‘Significant overload’ caused Norway’s timber bridge collapse
newcivilengineer.comThe homepage of the "Norwegian Safety Investigation Authority" is much more informative and the 2 documents have interesting photos, e.g. closeups of where the wood broke.
https://www.nsia.no/Road/Investigations/22-441
It does not state why they conclude the cause was overload, only that "Uncovering the technical causal factors for the collapse of the Tretten bridge has been challenging. The expert group has worked its way through several hypotheses to be able to exclude non-relevant fracture mechanisms, by connecting findings to computational analyses. There was significant consequential damage to the bridge structure, both from the collapse itself, the impact with the ground and the salvage work, and it has been challenging to separate these from each other."
Personally I would think that a bridge should be able to handle "A passenger car and a truck with trailer loaded with lime were on the bridge when it collapsed".
The headline is misleading. They are saying that it seems to have failed due to some kind of mysterious fatigue failure at a wood to steel structural interface. The investigators are struggling to work out how because the timber was damaged when it hit bottom of the river and then damaged further when it was hauled out for inspection. The overloading in the headline seems to refer to the ‘load shedding’ to other structural members that will have occurred after the joint (or joints) failed. (I’m not a structural engineer, but my profession is closely related.)
Apparently the Siemen's Sports Arena failure[1] was attributed to brittle failure in dowelled slotted-in-plate connections, which is also what they used for the steel-wood interface for this bridge.
[1]https://www.researchgate.net/figure/The-Perkolo-bridge-after...
>"... An overview of capacity models derived using the yield limit equations and following the European Yield Model (EYM) approach is provided in [20]. Major examples of structures collapse led by brittle failure in dowelled slotted-in-plate connections are the two trusses of the Siemens Sports Arena in Denmark [21] and the truss of the Perkolo bridge in Norway [22]. Uncertainty in the evaluation of connections' deformability is responsible for local damage in connections, and change of force transfer mechanisms compare to those considered in the design. ..."
> Personally I would think that a bridge should be able to handle "A passenger car and a truck with trailer loaded with lime were on the bridge when it collapsed".
The Norwegian Safety Investigation Authority seems to agree. https://www.nsia.no/Road/Investigations/22-441?iid=35881&pid...:
“Checked according to the recommended calculation methods in today's current regulations for the design of wooden structures, the degree of utilization in the connections on the relevant diagonals was around 200%. That is to say, the capacity of these connections was half of what they should have been when calculating the bridge's failure limit state, where safety factors for both loads and strength of materials were taken into account”
While the statement seems technically correct, it smells a bit disingenuous to me.
Most sophisticated structures are engineered using load factors in combination rather than traditional strength design methods because it leads to more efficient use of materials and hence lower cost.
Yes, strength design might require a 4x safety factor. That’s why engineering practice employs statistical methods and combined loads.
Keep in mind that the safety factors are never supposed to be the difference between stability and failure. If the structure relies on safety factors there’s a problem.
> leads to more efficient use of materials and hence lower cost.
I know that isn't always a bad thing, but this is the kind of sentence that always gives me pause. In the case were over-engineering something to 10x minimum tolerance can be re-configured to be only be 8x minimum, then sure, that sounds like there's some room to safely redesign for efficiency/cost sake.
My pessimism for the world today tends to make my first impression the "lower cost" phrase being used is always the "cheap bastards looking to cut corners at the expense of safety".
Most choices are not cake or bread.
Most choices are bread or going hungry.
Or a new bridge instead of an old failing bridge or no bridge at all.
The fact that the bridge failed catastrophicly rather than ductally suggests the fundamental problem was systemic in the design and the mode of failure was from unanticipated forces.
What I mean that if the force was 8x the design load 2x vs 4x safety factors would be irrelevant.
> If the structure relies on safety factors there’s a problem.
Maybe I’m misunderstanding your point, but I don’t think it’s unreasonable to have actual (large) limits on what a structure can handle? Otherwise, everything will cost X times more time/money to build.
The idea is to work out the maximum design limit, then add a factor of 2x or 4x to that to allow for things you can’t predict or model well. If the design is relying on some part of the above of the 2x to work, it’s not really a safety factor.
I’m a mechanical engineer and figuring out the appropriate strength a part needs, then making it twice as strong, is a common tactic. You can’t account for material differences, little casting voids, or just your math being off a little by the approximate methods we use to calculate stresses. The more perfectly we can model and then repeatedly destructively test a part, the closer we can get to a safety factor of 1.
Ah, gotcha. That makes sense!
I think they mean that this particular element was massively overloaded (200% more than its max, which would mean 400% of the design load), not that a few cars and a truck were overloading the bridge. It was more the structure self weight that is relevant. Somehow extra load was transferred onto that element which should have been taken by other elements. It doesn't seem good that failure of one element would cause the whole thing to fall down though.
There are a number of bridges where I live that cannot handle a truck with trailer. They have large signs warning what the maximum load is.
They are mostly on rarely used roads in farm country (as in a handful of cars per week cross), so a truck with lime is one of the more likely potential loads, but of course the local farmers know that bridge can't handle the weight of a truck and plan their deliveries to take a different route (which often more than doubles the trip time vs if the bridge could handle the weight)
The problem comes when those type of bridges don't collapse the first time someone overloads them; then people start using it because "eh it works" and eventually it fails.
