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Collapse of the Fern Hollow Bridge
- aaron695 5y ago
- Kon-Peki 5y agoComplete speculation based on reading the linked report: The bridge suffered a fatal compromise but stood with light traffic. Until some combination of speed and weight of the bus pushed it over the edge. Something happened in the minutes or hours before the collapse and it was doomed to fall.
- sgeisenh 5y agoThe weird thing about it all is that this bridge is very heavily trafficked during rush hour. Occasionally, traffic even backs up to the bridge. I'm really curious what could have happened to cause it to collapse so catastrophically when it withstood far more traffic just ~12 hours earlier.
- YXNjaGVyZWdlbgo 5y agoTime, weather and lack of maintenance.
- bobduke 5y agoFrom the article, the posted weight limit was 26 tons. A quick google search ("bus weight") shows that buses can easily exceed that limit and the bus on the bridge was a "double" bus. Maybe the bus wasn't supposed to cross that bridge?
- michaelrpeskin 5y agoI seem to remember that bridge weight limits aren’t as clear as just a number like that. I’d have to look at the DOT site, and I’m on my phone and too lazy right now. But I’d be willing to bet that a limit is per axle with other factors multiplied in for how close tandem axles are. It’s more about weight distribution than total weight. The posted signs are meant to be read by truckers who know the formulae.
- intrasight 5y agoA couple days before the collapse, there was a heavy snowfall. Deep heavy snow. I calculate that the weight of the snow was about 200 tons.
- Johnny555 5y agoHow did you calculate that? I don't know how much snow there was, but 10 inches of snow has around 1 inch (2.5cm) of water. The bridge was 136m long, by about 13m wide and assuming 2.5cm of water content: 136m * 13m * .025m = 44 m^3 of water, or 44,000 kg, roughly 50 US tons. A newspaper report said there was a light snow with 1 or 2 inches accumulated since 2am the morning of the collapse. Photos of the scene showed grass and small rocks on the ground visible through the snow, so it doesn't seem like there was a lot of snow on the ground. Even if there was heavy snow a couple days before the collapse, most of it would have been plowed off, I don't see big berms of snow on pictures of the collapse. So if there was just a couple inches of snow, then it was probably closer to 10 tons worth of snow. https://www.cnn.com/2022/01/28/us/pittsburgh-bridge-collapse/index.html https://www.cnn.com/2022/01/28/us/pittsburgh-bridge-collapse...
- intrasight 5y agoWe had almost 8 inches of heavy snow. But Still my math must have been off (did in my head). Doing again I get 10 lb/ft * 450ft * 45ft / 2000 = ~100 tons
- beerandt 5y agoThis is a failure mode already accounted for in determining the posted weight limits.
- alksjdalkj 5y agoLooks like the bus named in the NTSB report is 39000-45500 lb, so under the bridge's weight limit: https://en.wikipedia.org/wiki/New_Flyer_Xcelsior https://en.wikipedia.org/wiki/New_Flyer_Xcelsior
- mikeyouse 5y agoPerhaps it had been in a slow collapse for awhile and the 'point load' of the heavy bus in that moment caught it just right. There are pictures from several years ago of major structural crossmembers completely rusted through to the point they weren't even attached; https://twitter.com/gpk320/status/1078885655634157569 https://twitter.com/gpk320/status/1078885655634157569
- ratsmack 5y agoJust from that picture alone, the bridge should have been condemned long ago.
- bobthepanda 5y agoThe odd thing to me is that a bus route is regular. I would bet that a bus this size runs on that bridge fairly regularly throughout the day; a bendy-bus is never used on a route that doesn’t need it, it’s harder to drive and uses more fuel.
- jffry 5y agoI could imagine that having periodically changing loads like rush hour traffic could cause a critical failure state. The slow flexing between day and night eventually causing something to deform/crack/shift to the point that the next big traffic load causes a collapse. If prior NTSB reports are any predictor, we'll eventually learn all about the confluence of things that led to the collapse.
- hydrogtehd 5y agoIt was a nasty night, weather wise, if I recall. Perhaps a freeze thaw cycle that split one to many bolts?
