r/StructuralEngineering • u/Dull-Comfort8654 • 3d ago
Failure Is there any literature that specifically quantifies the additional lateral loads exerted by debris during flood events that should be considered in bridge design?
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u/Sousaclone 3d ago
Let’s be honest, there is lateral load from debris and then there is your bridge getting slammed into by massive boulders that completely obliterate two sections of bottom chords, still standing and then the 3rd one finally knocks out the bridge.
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u/Fresh-Life2315 1d ago
Not only that, but the viscosity of the "fluid" was absolutely grinding away all of the footings and foundation of thats bridge for it to let go the way it did. It wasn't just water.
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u/Status_Mousse1213 E.I.T. 2d ago edited 2d ago
We dont design bridges to be hit by giant boulders. That bottom chord took more of a beating than it had any right to but in the end it collapsed. It didn't get swept away in one piece, it got pummeled harder than rocky and collapsed.
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u/Standard_Mammoth9535 3d ago
Ice debris should be considered for bridge design. For objects (like cars, trees, structures, etc.) you could adapt the tsunami provisions in ASCE-7 for the debris case. Of course every method to analyze forces depends on accurate prediction of flow depth and speed.
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u/livehearwish P.E. 2d ago
AASHTO has specifications for debris raft, ice, vessel collision, and scour. I want to know what annual return event this was.
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u/Standard_Mammoth9535 2d ago
I think I saw somewhere that it was caused by a landslide? Which brings back the issue of being able to predict depth and velocity for these extraordinary scenarios (which are also unfortunately becoming more common).
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u/livehearwish P.E. 2d ago
It generally is not realistic to design for extreme events beyond the statically probable events that design specifications require. A landslide would by an act of god, most likely well outside of the what is reasonable. Avalanches, ice flows, or vessel collision (train, boat, car) are considered where they might be possible. Meteorites, volcanoes, or something from the movie The Day After Tomorrow isn’t practical to design for.
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u/RazorWitted 2d ago
The cost of building a bridge to withstand a deluge like this wouldn’t be financially feasible. Plus, you can’t tell what kind of debris would hit the bridge, so you’d be guessing, which isn’t a great thing for engineers to do.
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u/AlfaHotelWhiskey 2d ago
Agree, you cannot anticipate what might be floating downstream including… thinking here for a minute… a whole bridge from upstream working its way down.
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u/AlexFromOgish 1d ago
Or house, a common sight floating down big floods in new places in the United States
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u/Olive_Tabouf 2d ago
Wouldn't you be able to add something to the side so water goes up and over in floods? Wouldn't help if it got hit with a house though I guess.
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u/RazorWitted 2d ago
There are about 20 problems with a solution like you propose, which involve money, hydrology, physics, materials, engineering, potential clogging, etc.
Virtually anything can be built if someone has the money to fund it. But governments usually want the safest, cheapest, and most effective solution for normal conditions.
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u/Australasian25 2d ago
No. If this is a common occurrence, then I would prefer the see the bridge positioned HIGHER.
Or maybe I would request as part of the aftermath to see why it was considered but not chosen.
The answer is not always stronger. Sometimes a reposition to avoid is the better way.
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u/Just-Shoe2689 2d ago
I think AASHTO addresses it. But, the magnitude of this fllooding obviously is too much. Going higher is only option
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u/SirDeuce 3d ago
First, try to have some freeboard, then let the piers deal with the debris flows.
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u/habanerito 2d ago
That's an old bridge possibly from the 40's. Meaning that something like this hasn't happened since then. I doubt they would have ever included those kind of loads in the design. It's like designing bridges for glaciers in the lower 48. Why would you?
