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Comment by sgeisenh | Hacker News Reader

The 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.

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Perhaps 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

Just from that picture alone, the bridge should have been condemned long ago.

The 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.

I 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.

It was a nasty night, weather wise, if I recall.

Perhaps a freeze thaw cycle that split one to many bolts?

Time, weather and lack of maintenance.

From 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?

I 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.

A 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.

How 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...

We 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

This is a failure mode already accounted for in determining the posted weight limits.

Looks 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

But what if it was at that top range, 22.75 tons. The math would add up, with 5 other cars on the bridge weighing an average of 1.4 tons each, we would have 29.75 tons or so on the bridge. Before the 10 or so tons of snow is added.

Bridge weight ratings are per vehicle.

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