- R0b0t1 5y ago
- gameswithgo 5y agoI think you are right, something happened.
- mdoms 5y agoYour speculation is that the bridge suffered a failure prior to the failure of the bridge? Thanks for the insight I guess.
- tomcam 5y agoIt was probably me. I really need to get back on my diet tbh
- nja 5y ago> The Port Authority bus was traveling eastbound at the time of collapse. The bus was equipped with seven cameras: one forward-facing camera, one right side aft facing camera, and five interior cameras. Video data from these cameras have been recovered, and the initial assessment of the video data is consistent with the initial assessment of the bridge components. The National Transportation Safety Board (NTSB) will convene a group of specialists to analyze the video for further information useful to the investigation. I hadn't thought about bus cameras capturing the collapse -- wonder if this'll ever be released (or leaked)?
- CaliforniaKarl 5y agoResearch Pennsylvania's Right-to-Know Law. There may be an avenue that would let you request copies of the video.
- rhapsodic 5y ago> There may be an avenue that would let you request copies of the video. If that's true, my guess it that the mainstream media is aware of it and will have copies of the video.
- masklinn 5y agoYe folks seem to be way jumping the gun here, the ntsb generally makes their evidence public.
- petee 5y agoNTSB usually puts those videos in it's final reports, or as side footage on youtube if nothing is obviously graphic
- olliej 5y agoTLDR is that there is no obvious cause yet. The report says that there were no failures in the fracture critical components. There is no information: on state, maintenance, geologic activity, or initial failure point. While interesting any claim of cause is at absolute best an “educated guess”.
- ehnto 5y agoI wonder if maintenance schedule and state information should be gathered in a public database. I know US has thousands of entities of infrastructure but some manpower and some tablets is all you would need.
- Someone 5y agoYou mean like the National Bridge Inventory (https://www.fhwa.dot.gov/bridge/nbi.cfm https://www.fhwa.dot.gov/bridge/nbi.cfm)?
- engineer_22 5y agoMoney is the limiting factor. Tax dollars are already stretched thin. In many areas of the country infrastructure maintenance is at unsustainable levels with no easy solution. You cant squeeze blood from a stone.
- mrguyorama 5y agoExcept the stone in this case is absolutely dripping with blood being slurped up by fat scavengers and building aircraft carriers (because we NEED 12 of them of course) while some asshole wags their finger at us and tells us this pinch of sand is all we can afford.
- engineer_22 5y agoI take your point in good spirit, but there may be an argument to be made that military power is a net asset. But it's beside the point of my original comment. Local infrastructure budgets are not equipped to keep up with the demands.
- Yoofie 5y agoThe most surprising the thing from the article is the posted weight limit of 26 tons. I'm no structural engineer but that seems very low to me for the size and length of the bridge? From the looks of the picture, shouldn't that bridge support like 5x to 10x that weight? A quick google shows that the average car weight is 4156 lbs [1]. So the bridge can only support roughly 5.5 cars before its over limit? And that is not considering common heavy vehicles likes semis, buses, etc. And with auto industry going to toward EVs, vehicles are only going to get heavier, not lighter. The upcoming Hummer EV will be 9000 lbs [2] for example. This bridge seems awfully under-spec'd for its purpose. Someone please tell me that I am interpreting this number wrong. [1]: https://www.bankrate.com/insurance/car/average-car-weight/ https://www.bankrate.com/insurance/car/average-car-weight/ [2]: https://www.google.com/search?q=hummer+ev+weight https://www.google.com/search?q=hummer+ev+weight
- pwr-electronics 5y agoBtw, 1 ton = 2000 lbs, not 1000.
- titzer 5y agoI think the posted limit is supposed to be per vehicle.
- petee 5y agoLikely derated with time; a lot of bridges are given lower capacity as they wear out
- hackcasual 5y agoWeight limits are per-vehicle.
- pwr-electronics 5y agoImagine having to size up the other vehicles before crossing a bridge instead of just worrying about yourself.
