This is not how a suspension bridge works

27 Jun

I noticed this in the trailer for Dark Knight Rises:

In my screen capture, you can see the Williamsburg Bridge in the foreground and the Manhattan and Brooklyn Bridges in the background. (It’s worth nothing that Manhattan and Brooklyn are mirrored for some reason.) Seconds ago, all three were blown up at their center spans by Bane, a villain who is probably smart enough to know that this is not at all how suspension bridges behave.

A suspension bridge works by draping two very thick cables over two (or more) towers, and then anchoring each of these cables in a large abutment.  Vertical suspender cables are then hung from the larger draped cables, and the road deck is then attached to the vertical suspender cables. Everything is hanging on the draped suspension cables, which are under tension. If these cables are severed then they would have no way of staying in tension, and they would fail entirely.  The full span would collapse into the river, though the towers would remain standing.

Bane has clearly cut the main suspension cables with his demolition charges, but only nice, neat section of road deck drops into the river below. The span remains impossibly intact.

Picture a clothesline carrying a bunch of laundry. The line would be equivalent to the draped suspension cables, the clothespins would be the vertical suspender cables, and the laundry would be the road deck. If you took a pair of scissors and cut the clothesline in the middle, you wouldn’t have a clothesline anymore, you’d have a pile of dirty clothes on the ground.

Botching bridge physics is pretty much par for the course in movies, but this is still pretty surprising coming from a detail-oriented director like Christopher Nolan. Either he and his entire special effect crew misunderstood how suspension bridges work, or they understood fully that this was a mistake but wanted the bridges to be destroyed in this way for whatever artistic reason.


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