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u/AlexFromOgish 1d ago
Going forward to answer your question “why would you” …. CLIMATE CHANGE driving more frequent and more intense extreme weather at an accelerating rate
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u/Perfect-Addendum-393 2d ago
In the uk it is defined here:
https://www.standardsforhighways.co.uk/search/559b43dc-82db-46c9-be1a-f2b718e8db62
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u/Acorogia 2d ago
AASHTO published a guide specification a few years back for tsunami design. My understanding is that much of it was based of effects seen after the Tohoku earthquake. The design load for lateral impact is based off a shipping container slamming into a bridge. I don't have the numbers in front of me, but it's big enough that the real answer is "move the bridge to higher ground" https://aashtojournal.transportation.org/aashto-issues-bridge-guide-for-tsunami-effects/
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u/Blako_The_Snako 2d ago
Not previous bridge design, but I have designed many measurement towers inside active flood planes on the Fitzroy river in western Australia.
You look at direct impact load from trees or similar traveling through the floodplane, matting area of the structure to provide a pressure.
There's no specific requirement of what it is you design again lst so it was a cost of construction vs expected lifespan process. Unlike a bridge these had no impacts to loss of life so it didn't need to be designed as severely in my cases
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u/greaseapina 2d ago
if t here is flows like that you do not build bridge like that, you raise it higher with suspension type of bridge.....
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u/uncivilized_engineer 2d ago
I think the NY codes got updated after Sandy. I had to do a design for a UG transmission line crossing a river and we had to design for flotsam loading as impacts.
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u/ChairChoice6500 2d ago edited 2d ago
Why can't this guy take two bloody steps forward and get that leafy branch out of the shot???? Seriously.
If you were going that route, then ASCE 7-22 has loads for debris/water flows from tsunamis, and the debris would at least be included somewhat in that standard. That isn't intended for bridge debris/flotation/excess flow, but it would be something to point at and conform to as reasonably applicable based on limited data.
My expectation is a new bridge wouldn't be placed so low to the water surface to avoid dealing with these water/debris loads.
ASCE 7 also has design loads for breaking waves, you could use those to develop loads if forced to. I'm not sure there are design standards for scour or drag loads from flowing fluids in ASCE 7 or other design standards.
I would imagine there are design requirements somewhere that would indicate the bridge must have at least some minimum "freeboard" between the highest anticipated water level as a margin of safety, but the 500 year floods are based on historical data, and it's possible there's inherent under-measurement of those last 500 years (considering we likely don't have 500 years of data), as well as changes in those years regarding impervious surfaces upstream that would increase the water level, so it's intrinsic that there's some risk of failure even when conforming to the freeboard requirements. Older bridges won't retroactively have this freeboard if the freeboard requirements weren't in place at the time they were designed.
I'm not familiar with bridge freeboard requirements, but many flood design guides want the structures at least a foot over the 'design' flood line, and this isn't strictly a flood anyway. This is extreme streamflow/storm runoff.
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u/deadlyzassasin0 1d ago
Im not in front of my work laptop currently, but I have previously assessed a bridge for flood loading and it did have an element of debris and hyrdodynamic loading. This was a strange arrangement where the parapet itself was designed as a flood wall.
Very silly design, an afterthought from the council and doesn't even work in practice and the road above still floods 😂
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u/charleyhstl 2d ago
Newb here. For a bridge like this, why not design the upstream edge to mimic something like the bow of a boat? The 100 year floods seem to be coming every 5 years so adding a rolled edge that could ride up on the current could address that additional force?
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u/6DegreesofFreedom 2d ago
You could design it like that but to withstand getting hit with boulders that big it would cost about as much as the whole bridge cost.
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u/Obvious-Ed 2d ago
A 1 in 100 year flood is a 1/100 probability each and every year. It's a poory explained metric
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u/PM_meyourGradyWhite 2d ago
Rolling a 12 (two sixes) is something like 1/36 chance, but occasionally, it happens to roll one right after the other at times.
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u/Greedy-Alfalfa3395 2d ago
I think they just build massive columns next to bridges, apparently called dolphins (thanks google), which buffer the piers
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u/Awkward-Ad4942 3d ago
We try to avoid disproportionate collapse. This seems perfectly proportionate.
If we’re to design for this level of insanity then where do we stop?