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- snt 5y agoAnimation of the collapse and causative factors: https://www.youtube.com/watch?v=zKfmEBcxs8A https://www.youtube.com/watch?v=zKfmEBcxs8A
- masklinn 5y agoThis video doesn’t start with “hello I am grady and this is practical engineering”, it’s unwatchable (also the sad / doomy music is distasteful, and so is claiming knowledge of causative factors when the incident is under active investigation).
- rhapsodic 5y ago> The bridge was an uncoated weathering steel, three-span... The somewhat larger New River Gorge bridge is also made from that type of steel.[1] [1] https://en.wikipedia.org/wiki/New_River_Gorge_Bridge https://en.wikipedia.org/wiki/New_River_Gorge_Bridge
- badsoftware 5y agoVery interested to hear the outcome of this investigation. As a Pittsburgh resident, it's pretty common knowledge that our ~450 bridges are not in great condition, and this just confirms it. The last inspection on this bridge was done in September of 2021 - Although it was determined to be in "poor" condition, it wasn't deemed unsafe. Less than 6 months later it collapses. How many more of our bridges are this close to collapse?
- athrun 5y agoIf I remember things correctly about 45,000 bridges across the US are in poor condition and the original Infrastructure Bill targeted 10,000 of them. The Bill got watered down further. There is simply too much infrastructure to maintain, and more and more stuff is being built every day with no realistic plan for long-term maintenance. It's a losing battle.
- spoonjim 5y agoStrong Towns makes the case that there is just too much infrastructure in the US and the level of sprawl that we are built at is economically unsustainable in the long run.
- dahfizz 5y agoExcept for a very generous interpretation of "the long run", I don't buy this. We pay more than enough money in taxes to maintain our infrastructure. The problem is governments misallocating and wasting funds.
- phonebanshee 5y agoMostly to corporate subsidies labeled "defense" spending.
- ars 5y agoThe world Strong Towns wants is emotionally unsustainable. Maybe there are people who can handle it, but I would go insane in the dense cities it suggests. Let the people who like dense towns live there, and stop trying to tell the rest of the people how to live. Strong Towns also tries to creates cities where no one from the outside ever comes in, since there's no way of getting there (no parking, not enough roads). People in large cities also like to "get away" during the summer - but there will be nowhere to go in his world.
- basicplus2 5y agobefore picture of bridge from below on googlemaps https://maps.app.goo.gl/prRF494MztYAhXoW6 https://maps.app.goo.gl/prRF494MztYAhXoW6
- everyone 5y agoI'm assuming Corten steel would never be used structurally nowadays? I know it's sometimes used as a fancy cladding material, but I'm assuming thats all?
- pvaldes 5y agoPoor maintenance, bad decisions to remove part of the bridge support instead add another strip or just weld it again and ... salt in the roads that maybe end in the water or drip from the road above. Corten steel is very durable, add salt and this is a different question.
- D13Fd 5y agoCorten steel is often used structurally, even in recent structures, because the idea is that the rust forms a protective layer. As long as water is not allowed to pool, it shouldn't rust through. Some pictures of Corten steel structures: https://www.corten.com/what-is-corten-steel.html https://www.corten.com/what-is-corten-steel.html I'm familiar with the structures shown in that link. The U.S. Steel Tower seemed to be holding up well when I worked there, although looking out some windows you could see slightly worrying depressions in the steel beams where water had pooled over the years, and the steel had rusted away a bit (leading to more pooling). The Delaware footbridge was constructed much more recently and still looks new - no signs of wear that I've noticed walking across it. If Corten steel is not durable enough for structural use, we're in trouble. It's used quite a bit, at least in the northeast.
- MisterTea 5y ago> 447-foot-long bridge > The posted weight limit on the bridge was 26 tons. Is it just me or does this weight to length limit seem incredibly low? That articulated bus empty likely weighs 20 tons. An artic bus plus a few cars is already well over the 26 ton limit. I'm sure this bridge has also seen a fair share of truck abuse as the abysmal weight limit combined with the likelihood that truckers only see the weight limit once they're at the bridge with no way to turn around and no easy bypass route so they say screw this and drive their >26 ton rig right over it.