One day out of the blue…the hundreds of wires making up the support cable start snapping. Eventually, a cable breaks, and the bridge plunges into the river.
Could this happen? The cables are now over 120 years old, and have been exposed to weather , freezing/thawing, etc. Is the bridge really safe?
I wonder if modern materials experts have analyzed the old cable, and what they have to say about its safety (remember, it was made in the era of blacksmith shops).
It sure could…but only if it’s not regularly maintained (the cables are all galvanised, inspected, painted and replaced when needed). Life After People (skip to about 3:21) covered the Brooklyn Bridge, they estimate that it would last about 100 years without maintenance.
Anything could fail catastrophically at any time. As for the Brooklyn Bridge, it’s perfectly safe, but it has a few problems, mostly with the approach ramps to the bridge. (The span itself seems to be fine.) The City does a complete inspection of all city-owned bridges every two years.
Are you sure about that?
I remember checking out (doing the curious tourist thing, not an official inspection) the suspension cable at the anchors a few years ago. The strands of the cables didn’t look galvanized to me; they looked like ordinary mild steel.
Also, how would they replace a deteriorating cable strand anyway? The strands are all spun, then bound in a bundle with another spun covering.
I can see replacing or repairing the hangers, but the actual suspension cables?
You just make a new one and swap it out. The bridge has plenty of redundancy; one missing cable is not going to cause it to fail.
I’m not sure we are both talking about the same thing?
The suspension cables go from shore to shore and over the central pillars; each of these is absolutely critical to the functional integrity of the bridge. If any of these were to fail, the bridge deck would fall.
Each of these suspension cables is constructed of dozens of individual strands of steel wire, spun around each other into a bundle.
Each of these individual strands spans from shore to shore, and is individually anchored at each end of the bridge.
The entire composite bundle is wrapped in a “sheath” of wire, which is weather proofed with a coat of paint.
That being the case, I find it hard to visualize how an individual strand could be replaced.
I speculate that the suspension cables have enough redundancy to be able to withstand a lot of deteriorated or broken strands. At some point, enough of them would become unserviceable, and the whole bridge would have to be replaced.
On the other hand, the individual hangers go from the suspension cable to the bridge deck; I imagine these would be no big deal to replace.
No answer, but I’ll mention that the Brooklyn Bridge undergoes a missile attack in the movie I Am Legend and is struck by the tail of the monster in Cloverfield, and collapses both times.
I think you guys are talking about different cables. The main cables go over the arches and can’t be replaced easily, if at all. The “suspender cables” (edit, ‘suspendor’, not ‘suspension’ cables) are the vertical cables holding up the road which I assume can be replaced as easily as RealityChuck said.
All the cables were replaced on the Golden GAte some years ago. Do not know how they did it but it wasw not easy and took some time.
The vertical suspender cables were replaced in the '70s. He’s talking about the ones that span “from shore to shore, and is individually anchored at each end of the bridge.” Those are hard enough to replace on a large suspension bridge that it may as well be called impossible.
Okay, I’ll bite. What’s so hard about replacing them?
Because they are holding up the bridge. And they are all bound together and anchored at each end in a permanent way. The whole cable is one piece, so you can’t replace it strand by strand.
Here is a schematic of an anchor that would be at each end of the main cables.
But they protect those cables pretty well. They maintain the coating so water can’t get in, they are sealed in there and probably don’t undergo much degradation, if any.
I Am Not A Structural Engineer, but it looks like there are only four main suspension cables on the Brooklyn bridge. I’d imagine all four are needed to hold the thing up. So to replace one, you’d need to install a new one before removing the old one, and it doesn’t look like there’s much space to play with. Of course, I have no idea how you’d go about even building a suspension bridge in the first place, and how you’d string that cable across. Big structures like this boggle my mind, especially when you think this was built in the 1870s.
Well yes. 
But isn’t the bridge strong enough to cope with the loss of a strand or three?
Surely you can? Either you use the old strand to pull through the new strand or the new strand goes outside the main cable bundle; replace all the strands and you have a new cable bundle.
Here is an example of what the inside of a suspension cable looks like. Each one of those colored hexagons contains 127 individual strands. Given that they’re constantly under enormous tension, I don’t think there’s any way that you could replace one strand at a time by pulling it through. There wouldn’t be any room.
Instead of replacing existing strands on the cables, couldn’t new strands be added to the outside of the cable, since the strands are anchored individually? I would think the new strands could be made of something stronger and more corrosion resistant, like carbon fiber.
I don’t know that the Brooklyn Bridge cables are the same (I assume they are), but here’s the info on the Golden Gate Bridge. The cable is bound every few feet under great pressure. I don’t think they can replace just a few strands.
Yes – that was what I was talking about.
The shore-to-shore suspension cables are built in place, strand by strand. It’s not possible to have someone spin up a couple miles of 3 foot diameter cable and pull it across - it’s just too massively heavy.
The main cables on the Golden Gate Bridge are 3 feet in diameter and each is composed of over 27,000 individual strands of wire. In total, each cable contains 40,000 miles of wire. The Brooklyn Bridge’s cables are 15" in diameter and made of 5400 wires. That’s still going to be very heavy.
I don’t know about the Brooklyn Bridge, but every ten years, inspectors pick a section or two of cable on the GGB for intrusive testing. They unwrap the outer layer, then drive wooden wedges in so they can see the inner bundles of wire. So far, its holding up well.
And too much friction.