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Originally Posted by OldDartmouthMark
2) Bridge construction: I think most people were surprised at how quickly the Key bridge in Baltimore collapsed into the harbour. I know I was, but with each agonizing sequence played over and over again on the news channels (how many times can we watch those poor construction workers die?), it became clear that the integrity of the truss construction of the bridge was completely dependent upon both support points being intact. I can't say for sure, but I imagine the bridge deck on Halifax's suspension bridges would be more difficult to bring down completely due to their method of support and the nature of suspension bridge configuration.
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The Key bridge despite being built in the '70s was an unusual and now obsolete design called a continuous truss. It was what is called "fracture critical" in the loads are carried through the entire structure and it has no redundancy should a single part fail - thus why it is called "fracture critical". We can see the sad results of that in this case. It was quite shocking how quickly the entire structure failed. Suspension bridges like those in Halifax are not immune to failure but would typically not have the sudden result we saw here. Plus as Saul noted the tugs that are employed here every time a large ship passes under our bridges were only used in Baltimore to get the ships off the dock and then peeled away, another unfortunate choice.
I learned this from watching the impressive Jennifer Homendy, head of the NTSB, whose press appearances are posted on YouTube. She indicated that there are a number of such bridges still in service in the US. Also there you will find "B-roll" footage posted by the NTSB on their YT channel of what they found after they arrived. Worth a watch.