Is there some well established way to make some very visible but not structural part of a bridge to collapse after an overload, to scare people and make them report the problem?
You probably could, but the amount of engineering and cost required would be simpler to just replace the bridge with a stronger one.
Often these "car only" country bridges are just some large wooden beams over a creek; even a simple concrete culvert replacement would be truck-proof.
Some of them do things like this: https://www.alamy.com/stock-photo-small-car-suv-crossing-the...
A really simple catch all, that isn't quite accurate for all cases would be to put a sign over the top of such a bridge,that limits the height of vehicles crossing, excluding trucks with the tall trailers. Would not work for loads that aren't tall though.
I would have thought a few strain gauges, maybe some logic to filter false positives (if that's a risk) and a light would do a reasonable job.
Wait until you see the known issues and reports on bridges in the US that did collapse. Nobody monitors this stuff even on important ones.
Here that doesn't seem to be the case. If you look at the western approach to the bridge in StreetView (which is from 2019, the eastern part has imagery after the collapse), there is no weight limitation sign to be seen. As such, you would expect the bridge to be able to carry the load in the worst case scenario of both lanes being filled to capacity with heavy vehicles, so failing because of one single truck and one or two cars is really bad...
Reaction: +1...but there is still an "on any public road or bridge" weight limit for trucks, buried somewhere in Norway's regulations for trucking & highways.
The stereotype would be that the law-abiding Norwegian truckers are always careful to keep their trucks below the legal weight limits...but stereotypes aren't always true.
This Norwegian bridge is practically brand new.
Are these bridges you are referring to from your area really old and poorly maintained bridges?
Mostly, some date back to the 1800s. Some are a bit more modern, but still mid 1900s. Though in farm country it isn't unheard of to save money buy not building one up to the weight of a loaded truck. For that matter, there are farmers who run more than max weight of the road (this is sometimes even legal, it damages the roads, but replacing a road a bit early is considered cheaper than building to higher capacity - bridge weight limits cannot legally be exceeded though, and farmer going overweight know where they can't go)
I read it to imply there was a design or construction problem that meant the part that failed experienced much more load than intended.
Building Integrity on YouTube is a fabulous channel to watch if you're interested into deep dives into failures such as this. The host is a civil engineer who specializes in performing structural audits of older buildings (specifically condos, I think).
He did a an episode on this bridge. It is excellent, and it provides some context about other timber bridges that is not discussed in this article.
I think a fairly good argument can be made that the design of this bridge was novel to the point of being negligent as per https://www.youtube.com/watch?v=FSPI0xkTifI
I can’t get really past the guy’s attitude. From the get go he’s mocking the work from the marketing to materials, and he’s not doing it in a fun way. Just plain denigrating.
I thought you linked to B1M or something.
I’ve been watching his videos since Champlain Towers South, and this was the first video that I found off-putting. I would recommend giving him a chance or two, I especially found the CTS videos interesting.
He's correct, though, a bridge that collapses after only 10 years of use demands mockery (and worse) for the people that designed it. Especially as this new bridge replaced an existing one which had, indeed, been up for more than 100 years. It's very fortunate that no-one died because of the incompetence of the people involved in building it.
My dad is a retired civil-engineer and he displays the same type of mockery (and worse) when he sees badly built stuff. Because he very well knows that badly built stuff can cause people to, well, die. It's not "they made an ooopsiee, let's not make it worse by denigrating the designers/builders", it's "they really f.ed things up, people could have died, they're morons (and worst)".
This is a shit attitude that results in more shit collapsing because everyone’s busier finger-pointing, calling each other morons, and trying to avoid being called morons than they are figuring out why shit collapsed in the first place.
A bridge doesn’t collapse because someone was incompetent or fucked something up. It collapses because multiple different things fucked up in lots of ways, and the processes responsible for checking they didn’t fuck things up also fucked up. It’s a systemic failure, and by far the most important thing is figuring out how this failure happened and how we can prevent it from happening again in the future.
> than they are figuring out why shit collapsed in the first place.
That's what the guy from the YT video is trying to do in the first place, while also naming things for how they really are.
> A bridge doesn’t collapse because someone was incompetent or fucked something up.
Yes, that's why a thing like a bridge collapses, because key people that were part of the process managed to fuck up. It's as simple as. That's what my dad's structural engineering professor told him and his student colleagues back in the '70s: "don't fuck up in your future job or people will die. You'll also most probably go to prison". Said professor didn't mention anything about systemic errors or anything like that, which actually means diluting the blame ("it was not me, it was the system").
Civil engineering is not like aeronautics, which has lots of moving parts, civil engineering is (mostly) a tried and tested discipline (like the 100+ years old bridge that was replaced by this new, fancier bridge can attest to). Yes, learn from mistakes, obviously, but also call people out and throw them in prison if people have died because of their incompetence.
The idea of a risk management strategy that involves depending on people to not fuck up is plainly laughable.
