‘Significant overload’ caused Norway’s timber bridge collapse
newcivilengineer.com
newcivilengineer.com
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".
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)
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...
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.
Are these bridges you are referring to from your area really old and poorly maintained bridges?
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”
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.
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.
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.
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 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.
[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. ..."
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 thought you linked to B1M or something.
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)".
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.
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.
You don't say...
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.
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.
[0] https://triblive.com/local/frick-park-bridge-collapses-natur...
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.
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.
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.