Every time I cross the Brooklyn Bridge, I am astonished by its apparent poor condition, with abundant deep rust and so on. With all the traffic it carries and its condition, how is it that it is still standing? Is it really in as bad shape as it appears to be? And yet, I saw a bit on TV a while back that said the Tappan Zee Bridge (built in the early 1950’s, IIRC) is in the worst shape of all the area’s bridges, and at the end of its practical (safe) service life. So is there something about the Brooklyn Bridge that belies its otherwise decrepit appearance, or was the Tappan Zee made out of pressboard and Ikea leftovers?
What makes you think the rust is so deep?
Anyway, the Brooklyn Bridge was insanely over-engineered and will likely live for a long time yet. It was designed and built to last forever. The Tappan Zee was designed to be a piece of junk. It was built during the Korean War when there was a steel shortage, and it was always intended to be replaced. (You know how governments are about “temporary” structures.) It’s basically a big Erector set and about as sturdy.
Anyway, they’ve fortunately started work on a replacement.
Gotta love temporary wartime structures. In the late 70’s I drove over a highway (I-94) that had been built as a “temporary” road between Chicago and Detroit (or, nearly Detroit), that never got a proper foundation, so was always needing fixup work. They finally redid it and put down a proper roadbed around 1980, IIRC.
Better a highway than a bridge, though!
The brooklyn bridge was designed before we had the ability to analyze it fully. In those cases you should design conservatively. It’s extremely over designed. Same with Hoover dam.
Looking at rust like this, likely similar to what you saw from the walkway, I have two observation:
The rust is superficial; its basically the just the paint flaking off, and a bit of oxidation occurring on the temporarily exposed surface. I walked over the bridge last summer, and they were in the process of repainting the bridge, I believe. Bridges need basic maintenance, and even an over-engineered hulk like the Brooklyn Bridge needs some TLC if it’s going to last forever.
The second is that the rusty pieces no longer support any weight! The bridge originally had trolly lines that paralleled the wooden walkway that were supported by the rafters. They were dismantled sometime after the Manhattan Bridge, allowing the subways to be extended into Brooklyn/Queens.
A lot of bridges (and other structures) are designed to rust, so rust alone isn’t an indication that things are necessarily bad.
In plain old vanilla steel, the surface rust will flake off and expose metal underneath, which then rusts and flakes off, etc. until the entire structure gets eaten away. Certain alloys of steel don’t flake so much when they rust, though. This allows the surface to rust, which then shields the inner metal from oxygen in the air and prevents further rusting. While the surface ends up looking pretty crappy and rusty, it doesn’t require constant painting for protection and therefore requires a lot less maintenance over the life of the structure.
I have no idea if that is what is used on the Brooklyn bridge though.
Basically, the Tappen Zee Bridge was built was built dirt cheap on a pile of mud.
The bridge’s piers sit atop of wooden poles driven deep into the Hudson River bed. The piles are now settling under the weight of more traffic over a longer period of time than ever expected, and the bridge is starting to experience extra strain.
It’s still perfectly safe, but requires a lot of structural maintenance to keep it from outright sinking! Maintenance is expensive in both labor and delays for commuters. The new bridge is expected to last at least 100 years, with only routine maintenance (http://www.newnybridge.com/about/index.html).
I like the Project Updates at that site. “Construction is expected to start in early 2013. Please check back for updates and alerts.”
Bridge engineers, please feel free to correct, but as a wee lass growing up in brooklyn, I learned that John Roebling was very aware of the limits of his knowledge; after calculating the weights and loads that could be anticipated, he multiplied by six.
In other words, every aspect of the the Brooklyn Bridge was designed to be at least six times stronger than it needed to be, by any calculation known at the time.
One of my favorite quotes:
Construction has begun, and updates can be found on http://www.newnybridge.com/news/index.html though the website is clearly a bit lacking.
If true, that’s not as excessive as you may think. Traditionally, most structures are built with a factor of safety of “3”, rather than “6”. The Brooklyn Bridge was new technology, so he doubled the factor. As knowledge of materials improves, the factors of safety can be narrowed.
This was definitely true in the case of the four main suspension cables carrying the bridge superstructure. Luckily so, as when spinning the cables was well along it was found that the contractor supplying the wire had provided shoddy steel for the wire used in these cables. Roebling then calculated the cable strength using the strength for the poor steel together with the strength of the remaining wires that would be of the correct steel, and found that the safety factor was now four instead of six. It was decided that this was good enough and the bridge was completed with these 4X cables. Obviously, this was a good decision.
BTW, I rather expect that to a modern bridge engineer, Roebling was a God. Certainly is to me.
Some people in my university are doing tests on the cables of the bridges. My class briefly talked to them, and they told us to not go on the bridges (or perhaps it was a materials science professor who told us that).
What was the quality of the steel made back then (in the 1870s-80s? We now know that the steel used to construct the RMS Titanic was pretty crappy-it would shatter like glass, when cooled to the freezing point of water.
Indeed. I recently read The Great Bridge (1972), by my favorite historian David McCullough. The story of the Brooklyn Bridge. Very detailed. I recommend the OP read it. The wife and I walked both ways across the Brooklyn Bridge last year and thought it magnificent.
The bit I heard on the radio about this said it was the rivets that were brittle in the cold, not the plate.
Well, nobody really does allowable stress design anymore, but I believe that going by the steel design manual (I’ve never designed a bridge) you’re required to design for an equivalent of a factor of safety of around 1.7. A factor of safety of 3 is often used in foundatiin design, but that’s because soil properties are far less well known than what comes out of a steel or concrete plant.
Or the Roman bridges and aqueducts. According to a history of engineering book (from the 60’s, but still an interesting book), the Romans didn’t know about the parallelogram of forces, so they couldn’t compute or predict transverse loads. (It also says they didn’t know about trusses, which is hard to believe. You can’t make a stone truss, but one assumes when building wooden roofs it would be obvious …)
Anyway, as a result of that, they overbuilt. They didn’t overbuild because they wanted things to last forever. Emperors spending money on public works wanted to get the most for their money, but without running the risk of an embarrassing failure.
Romans also didn’t know that elliptical arches can work. They’re all circular, and possibly even all semicircular, even though smaller arc sections can work. I find it a bit surprising, given all that the Greeks had figured out about conic sections. I guess the Romans just couldn’t do the math.
Well, it’s not easy doing multiplication or division using Roman numerals.