Baltimore Bridge accident.

Hit by a massive cargo ship and collapses in its entirety.

Astonishing footage, how can a bridge over a mile and a half long collapse as quickly as that?

Had it happened during rush hour there could have been hundreds of fatalities. As it is, there were still vehicles on it at the time and a rescue operation is underway.

The spans were very long and there weren't many supports. That's why it collapsed. Why did it collapse so quickly? Take away the support and all that's left is gravity.

Probably a power failure on the ship.  Bridge supports are obvious single point of failure, surprised there wasn't more protection, especially as ships are bigger (heavier).  

A 150,000 ton container ship travelling at only 5 knots is a lot of momentum.   Bridge supports are designed to support a set number of vehicles from from above, so it's no surprise that a container ship would caused this.   It's not even the first time it's happened in the States.   Id have to Google the last one but that didn't end well either.

The mitigation here should be in the ship's design.   There should be enough redundancy to allow steering of the ship in a power failure scenario.   There probably is, to be honest, but maybe it didn't play out that way for whatever reason.   Professor James Reason's Swiss Cheese usually has the answer.

Recent reports say that the ship managed to contact the port authorities, who in turn managed to stop more vehicles getting on the bridge as soon as they could. So it could have been much worse, even at that time of night.

4 hours ago, Fly-By-TheWire said:

It's not even the first time it's happened in the States.

Sunshine Skyway bridge in Florida.  

The Scientific American explains:

https://www.scientificamerican.com/article/francis-scott-key-bridge-collapse-engineering/