Auckland Harbour Bridge damage: How it happened and what needs to be done to get it fixed
Sunday, 20 September 2020
“Hurry up and fix that s***.”
That was the blunt message from one Auckland motorist to a group of NZ Transport Agency (NZTA) managers and engineers on Sunday afternoon, as they strolled up the closed inner lanes of the Harbour Bridge.
They were escorting media onto the bridge following what the NZTA is calling a freak incident on Friday, which blew a truck into a load-bearing steel beam, rendering the middle lanes unusable.
The effects of the closure rippled out across Auckland's already congested motorways and continues to cause long delays and gridlock.
**READ MORE:
* Auckland Harbour Bridge damage: 'You can't build your way out of it forever'
* Auckland Harbour Bridge middle lanes 'not safe' after crash but no risk of collapse
**
A possible temporary fix allowing some inner lanes to open to lighter vehicles won't be in place for about another week, if it goes ahead at all. The permanent fix will take even longer as a beam needs to be designed and manufactured from scratch.
The NZTA is asking people on the North Shore to work from home if possible, or to use the Western Ring Route or buses if not.
It is also investigating ways to create express lanes to the city end of the bridge, which exist on the shore side with the Northern Busway.
Brett Gliddon, NZTA transport services general manager, said a team of about nine structural engineers were working on a fix.
If the previous week’s traffic patterns repeated on Monday, serious congestion would be the result, he said.
But Gliddon struck an optimistic tone as he defended his Agency’s operation and maintenance of the bridge, and explained how the situation came about and what is to be done.
How exactly did this happen?
About 11am on Friday, strong and sudden gusts toppled two trucks as they crossed the bridge, Gliddon said.
One was at the peak of the bridge when it was knocked over by a gust and slammed into a steel beam, twisting the metal and shearing it clean off where it connected to the bridge below the road surface.
Gliddon said the wind was an acceptable 60kmh before it shot up to a gust of 127kmh before dying down again.
“We were hit by an absolute freak gust of wind which caught a truck as it was right on the top of the bridge.
“That's really hard to design for.”
The beam did what it was supposed to, shearing off at the bolts rather than pulling on and damaging other support member, with its load transferred to other supports on the steel truss.
The bridge continues to be able to support itself, but engineers are unwilling to allow traffic over the original inner lanes, which opened in 1959.
The outer two lanes on each side, clipped on 10 years after it opened, have their own support structure and do not rely on the damaged truss.
When will a temporary fix arrive?
Not for about another week, if it's possible at all.
Gliddon said a temporary fix would involve cutting out half of the damaged section, adding some more steel, then bolting it back to the bridge.
That would allow more load on the bridge, but not a full load, meaning it's likely not all lanes would open, and heavy vehicles would be confined to the clip-ons.
Engineers continue to crunch the numbers to work out whether that approach will fly, Gliddon said.
“We're working hard to get that in place, but we've got no guarantees yet that it's going to work.”
What about a permanent fix?
That's a bit more involved.
About nine structural engineers are working on the problem, which requires replacing the entire damaged beam from top to bottom.
Gliddon said the work can be done entirely with materials and expertise already available in New Zealand, and no foreign experts need to be brought in.
But it's not as simple as finding a new beam and bolting it on.
One has to be manufactured from scratch, taking care that its material and design will work with the six-decade old bridge, Gliddon said.
”Because the bridge is dynamic. It heats up, it shrinks, it moves constantly, so we've got to make sure whatever new piece we use, with the new materials and the new steel, is going to work in the current environment.”
Fitting the beam will mean using jacks on either side to tension it up and push load back into the support, before tightening the bolts.
“The real challenge at the moment is getting the design right,” Gliddon said.
Once that work is finalised the design would need to be peer-reviewed by independent specialists.
“That's why it’s going to take a little bit of time.
“We can’t take any risks with any of this sort of stuff.”
Do changes need to be made to how the bridge is operated and maintained?
In Gliddon’s view: no.
“I don’t think we'll need to make any [changes]. We look after this bridge incredibly closely.
“We’ve got a team that works 24/7… maintaining and operating it.
“It was a freak situation.”
Nor did he think the incident meant the NZTA should have spares on hand.
“Every piece in this bridge is slightly different.
“We can't carry every single piece of steel in a shed somewhere, that's just unrealistic.”