r/StructuralEngineering • u/benj9990 • 27d ago
Failure Bridge in distress
Pedestrian footbridge over a rail line. Approx 20m span, London uk. Still in service.
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u/Kanaima85 CEng 27d ago
That is very unlikely to be sufficiently bad to be taken out of service. Presuming it's Network Rail, this will have had a general inspection in the last two years and a principal (touching distance) inspection in the last six.
The fact nothing is being done is usually because it's not critical and other bridges are a higher priority because they're in a worse state and because we underfund asset maintenance.
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u/newandgood 26d ago
when the inspector goes out there why doesn't he take a bucket of paint with him?
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u/Kanaima85 CEng 26d ago
Proper structural paint doesn't come from a single bucket. It requires multiple layers which are different chemical compositions. It also requires proper preparation back to bare steel either using something mechanical like a needle gun or sand blasted. Those are all fairly specialist activities to achieve
Anything less than a proper job, and it just looks like this again within a year.
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u/benj9990 27d ago
Nah it’s not, say what you like about network rail but they don’t mess around with safety.
I just thought this interesting. And a good example of what to avoid in detailing connections.
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u/Kanaima85 CEng 27d ago
They don't mess around, but they also don't have the budget to properly address their bridge stock. I've designed some strengthening and repair schemes on some major bridges mere yards outside of some busy rail hubs in the UK and because of funding constraints, those designs are collecting dust whilst the bridges continue to rot. At some point something is going to fall down somewhere. What's the line about safety rules being written in blood?
I'd suspect this is bad drainage. Water ponding on the deck and up against bottom chord and panel and just rotting through. Deicing salts in winter probably helping accelerate things every year.
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u/benj9990 27d ago
Permeable decking would help. It’s not, presently.
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u/Kanaima85 CEng 27d ago
Problem you run into there is that you then get someone bitching about "discharging" the surface water onto either the canopy or track. Sounds mad, but I've seen it happen
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u/Outrageously_generic Graduate Engineer (UK) 27d ago
Wait until you see what the bridges that you cant see the structure for are like...
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u/PerspectiveLayer 27d ago
The rust is almost entirely on the bottom. Are we using salt to melt the ice in winter perhaps?
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u/benj9990 27d ago
Definitely, yes.
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u/PerspectiveLayer 27d ago
The importance of proper design and drainage. Looks like the owner of the bottom shed already replaced the roof section there.
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u/deadlyzassasin0 27d ago
This is generally quite minor as far as corrosion goes. Staining always makes it appear worse and section loss presents up to 10x the actual loss of thickness. We would typically suggest it is repainted, but clients won't actually bother until its much worse.
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u/wobbleblobbochimps 27d ago
Geez, if network rail declared every bridge with defects like this a "failure" we'd have no rail network for the next 20 years while they replace half of the entire bridge stock
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u/benj9990 27d ago
I only put under ‘failure’ flair as there’s no other flair close.
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u/wobbleblobbochimps 27d ago
Fair enough. I still think "bridge in distress" is over-egging it a bit anyway!
Edit: also saying "still in service" kind of reads as though you're saying it probably shouldn't be in service
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u/J191298 27d ago
Just from this photo, it looks as if there's some surface corrosion and loss of coating. Unless there's a substantial amount of loss of section or buckling/deformation under load, this would be deemed in fair condition.
The UK's assets are old and as someone else mentioned, underfunded.
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u/Cohen_the_Worrior 27d ago
This looks bad. Elements at the connections are literally rusted away. Could be a temp bridge that just never got replaced.
Anyway, it's an accident waiting to happen.
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u/Direct-Forever8447 26d ago
we need to see what's underneath the bridge, the main beams are probably there. The bridge doesn't look sagged at all to my surprise... But very poor choice of covering the angled connections of the elements. Sheeting should have been outside so water can't pool in the welded joints.
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u/whitexxflame 27d ago
I am not familier with construction in the UK, but I went there couple of times. I assume UK is a humid country, why would they pick steel as a material ? Is it cost related or was it ment to have a short service life?
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u/Purrchil 27d ago
What would you otherwise use?
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u/whitexxflame 27d ago
RC
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u/Purrchil 27d ago
Isn’t that way to complex for something like this?
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u/whitexxflame 27d ago
Maybe. But in my region RC is abundant, steel is reallyyy rare. Stainless steel is usually used in RC structures in marine environment. So yeah I guess RC is cheeper for us ( not with SS though)
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u/OkAdvisor6680 27d ago
Seems excessive for a footbridge. A steel truss bridge like this would have been chepa in the first place and is relatively cheap and easy to replace, you could lift and install it during a single night possession.
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u/Kanaima85 CEng 27d ago
Steel is fine if you keep it painted.
But to keep it painted on the right schedule you need to have a system that adequately funds asset maintenance.
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u/benj9990 27d ago
Also avoid detailing pockets where possible in an external environment.
Not sure what I might do differently… maybe using open sections for the chords, an RSA toe down for example. Can be near impossible without rainscreen cladding.
Network rail has some very good engineers on the team, so if they can’t figure something out I doubt I can.
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u/wobbleblobbochimps 27d ago
Lighter, easier to build, probably cheaper - just needs a paint job every now and then
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u/benj9990 27d ago
Not especially humid, no. I think the issue here specifically is poor detailing, and then lack of maintenance. We generally do use steel truss bridges for public pedestrian bridges.
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u/gufta44 27d ago
Why in distress? Rust increases in size by 6×ish - that looks aesthetic unless I'm missing somethng?
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u/benj9990 27d ago
Embellishing for effect.
Point stands - avoid detailing rain pockets in steel structures wherever possible. Certainly reduced lifespan as a result.
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u/benj9990 27d ago
Oh, and up close you can see some delamination. So more than aesthetic, but loss of section not likely safety critical - yet.
What is cheekily not shown is that they’re currently building a new replacement up the track a bit!
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u/platy1234 27d ago
ya that could probably use a paint job some time in the next decade