It’s a while since I’ve dug out my copy, but this book by Stanley Greenberg has some neat photos of the anchorage point of one of the main cables.
In Scotland they were considering replacing the main cables on the Forth Road Bridge (a fairly big classic-looking suspension bridge) and found that laying new cables alongside the existing ones wasn’t structurally feasible; more likely was a plan to lay the new cables above the old ones (presumably on new anchorage points built on the towers above the existing cables) and then remove the old ones.
Lots of detail in this (pdf) feasability study (scroll down to 4.1 or so for their findings)
In the end they decided to build a new bridge just upstream and reduce the traffic load on the original bridge!
Now, he’s a question for you: Let’s say that the entire population of Brooklyn gets turned into Zombies, and those zombies all start wandering across the bridge looking for sweet sweet human brains. Could the bridge support all that shambling weight?
Almost certainly. Trucks are much heavier than people. If you got them to all march in step with the resonant frequency of the structure, then maybe.
Regarding liquefaction, it seems unlikely to me that it would have been founded on liquefaction susceptible material, but I guess it’s possible. If they’re worried about that, it’s a much easier fix than replacing a cable anyway.
You’d be surprised how heavy a mass of people is. Trucks are spaced out as they pass over the bridge, but people are much more densely packed.
On the Golden Gate Bridge’s 50th anniversary, 800,000 people crammed onto the bridge, and the deck flattened out under the weight. In this pic, the deck has actually gone past “flat” to “Ah, let’s get these people off before it collapses.”
We know also have empirical evidence that it would be safe. Plus, the Brooklyn Bridge was way overdesigned due to analysis limitations at the time. It’d handle the weight much easier than the Golden Gate.
I find it interesting that both of the towers are standing on wood, albeit a thin layer compared to their overall height. The caissons were in effect upside down wooden boxes in which workers dug out the floor as the towers were built upon them. When they reached the desired depth, they were filled with concrete.
IIRC, the Brooklyn Bridge has a safety factor of 5 or 6 on the main cables. (apparently this takes into account the small quantity of substandard wire spun into them)
Also, the Brooklyn Bridge has 4 main cables.
Additionally, the bridge also has a secondary set of stay cables (the diagonal wires radiating from the tops of the stone towers).
I’m thinking we don’t need to worry about a cable snapping for a few more centuries . . .
Concrete is beneath the wood- the caissons were pumped full when they got to bedrock- at least on one side. On the other side (I don’t remember which- it’s been a while since I read The Great Bridge by David McCullough) they never did reach bedrock and said, in effect, “good enough”. But I’m pretty sure that where they stopped on that side wasn’t soft soil.
Ok. I assumed the fact that you found that interesting implied that it was risky or something. Many many building have entirely wooden foundations, btw.
Ok. I assumed the fact that you found that interesting implied that it was risky or something. Many many building have entirely wooden foundations, btw.
