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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 bri

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?

michaelrpeskin 5y ago
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.
intrasight 5y ago
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.
- Johnny555 5y ago
  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/index.html https://www.cnn.com/2022/01/28/us/pittsburgh-bridge-collapse...
  - intrasight 5y ago
    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
- beerandt 5y ago
  This is a failure mode already accounted for in determining the posted weight limits.
alksjdalkj 5y ago
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 https://en.wikipedia.org/wiki/New_Flyer_Xcelsior
- mosseater 5y ago
  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.
  - jachee 5y ago
    Bridge weight ratings are per vehicle.

michaelrpeskin 5y ago

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.

intrasight 5y ago

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.

Johnny555 5y ago
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/index.html https://www.cnn.com/2022/01/28/us/pittsburgh-bridge-collapse...
- intrasight 5y ago
  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
beerandt 5y ago
This is a failure mode already accounted for in determining the posted weight limits.

alksjdalkj 5y ago

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 https://en.wikipedia.org/wiki/New_Flyer_Xcelsior

mosseater 5y ago
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.
- jachee 5y ago
  Bridge weight ratings are per vehicle.