Truss Bridge Collapse 3

[CANPRO]

This just happened yesterday in rural Nova Scotia. The bridge was scheduled for replacement. Collapse took place while the contractor was moving equipment in to start the work. One worker in hospital (assuming the truck driver).

I of course don't want to see anyone get hurt, but I find these failures fascinating. The video and sound quality isn't great, but it sounds like there was indication the bridge was failing. The worker walking on foot on the far right seems to get off the span quickly just before the collapse. We will need to get more info, but my initial speculation is that one of web members failed.

https://www.cbc.ca/news/canada/nova-scotia/bridge-collapse-guysborough-county-worker-1.5641319

 

[CANPRO]

Euler buckling means that there is little deflection and then BOOOM!

I don't think it was buckling that caused this, I think a tension member failed. The failure initiated on the far side truss - it looks to me like the tension member failed, then the top chord turned into a bending member, and it failed. See screenshot below.

The picture below gives you a pretty good look at the equipment being moved. The driver is lucky to be alive.

 

[CANPRO]

saikee119, we posted at the exact same time. Mid-span wouldn't be worst case for shear. It looks like it failed in what would be (or close to) worst case shear - when the rear axles of the trailer start to load the bridge.

 

[3DDave]

CANPRO - BOOM! was only my first suggestion. The second should have been "Twang!" but just said a tension member letting go would also not give much warning.

Good observation on shear; not sharing much load with the far end could overload it, but it looks like the part most exposed to shear is still attached to the supports.

The way it's crumpled to the side suggests that one of the diagonals across the top let go first, letting the sides fold over.

They are attached on top of the truss and therefore difficult to inspect for corrosion; I suppose the investigation will explore these options based on a much better inspection of the wreckage.

 

[CANPRO]

For sure the last couple of web members were seeing higher axial loads, but they might have been in better condition. Also, the last diagonal is a compression member.

The diagonals across the top would provide lateral stability to the top chord and handle lateral loads, I can't seem them being highly loaded here or leading to sudden collapse. I think the top chords are bent so much laterally because the far side truss lost all capacity when the tension member failed and pulled the near side truss downwards and sideways.

If/when they confirm the load was over the rated limit I'm not sure if they would investigate so far as to determine the exact origin of the structural failure. This might just all remain speculation.

 

[bimr]

Hard to tell from the pictures, but that crane appears to be a Terex HC 80. 88,000 lb transport weight includes side frames and boom inner.

 

[TenPenny]

Reminds me of a case here in New Brunswick two or three years ago, a company hired to do rehab on a wooden covered bridge hauled an excavator across it, and it collapsed. Now replaced with a Bailey bridge.

If your job is to replace a bridge that is unsafe, why would you haul a heavy crane across it? You already know it needs replacing, that's what your contract is for.

 

[saikee119]

Perhaps the operator did check the Terex HC 80. 88,000 lb transport weight includes side frames and boom inner. He could have forgotten the transporting truck's own weight. 88,000 lb is about 39 tonnes and the bridge was certified to carry 41 tonnes.

 

[LittleInch]

Bit difficult to find the unladen weight of a tractor unit and low loader but seems to be somewhere in the region of 15 to 20 tonnes.

So total of maybe 60-65 tonnes.

But 41 tonne limit for that thing?

Truss bridges like that don't like uneven loading so it might have taken 40 tonnes evenly loaded ( i.e. two 20T trucks spread apart) but not when only one side is loaded up.

I think one of those tension bars or rods snapped or came away on the far side from the camera then pretty much instant collapse same as the miami bridge. Loose any one component and that's it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

I looked at some trailer specs.
That type of trailer runs from 20,000 lbs to 30,000 lbs tare weight.
Weights for a tri-drive tractor start at around 20,000 lbs for the lightest and go up with heavy duty specs.
50,000 lbs for the empty tractor and trailer is reasonable.
The truck and trailer are both three axle rigs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dgallup]

That bridge was also 70 years old in a salt water environment. The odds of it still holding the original weight rating are slim and none.

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[saikee119]

That bridge was also 70 years old in a salt water environment. The odds of it still holding the original weight rating are slim and none.

The above could be viewed as an attack on the entire Department of Transport and its established procedure of certifying a bridge in the US. Hard evidence and scientific proof will to be needed to convince others.

However for a simply supported bridge the worst loading condition should be when the load reaches the mid span. This wasn't the case and suggested the load could be substantially and not marginally above the limit. In general there is a load factor on dead load as well as another load factor on live load built into the safety of every bridge.

On the other hand if there were no overload and the bridge simply fell because its capacity has been compromised by the salt water environment, a condition never considered relevant by the bridge inspecttorover 70 years. In such a case when the load was gradually introduced to the bridge would we not expected to see some warning signs like material yieldings and progressively large deflections/distortion instead of a sudden collapse? Afterall we are talking here metal plates losing thickness from corrosion. ALso the bridge inspection is done periodically and a mecanism exists to downgrade its load carrying capacity to suit the changed condition. The current capacity dated Apr last year apparently.

In conclusion I don't think it is productive for the discussion to pursue the line of enquiry on faulty certification of the bridge capacity because it does not seem to play a significant role in the current collapse.

 

[JohnRBaker]

The above could be viewed as an attack on the entire Department of Transport and its established procedure of certifying a bridge in the US.

Why did you bring up the US Department of Transport(SIC)? This bridge was located in Nova Scotia, CANADA.

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

 

[CANPRO]

However for a simply supported bridge the worst loading condition should be when the load reaches the mid span

This is true only for some components of the bridge. The web members at the end of the bridge would see their maximum demand when the load is applied near the end of the span. Similar to a simple span beam having its maximum shear when a point load is located near the supports. It looks to me like one of the web members broke just as the rear axle of the trailer hit the bridge.

In conclusion I don't think it is productive for the discussion to pursue the line of enquiry on faulty certification of the bridge capacity because it does not seem to play a significant role in the current collapse.

Agreed. Seems to be a pretty clear case of the bridge being loaded beyond the rated capacity.

 

[saikee119]

JohnRBaker (Mechanical),

My mistake. I did use generic term but inadvertently added US at the end.

 

[dgallup]

In conclusion I don't think it is productive for the discussion to pursue the line of enquiry on faulty certification of the bridge capacity because it does not seem to play a significant role in the current collapse.

It was already condemned and scheduled for replacement. You think they did that because it was in like new condition?

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[IRstuff]

Speaking of SWIFT; there's a new one (sort of; 6 yrs old) in town:

https://appleinsider.com/articles/2...-everyone-can-code-develop-in-swift-resources

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[JStephen]

It's curious to me that there don't appear to be any weight-limit signs on the approach, are they not used up there?

 

[FacEngrPE]

Don't know if Nova Scotia bridge uses weight limit signs often, but their traffic sign regulations include them. They look similar to the US signs but use metric tonnes (2205 lb)

https://novascotia.ca/just/regulations/regs/mvtrafficsigns.htm

Fred

 

[IRstuff]

The bridge was pretty rusty from the 2012 imagery that Google Maps has of that bridge.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[bimr]

The Contractor should have had a permit to transport the load.

Antigonish-based Alva Construction has the contract to build the replacement. No one from the company was available to comment Wednesday.

The bridge had a maximum permissible weight of 41,500 kilograms.

The side of the crane visible above the water bears the markings Terex HC80.

According to the manufacturer’s specifications, the crane has a transport weight of 39,916 kilograms.

A truck hauling a flatbed trailer can weigh an additional 14,000 to 17,000 kilograms.

Link