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How much time would the people on the Brooklyn Bridge have to get off if all four main cables failed simultaneously? Are we talking about an immediate collapse the second the cables are severed, or are we talking some swaying back-and-forth for several seconds/minutes before it collapses?

The only examples I can find are the Morbi Bridge Disaster, and another pedestrian suspension bridge collapse that occurred shortly after, which lead me to believe the collapse is rather immediate; however, these are highly imperfect examples as the Brooklyn Bridge is worlds away from that bridge in terms of design and robustness. The only other suspension bridge collapse I have found is the Tacoma Narrows Bridge, but if I understand correctly, the cables on that bridge never snapped, the deck simply tore itself apart from the shaking of the bridge. I did find this video of the Fort Steuben Bridge being brought down by a controlled det, but because of the way it was brought down, all at once, it doesn't tell me about the stability of a suspension bridge without its cables.

So, the question is, if all four main cables of the Brooklyn Bridge simultaneously failed on one side where the cables connect to the ground, how long would it take for the bridge to collapse? If the bridge does not immediately collapse, I imagine how long it stands is affected by the load on it, so simulate any load you want, just make sure you state what you used in your answer. Also, the answer does not have to be specifically for the Brooklyn Bridge, I will accept the answer for any major suspension bridge; however, if you do answer for another bridge, please explain how the bridge generally compares with the Brooklyn Bridge, i.e. how many main cables does that bridge have and how big are they in comparison, as well as the spans of the bridge.

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    $\begingroup$ Engineers design bridges to be built and used, not how they would fail in a terrorist attack. $\endgroup$
    – Tiger Guy
    Oct 4, 2023 at 13:23

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If the Brooklyn Bridge was a familiar suspended-deck suspension bridge, it would probably collapse in less than a minute if all the main cables at one end failed simultaneously. The Brooklyn Bridge is, however, a hybrid cable-stayed suspension bridge, so its time to collapse is not so easy to estimate and would probably require a detailed simulation. Such a simulation is not immediately found online, and if it exists, it likely is kept confidential for very understandable security concerns.

We can, however, look at somewhat related suspension bridge failures and how the Brooklyn bridge has a redundant design that might make it surprisingly - albeit far from completely - robust against sudden main suspension cable failures.

Suspended-deck Suspension Bridge Failures

There are at least a couple of suspended-deck suspension bridge failures where one of the main cable supports on one side failed and the bridge collapsed:

The Hidalgo-Reynosa Bridge failed on 12 November 1939 due to electrolytic corrosion. This bridge had two main cables and a centre span of 137 m and a roadway width of 5 m. The upstream cable's mooring broke suddenly which caused the downstream cable to overload and to fail rather slowly allowing the roadway to gradually settle into the water.

The Silver Bridge failure on 15 December 1967 due to stress corrosion. This two main chain-cable bridge between Ohio and West Virginia had a centre span of 210 m and a total length of 681 m, compared to the four main wire-cable Brooklyn bridge's 486 m and 1834 m. One of the cables failed near the top of the Ohio tower. According to "The Silver Bridge Disaster of 1967", "the bridge start(ed) to fall on the Ohio side followed by a wave of destruction spreading to the West Virginia side". Eyewitnesses estimated it took from 20 to 30 seconds to a minute after the initial extremely loud crack of the cables failing until the collapse was finished. The loud crack gave just enough warning for at least one person who had just entered the bridge from the West Virginia side to reverse their car off the bridge, but 46 others on the bridge were killed.

Brooklyn Bridge Robust Hybrid Design

The road-bed of the Brooklyn Bridge is supported by a interconnected web consisting of both vertical suspension cables hanging from the main cables as well as diagonal stays from the tops of the towers. The designer, John Roebling built a number of such hybrid bridges, and the stays were typically intended to support 1/4 to 1/3 of the load and the rest by the suspension cables. Roebling designed a safety factor of five for the Brooklyn Bridge, and he apparently intended that the "stays be strong enough to support the roadway if the main cables should be removed or damaged." This suggests that if the main cables were suddenly cut, the roadway might not actually collapse, or at least not immediately or completely, but I certainly would not want to be one of the drivers on the bridge who would find out personally.

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