Indeed, the whole bridge failed because the impact didn't exceed the impact strength of the bridge's material, which means the whole bridge bent until the deflection exceeded the tensile strength of the material. A missile impact with the same total energy would exceed impact strength and would probably have destroyed only part of the bridge.
Did the math, this seems surprisingly spot on, at least from the energy perspective. 100k tons going 7.5 kts is about 160kg tnt equivalent which is a little less than the explosive power of a Mk83/GBU-32, probably not enough to take down an entire bridge, just one span if you're lucky. That said, the issue is the bomb expends most of its energy moving air around, the freighter expended nearly 100% of its energy on the bridge.
There's also the loads are balanced on the pylon. When the collapse took out the bridge on one side of the pylon, the other side failed as well. You can see that in the video as the bridge on the right side of the pylon is no longer counterbalanced by the bridge on the left side, and collapses.
Imagine you're holding up a barbell with heavy weights at each end. If the weights on one end drop off, the bar will tilt strongly in the other direction, dropping the weights off on the other end.
Thank you for breaking it down like this. As someone without an engineering background, your explanation (along with some Googling for terms and definitions) really helped me grasp what happened.
You got me wondering:
> the whole bridge failed because the impact didn't exceed the impact strength of the bridge's material
When I first read this, it initially threw me off. The cargo ship's impact not exceeding the strength of the bridge sounds like a positive thing, but upon closer reading of your comment, it sounds as if it was the catalyst to the entire bridge collapsing.
So, how do engineers balance these properties of impact strength and tensile strength, especially considering large ships channel through these bridges near their pylons frequently? How much engineering goes into the possibility of large structures hitting their pylons?
> The cargo ship's impact not exceeding the strength of the bridge sounds like a positive thing, but upon closer reading of your comment, it sounds as if it was the catalyst to the entire bridge collapsing.
I had intended to add that exceeding the impact strength typically results in a local failure near the impact point. A tensile strength failure could happen far from the load and so could be more catastrophic.
I'm not sure if it would have made a difference in this case though, as destroying a main pillar by exceeding impact strength would have by itself transmitted most of the full bridge load to the remaining pillars and that alone may have been enough to exceed the safety margins on the tensile strength that are built into all structures. Unclear without more data, but there was a chance it could have survived in that case, but no chance with the ship consistently applying more and more shearing load.
> How much engineering goes into the possibility of large structures hitting their pylons?
Good questions, I'm not familiar enough with it to provide any further insight, except to say that I believe this bridge was designed long before these huge container ships existed. If they factored ship collisions into the bridge's design constraints at the time, they've no doubt been dramatically exceeded with these huge ships.
I don't think impact strength is factored very much into static structures, tensile strength is more important. It only comes up in very unusual situations like this or 9/11.
Bridges were not designed for being hit. They were designed to span. If those designing and engineering it had considered an impact, the design would not be catastrophic but rather sectional. If sectional, each span would remain standing rather than collapse like dominoes. It takes 3 miles to stop a 100k ton stacked container ship. The things nobody has mentioned are: Current direction and speed. Wind direction and speed. The entry of the vessel in relation to the bridge. The fact that dragging anchor would have on the starboard side pulled the bow right into the pilon, the worst case.
