Big wooden bridge fail.

[Oldgreybeard]

Hard to know for sure from the photos and video exactly what failed first, but the design of the stone pier end support looks seriously iffy, as does the design of the fastenings in the upper member. Looks to me as if the designer took a steel cantilever design and translated it to timber without regard for the way the loads changed between the dead and live load cases. The upper members are subject to load reversal in use I think, as the live load application point moves along the bridge deck.

Wouldn't surprise me if analysis shows that the upper members could see a tensile load under some conditions, and it's clear from the first photo of the intact bridge that the fastenings simply weren't up to taking a serious tensile load. Transferring tensile loads into timber is tricky and best avoided by design, really. Timber is a great material if used properly, but is far better if the design ensures that it's kept in compression for all possible load cases.

If I had to guess how the bridge failed and that stone pier punched through the deck, then I think the car will have been on the land side of the stone pier at the start of the failure, with the truck past the mid-span point. The upper timber members immediately above the deck failure point would then probably have been in tension, due to the cantilever effect around the stone pier. I suspect the fastenings failed and pulled out, removing the support from the upper members to the deck and then the deck failed by a combination of bending and punch through shear right at the point of maximum stress, which would be where it was bearing on the stone pier.

The steel pillars were OK as they didn't see much of the live load, the whole structure was effectively cantilevered over the stone pier. Had there been proper tensile members along the top then this probably wouldn't have failed. Almost certainly a design failure caused by the designer failing to appreciate the difference in allowable stress in timber depending on the grain direction. If he was used to working with steel, rather than timber, maybe he just didn't appreciate the anisotropic nature of timber, and in particular failed to understand how the fixings need to be very different depending on the direction of the load application.

[Moxi]

Do we know who the designer was ?

Moxi

[billi]

Do we know who the designer was ?

Moxi

Maybe the Lorry driver , just for the craic

[Moxi]

I reckon he (lorry driver) is still traumatised - he was grabbing at everything to stay in his cab, the helicopter veritably ripped him from the window frame and the way he grabbed on to the recue winch man afterwards made me wonder if they ever managed to transition him into the helicopter or if they had to land with him and the winchman still hung of the line?

Moxi

[Oldgreybeard]

Do we know who the designer was ?

Moxi

There's been some suggestion that the architect that designed it hadn't designed a timber bridge before. The design seems to have been selected on the basis of its aesthetics. Quite why it was designed to be so asymmetric, when logic would dictate that cantilever structures are almost always better if they are symmetric, is a mystery. The report should make interesting reading.

[AE-NMidlands]

Do we know who the designer was ?

Moxi

There's been some suggestion that the architect that designed it hadn't designed a timber bridge before. The design seems to have been selected on the basis of its aesthetics. Quite why it was designed to be so asymmetric, when logic would dictate that cantilever structures are almost always better if they are symmetric, is a mystery. The report should make interesting reading.

That sounds like the Genoa collapse, where it looked to me as though they had copied the Forth Rail Bridge, but all in reinforced concrete. The bits in compression worked OK, but using thin (un-inspectable) steel inside concrete for the tension members was mad.
A

[Oldgreybeard]

That sounds like the Genoa collapse, where it looked to me as though they had copied the Forth Rail Bridge, but all in reinforced concrete. The bits in compression worked OK, but using thin (un-inspectable) steel inside concrete for the tension members was mad.
A

There was a tendency years ago to assume that steel tendons grouted inside concrete were immune from corrosion or age related degradation, so there was no need to provide any means to inspect them. I think the lesson has been learned now, with so many tensile structures being found to degrade faster than anticipated by the designers. The big problem seems to be that testing using accelerated ageing doesn't always match what happens in the real world. One look at all the 1960s reinforced concrete structures that have suffered from corrosion and failure of the steel reinforcement shows that we're often far too ambitious and optimistic when using new methods of construction.

The worst culprits by far are the arty farty designer types that couldn't make anything with their hands if they tried, and who assume that an engineer will always be able to turn their completely impractical ideas into reality. All designers and architects should be forced to spend a couple of years actually making stuff, before they are let loose in the wild.

[smegal]

Do we know who the designer was ?

Moxi

Tretten Bridge, which was designed by Norconsult with architect PLAN Arkitekter.

[Moxi]

Thanks Smegal,



Moxi