[silvestrov]

The 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".

[ArchitectAnon]

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.)

[mc32]

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. ..."

[Someone]

> 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”

[brudgers]

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.

[dylan604]

> 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".

[brudgers]

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.

[yunohn]

> 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.

[MarkMarine]

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.

[yunohn]

Ah, gotcha. That makes sense!

[angry_octet]

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.

[bluGill]

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)

[bombcar]

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.

[pmontra]

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?

[bombcar]

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...

[yonaguska]

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.

[mannykannot]

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.

[bombcar]

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.

[rob74]

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...

[bell-cot]

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.

[beAbU]

This Norwegian bridge is practically brand new.

Are these bridges you are referring to from your area really old and poorly maintained bridges?

[bluGill]

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)

[h11h]

I read it to imply there was a design or construction problem that meant the part that failed experienced much more load than intended.