Cathedrals were not built based on “risk management strategy”, but on builders not fucking up. The civil engineers that I know of (basically my dad and his friends from his generation) looked up to those builders from the Middle Ages, not to today’s management consultants instructing them about “systemic risks” and “strategy”. Granted, I do not know what today’s generation of civil engineers looks up to.
Damn, this is a really high quality, high effort video, with a lot of nice pictures. Thanks for posting.
It's a fantastic channel. Watched all the videos on the Surfside condo collapse and they're all this high quality.
That's a lesson I wish more software engineers would internalize. Novelty is a reliable way introduce failure modes. That was my main takeaway from a book that was mostly about bridge failures: Case Histories of Error and Judgement in Engineering by Henry Petroski.
> head of the Centre for Wood Science ..., identified steel truss issues as a possible factor.
You don't say...
Wood community vs the steel folks. Worse than emacs vs vim
I'd like to hear from the Centre for Steel Science.
I think they'd agree that hydrodynamic effects on the concrete support columns should be considered.
And what of the fine gentlemen from Center for Concrete Science? What say them?
Surely corrosion of the rebar...
I frequently cross the Golden Gate bridge. This is my nightmare. Can you imagine plunging to your doom if that bridge failed? Yikes!
Pittsburgh allegedly has more bridges than any other city in the world. One collapsed last January.[0] I had driven over and walked under it many times. An uncomfortable number of other bridges in the city are in poor repair.[1] If one of the bigger ones over one of the rivers here collapse, it will be a tragedy of similar magnitude to your nightmare.
[0] https://triblive.com/local/frick-park-bridge-collapses-natur...
Ya but the bigger the bridge the better the monitoring. I used to use one that fell daily, but its not a bridge your average Yinzer crosses even on a yearly basis.
It just doesn't really go anywhere except a couple thousand homes.
The big bridges on the three rivers, on the other hand! It seems to me that they're always always under repair.
There's A LOT more traffic across the Fern Hollow bridge than if the end destination were a couple of thousand homes. Average daily traffic ca. 2005 was 14.5k vehicles including transit bus routes with articulated buses that nominally carry 70+ at capacity. At rush hour I've been on those buses and they're regularly SRO.
The Goldstar memorial bridge over the Thames river in Connecticut is the same type as that I-35W bridge that failed in Minneapolis (only Goldstar is older and higher). I worry every time I cross it..
You might be interested to know that the term for avoiding bridges is gephyrophobia.
The more you know.
One morning I drove across the Tacoma Narrows Bridge and it was quite foggy. I couldn't see the road surface very far ahead of me and had to just assume it would be there. Thankfully, it was. When I drove across the return span later in the day, I was happy to see the whole bridge before I started.
My intuition is that the huge iconic ones in globally significant metro areas probably receive closest to an adequate amount of attention and maintenance.
I'm much more worried about like, a random 40 foot bridge outside of the second largest suburb of the third largest city in a southern state or whatever. A short fall into icy water or just getting bonked by a piece of the falling bridge at the bottom is less dramatic but you'd be just as dead. And I'm pretty sure those bridges haven't seen an attentive engineer since whenever they were finished.
I'm much more worried about...
Various ancient bridges near my rural home have been "saved" by local morons (here they might be called "hillbillies") who organize elaborate campaigns to demand that the highway department not close them despite their ill repair. In some cases an alternative bridge has been built nearby at vastly increased expense, in other cases it just means that only morons use the existing rusted-out (or in the case of the "swinging" variety, multiple-broken-planked) bridge, carefully driving so they don't drop a tire into the holes.
How a utilitarian object riveted together out of commodity steel in 1910 is such a triumph of human will that it must be imposed on posterity forever, surpasses understanding.
Well you just sound delightful and open-minded about your neighbors I wonder why they don't value your opinion about it!
I have never expressed that opinion to any of them. I have a personal rule, which I'll break now because I am quite curious: why is a crappy old dangerous bridge better than a new safe bridge?
Suggested viewing: Final Destination 5
Anti-recommendation system has potential
Can someone provide background on this — what makes this accident and/or bridge notable?
It's notable because it's practically a brand new bridge - only 10 years into its 100 year life span. Bridges that fail are typically much further into their projected life spans, and the ones that aren't make headlines (like Tacoma Narrows). It's also a novel design, so lots of heads will be scratched while figuring out where the mistakes were made. Was the mistake in analysis? Materials? Something else?
I say novel design, but it's more like some novel techniques for combining wood and metal together. The basic bridge design is fairly common but typically built using all wood or all metal. When you look at it it's fairly similar to train bridges from last century.
TL;DR: One of the diagonal beams was only designed to be half the strength it needed to be, and it snapped.
This type of bridge needs every beam - so when one broke, the whole thing fell.
The mystery really is why it didn't collapse sooner - that's still under investigation.
Can't wait until all of the timber skyscrapers that they're building to fight global warming meet the same fate.
It'd be neat to integrate ChatGPT into a "bridge builder simulator" type game, and have it generate post-mortem news reports like this one for the bridge failures.
Considering what i saw posted about GPT thingies cost, you'd have to pay a subscription for that game.
Well, then it was underdimensioned.