Complete 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.
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;
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.
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.
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.
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.
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.