Posted on 08/03/2007 5:40:58 PM PDT by traumer
Engineers are trying to understand what caused the catastrophic collapse of the bridge over the Mississippi river in Minnesota.
Resurfacing work was taking place, but the bridge was last inspected in 2006 and no significant structural problems were found.
Such complete bridge collapses are a very rare occurrence.
If they happen, it is either because the load is too heavy, or the connections between the bridge's structural elements are too weak, Keith Eaton, chief executive of the UK's Institution of Structural Engineers, told the BBC.
"The engineers will have to see where the collapse started. Clearly a failure occurred somewhere which imbalanced the whole thing," he said.
Speculation that hot weather contributed to the accident by weakening the concrete or expanding the steel framework was not a likely explanation, he added, as modern bridges are built to cope with extremes.
A crack in the steel making up the bridge's structure was the most likely explanation for the disaster, he said.
Corrosion
The I-35W highway bridge (Bridge 9340) was built using a framework of rafters, posts and struts - a structure known as a truss bridge.
In 2005, it was one of thousands across the US rated as "structurally deficient" on the federal National Bridge Inventory database.
It rated 50 on a scale of 100 for structural stability in that study, White House press secretary Tony Snow said.
About 140,000 cars are thought to have used the bridge every day, but a 2001 report by University of Minnesota's civil engineering department found traffic levels were below those the bridge was designed for.
See graphic of the bridge collapse
The report went on to express concerns that a single crack in the main truss could "theoretically" lead to the entire bridge's collapse.
However, it also said that even if there was a crack, the load could "theoretically" be redistributed along the steel trusses or the concrete deck of the bridge, keeping the bridge aloft.
It added that no fatigue cracking had occurred, and that the bridge "should not have any problems with fatigue cracking in the foreseeable future".
File photograph of the Minnesota bridge The bridge crossed the Mississippi River near downtown Minneapolis
The state need not "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project".
The truss bridge was built in 1967, with eight lanes over a span of 581 meters (1,900ft). It had no piers in the water, allowing easy passage for river traffic.
While no longer the cutting edge of bridge design, truss bridges are relatively cheap to build, and were a very popular structural choice in the US in the 1960s and 1970s, Mr Eaton said.
They have a downside, however.
"They are made of lots of complex pieces of metal, interconnected bolts or rivets," Mr Eaton told the BBC.
"They have little corners between two pieces of steel where water can collect and cause corrosion."
Nesting pigeons could also be an issue.
"Their droppings are very corrosive, which can be a problem," he said.
It strikes me that the common denominator in these truss bridges is the inexpensive steel used to build them in the 1960s.
Post WW-II is when the interstate system construction kicked in to high gear, and thousands of these kinds of structures were built using steel that was pumped out of Pittsburgh and Youngstown, as fast as those plants could produce it running three shifts, 24/7/365 for years.
Of course Big Steel also drove big coal, the railroads, the iron ore boats on the Great Lakes, and everything in between....
I’m not saying that it was low quality steel per se, but these structures were never meant to last much beyond fifty years or so, and that was under the engineering standards that existed at the time that particular bridge was built in that particular location. The steel that we produced was high grade for the time, but due to process improvements over the years, the material used in 1960 likely would not meet the building standards of today. There are simply much better formulations for structural steel now, than then.
Everything has a service life, and a design envelope that is based largely on well grounded theory, but theory none the less. We know what the average life of a particular beam or bolt ought to be, based upon experience, and the fact that these failures are relatively rare.
But if you consistently push the edge of the designed load, or inadvertantly match the frequency that this kind of structure happens to harmonically vibrate at, (say with a series of jackhammers), the inherent qualities of old, inferior quality by modern standards steel simply may not be able to stand the further stress, and away it goes.
It certainly makes sense. I mean one second you have a standing bridge and all of a sudden a section completely collapses taking down other sections with it. Something catastrophic like a washout would fit the bill.
Investigating … could have been a span bridge rather than a suspension. Neither would need in water footing.
Good pictures here. There is a lot of rust in that third picture though.
http://www.visi.com/~jweeks/bridges/pages/ms16.html
“[bridges collapse] either because the load is too heavy, or the connections between the bridge’s structural elements are too weak, Keith Eaton, chief executive of the UK’s Institution of Structural Engineers.
Well, nobody can claim Keith is a dim bulb! Load too big or bridge too weak. I’ll be!
I’d think that salt on the road in the winter probably contributed to the corrosion, as well. There are other substances that can be used to keep ice off roadways, but cities and towns don’t use them because they’re more expensive and they don’t want to spend the money.
One ought to pause and realize how difficult the conditions must be for the divers with all that twisted sharp metal, heck, most of us would have a better chance of landing the space shuttle than swimming darkly in those currents, can’t imagine.
Thanks faq.
Sorta’ proves my point, no “in water” footers thus “scouring” was probably not a factor. It was designed to avoid putting piers in the water. And it is a “span” bridge if my research is correct.
Bridges, and anything else for that matter, was built with a much larger factor of safety 40 years ago than today. Quality of steel is irrelevant. They used 10 times more of it than was required. Nowdays the steel MIGHT be 10 percent better, but they use so little of it with a factor of safety so small, that the quality issue becomes a moot point.
A jackhammer on a bridge is like a flea on the ass of an elephant.
A pile driver might be something of measurable impact. Or a very large backhoe improvised as a battering ram to break up concrete. I think it’s more likely there were workers removing something they were not authorized to remove. Or there was a bridge engineer not quite up to speed in old fashioned bridge design and authorized something he shouldn’t have.
There is likely no problem with the steel. To be used it must meet AISC Specifications and have the specified yield strength that is rigorously tested.
The same is true for the high strength bolts used in the connections. There is a possibility that bolts were not the soecified grade, but unlikely.
The connections are subject to corrosion and that is most probable reason for failure.
You mean those eeeeevil SUVs, don't you? They must be implicated in this somehow.
And cigarettes.
And handguns.
Too many SUVs with handguns on board (makes 'em heavier, dontcha know) driving across the bridge, and no doubt the drivers smoking cigarettes, too.
/sarc
Absolutamenty! You got it.
A national gas tax would help
I doubt that a washout was responsible as the support structures for the bridge appeared to be above the water level, on the river banks. Truss bridges are like lattice work crane booms. I watched a crane taking a building down while I was going to college. The operator had the boom out nearly horizontal and was reaching for the far corner of the building's basement when he just brushed the boom against an interior brick wall. The touch was so slight it didn't even knock a single brick loose but the boom crumpled like a wet Kleenex from the base to the tip.
A truss is an exercise in minimalistic design. There is sufficient strength to carry the load but no redundancy. If one element fails the whole structure fails. It could be rust or brittle welds or fatigue cracks from loads exceeding the design limits assumed forty years ago, what ever it was brought the whole thing down.
Regards,
GtG
I am a novice at this but my guess the workers added to much weight in the repaving and it collapsed.
***Nesting pigeons could also be an issue.
“Their droppings are very corrosive, which can be a problem,” he said.***
How about salt for ice in the winter.
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