaikōura earthquake.

Jury said the focus was on prioritising buildings and getting the “bad ones” out of the way.

“Our assessment approaches are really finding the worst buildings, because we want to find the CCTVs.”

Christchurch’s CCTV building rapidly and almost completely collapsed, killing 115 people, when the 2011 February earthquake struck.

So, is Freyberg Pool safe?

Wellington City Council chief customer and community officer Kym Fell said he was confident the pool was not earthquake-prone following the final opinion of three technical experts.

“I’m comfortable with that, I don’t have any real concerns around that.”

Fell said there was a lot of life left in Freyberg Pool.

For the meantime, the pool will undergo general maintenance until there was information to suggest significant population growth in the area, Fell said.

Then the council could look at new facilities or an expansion, he said.

So, is Freyberg Pool safe? Yes it is, but Jury pointed out “safe” wasn’t an absolute term.

He also said he wouldn’t have a problem with his own grandchildren swimming there.

Source: Read Full Article