I’m thinking of bridges like Thomas Telford’s Menai Straights Bridge (ca 1827), or the Brooklyn Bridge (1888?). Presumably, the old-time engineers used 400-600% safety factors-but iron and steel rusts, and old cables in suspension bridges will fail in time. So how does one test such an old structure, to be sure that it is still safe to use? On something as old as the Telford bridge, is there any reason to thing it would fail unexpectedly? Or even the Golden Gate Bridge (ca 1936)-how do we know that the suspension cables are still strong enought to support the loads?
I can’t speak for the others, but there is daily maintenance work on the Golden Gate Bridge. The cables were inspected in 1998 but restoration has apparently been delayed.
Well, first you hire an engineer certified in bridge inspection to go over the bridge and check for obvious problems like bent truss memebers, rusted rivets, etc. You’ll also pull up the original plans (which are usually available somewhere,if not , older bridges aren’t horribly complicated) and see what loads the bridge was originally designed for. Often times you can also find the original materials tests from the steel mill so you can get a rough idea of what the original material strength was. You also have to get a rough estimate on the amount and types of traffic the bridge has seen in order to find your fatigue factors.
Then you take all that information and run some calcs to see what the current carrying capacity is (pretty much the same calcs you’d use for a new bridge just using different numbers). You’re correct, a lot older bridges were really overdesigned by today’s standards so it’d be really unlikely for an old bridge to fail unexpectedly unless it wasn’t properly maintained (the Golden Gate Bridge is rigorously inspected and maintained) or there was a incredibly weird design flaw that had never been spotted.
Yes, civil and mechanical engineers know how to assess the safety of bridges. It is up to politicians to act on that information. There are numerous accountslike this one from 2005 that indicate the politicians are doing a damned sloppy job of it.
I was part of a design team that developed a wireless system to test bridges. Here’s the article.
And here I though all they did was park a load on the bridge and measure how much the bridge sagged.
They subject the bridge to larger and larger loads until it fails. The weight of the second to last load is what the bridge could support.
All that tells you is what the load is and where it is on the bridge. But we knew that already, didn’t we?
Thank you, Calvin’s Dad.
Well, that’s a big part of it.
Five years ago I was part of a small team that observed live-load measurements on Ohio’s first all-composite bridge. A company that specializes in bridge testing (Bridge Diagnostics, Inc of Boulder, CO) performed the testing.
Basically, a big, heavy truck was slowly driven over the bridge multiple times. Measurements were taken each time the truck drove over the bridge. BDI’s system consisted of strain gages affixed to the bridge at certain locations. They also used LVDTs to measure movement at the abutment and sag at the center of the bridge.
Was I the only one expecting that by “extremely old” the OP meant 1000 or 2000 years old bridges?
Gee… I guess this illustrates the difference in perception in Europe and in the USA about what “old” means.
In America they think 100 years is a long time. In Europe they think 100 miles is a long distance.
Near where I live there is a stone bridge built in about 1750. There are no weight restrictions on it and 44 tonne trucks can drive over it. This compares to several concrete bridges I know about which were built 20 -30 years ago that had to be reinforced when heavier trucks were allowed on the roads.
Well, I am sure everybody has heard of the “twist” test of the Tacoma Narrows bridge in 1940.
The video (easily found) is amazing, and I understand engineers learned a lot from the incident
Well, this bridge near me isn’t that old - 40 years or so (I still remember crossing on the old ferrys!) but the individual strands of the main cables are starting to corrode and
from here and this update also.
There’s now a major debate about building a second crossing so the load on this bridge can be reduced. But, knowing our politicians, it’ll probably have fallen down before they decide between a new bridge or a tunnel, let alone actually build it!
The reason why some old bridges are so strong is that they were over-engineered. One case in point is the Forth Rail Bridge. This was built just after the Tay Bridge fell into the water, taking a train and passengers with it. Just to make sure on the Forth Bridge, the design engineer put (I think) twice as much steelwork into it than was necessary, just to be on the safe side.
And 100 years on, there’s always work being done: hanging scaffolding
There’s always some section of it swathed in sheeting or covered in scaffolding - and it’s not just for painting it!
Roebling designed the Brooklyn Bridge with some huge safety factor (four times, was it?), which ended up saving his ass when the corrupt politicians of the Tammany Administration forced him to take defective wire. :eek:
He’d reject a wagonload of wire; they’d drive it back to the wire factory, in the front door, out the back door, and right back to the construction site. He couldn’t disbar the supplier because they had political connections. Finally he got a good supplier, after a certain amount of bad wire had gone into the bridge, did his calculations, and found out he still had enough strength left, despite the bad wire. <<whew!>>
That’s for sure. They were awfully slow learning it, though. The bridge had problems with flexing and had the nickname “Galloping Gertie” quite a while before the collapse. The design didn’t call for it to move so much and this was a tip-off that something needed to be done. So one thing that was learned was, when something doesn’t act as it was designed to act, look into it.
Of course it’s possible that engineers did sound the alarm and were ignored by the political leaders.
Bridge engineers were well aware of the flexure of suspension bridge decks long before Galloping Gertie. In fact, the Brooklyn Bridge has an innovative system of cables below the deck to stabilize it. Gertie was a daring design, with a deck thinner and weaker than many bridges before it–and that’s why it got into trouble. Frankly, it looks a little like negligence on the part of the designers.