posted
Since I'm a registered professional civil engineer in two states, I'll try to answer Train Lady's question.
Lots of things can cause a bridge to collapse. We've all seen film of the Tacoma Narrows Bridge--sometimes called Galloping Gertie--as it failed in wind. No weight at all on the bridge. Certainly an earthquake could make a bridge come down, as happened in the East Bay during the Loma Prieta earthquake, which also took down the bell tower at my high school. What happened in Minnesota was probably the failure of some specific component on the bridge. That bridge was designed in the B.C. years--Before Computers. It's quite simple now to make a computer model of a bridge and allow the computer to simulate component failure. And then modify the design.
The Minnesota bridge was a steel arch. Arches are common bridge structures, the Roman aquaduct bridges were built with arches. The Roman bridges, though, used stone or masonry for their arches, an ideal material. Steel arches are generally made from pieces that are connected together, usually with rivets or high-strength steel bolts. These bolts or rivets can fail, particularly when the road maintenance crews use salt in the winter.
For these reasons, bridges have to be inspected carefully. Federal law requires inspections every other year. Many years ago, my first civil engineering job was as a bridge inspector. Inspectors now take a two week training class and regular update classes. I don't do that kind of work any longer, but my office mate does and he's off to Seattle next month for his refresher class. I guess it would take weeks to inspect a bridge like the one that collapsed.
Scoll up in this thread and look at the picture that Mike Smith posted of the St. Louis bridge. The white stuff that hangs down is call "efflouresence." I think I got the spelling right. It's a sign of concrete deterioration, and huge chuncks of the concrete have fallen off exposing the reinforcing steel, which is hanging down. The steel carries the tension load in the reinforced concrete, but to carry the load it needs to be bound to the concrete. Which it is not in that picture. If that bridge was to fail, would it be "weight" or bad maintenance? When I was riding the Acela from D.C. up to Stamford, CT, a few weeks ago, I saw lots of bridge like that, and I wondered how are we going to pay for that.
Generally, bridges are quite safe. Transportation management, though, needs to listen to the inspectors and act on the recommendations.
Sometimes the way a bridge acts can be deceptive. I remember being stopped in traffic on the Interstate 10 bridge in Baton Rouge and feeling the bridge bounce under the deflection caused by the eighteen-wheelers. Bridges deflect and it's perfectly normal.
It is creepy, though, to look out the window and see the water when you're crossing the river.
-------------------- Kiernan Posts: 155 | From: Santa Fe, New Mexico | Registered: Apr 2005
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posted
Kiernan: Are you sure it was an Arch? My impression was that is was a Cantilever, just by having the roadway on top was essentially upside down from the normal steel truss cantilever arrangment. You are the former bridge inspector, so I could well be wrong, but it sure did not look arch like to me.
Also in could well be in the early days of computer use in design. I began my engineering education in 1962, and we did have one course in the curriculum on computer (an IBM 1620. I may not remember the model of my first PC, but that I will not forget.) in which we actually got to be in the presence of the computer for a couple of classes. It was treated about like entering a temple and approaching the idol, only lacking the burning of incense to make it complete. George (currently PE in only one state)
Posts: 2810 | From: Olive Branch MS | Registered: Nov 2002
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The 35W bridge was a "spandrel braced" arch bridge, a combination of beam and arch bridge. The arch was very slight, which is probably why the bridge deck could fall 64 feet into the river while remaining mostly horizontal. It is amazing so many survived this fall, some even walked away. The truss hitting the river bottom probably absorbed some shock, along with car suspensions and tires.
I was an engineering school dropout who watched this bridge being built.
Posts: 1572 | From: St. Paul, MN | Registered: Dec 2002
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Thanks to both of you for answering the question. But George seeing that long range worrying and an over active imagination are some of my specialties I wonder if the freight couldn't weaken an already weakend spot and the addition of a passenger train cause the final blow. I realize that this is a very slim chance but couldn't it occur?
Posts: 1577 | From: virginia | Registered: Jun 2005
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Train lady, I don't know quite how to deal with your long range worrying other than to quote, "Sufficient unto the day is the evil thereof" which realize really does not solve anything for you.
Yes, it is a very slim chance, more like extremely slim. You must also understand that a lot of design and analysis effort deals with thing that approach the slim and none level of possibility. It must, because slim and none occassionally happens.
Katrina was a good example of this. The engineers involved wanted stronger and higher levees because they knew that they were only good enough for the likely largest likely storm in 50 years. The politicians basically told them go away, you are being worry warts and scare mongers. Of course they were aslo thinking, "50 years. So what? I will be gone long before then." (A "50 year" storm means the likely largest in any given 50 year period, but it also means that you have a 2% chance every year of having one.) Katrina was something like the 200 year plus interval storm, and by the nature of its path hit New Orleans hardest at the weakest point. The New Orleans levee system started off and and is still primarily oriented toward protection against flooding from the Mississippi River with Lake Ponchartrain being the relief from the flood, not the source of the flood. We may never again have a storm hit the New Orleans and Gulf Coast as hard as Katrina did, or we may at sometime, anytime, over the next century have an even bigger one. No one really knows and those that talk like they do are talking nonsense.
Back to your original question: As I said, yes, but highly unlikely. It was not a bridge, but the sort of thing that you describe is exactly what derailed the Texas Eagle near Marshall, Texas in 1983. Without going into a lot of details, the Texas Eagle was the third train to pass over what was a poorly done broken rail replacement. The new rail, not the old rail, shattered under the second or third car AFTER the TE's engines had passed over it. That is after carrying a few hundred axle loads of 20 to 32 plus tons, it shattered under an axle load of only about 15 tons. First and foremost lesson learned: The particular metallurgy of the new rail, a 1% chromium alloy, which was then being experimented with in the US, is no longer used, and so far as I know there is none left in track in the US. The improved hardness was gained at the cost of too much increase in brittleness. This is another of those things that supposedly worked great in Europe that some how rotted on the Atlantic crossing. It is no longer produced in Europe anymore, either.
Posts: 2810 | From: Olive Branch MS | Registered: Nov 2002
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