Posted on 08/04/2007 11:49:45 AM PDT by kathsua
I've been looking at some of the video showing the collapse of the I-35W bridge over the Mississippi River in Minneapolis and it looks like the collapse could have resulted from a major mistake by the company working on the bridge's surface. The bridge carries eight lanes of traffic, four each direction. To maintain an even load on the bridge supports during construction the contractor should have either worked on the two inside lanes of both sides of the bridge or the two outside lanes. Instead the contractor worked on the inside two lanes of one side and the outside two lanes on the other.
The result was the support on the side in which the outside lanes were carrying traffic had to support a greater load than the side on which the inside lanes carried traffic. I'm oversimplifing, but essentially the support on this particular side would have been carrying the load of the traffic directly above it as well as a portion of the traffic on the other side. The support on the other side would only have been carrying a portion of the load on its side.
If you've watched videos from the scene you know there is a school bus and semi truck next to each other in those outside lanes. There is another semi just behind them. These heavy vehicles went across the supports just before the collapse. The supports on this side of the river shifted toward this side with the roadway shifting toward the other side as if the weight of the heavy vehicles may have been too much for it to carry in a weakened state
My background is in math and physics rather than engineering so I'm not familiar the precise distribution of weight and how the supports carried it, but the basic math would indicate more stress on one side of the bridge than the other.
A computer analysis of the remains of the bridge and its design will be necessary to determine the precise amount of stress on the different pieces of the bridge and their physical condition. This analysis will show exactly which part(s) failed. Fortunately investigators have a computer program using Finite Element Analysis developed at the University of Minnesota.
So weight (by itself) wasn’t a cause, but removing the concrete from the surface seems to have reduced platform stability. The platform became less rigid as the concrete deck got thinner, and so the platform shifted (or moved the pins off their support at one end of one platform.
Then, as soon as one end of one plate was free (or had wiggled too far - the construction crew said the entire deck was getting more and more mobile as the concrete got thinner) the other concrete decks were able to fall. Then, the whole structure came down because it was not locked in place at both ends of every deck section.
My opinion? The connections (of one deck platform) must have failed first, as you can tell because the individual decks and beams came down in complete pieces.
A properly designed bridge will bear a full asymmetrical load with a safety factor of 2.
Other factors are more at play here, mostly (lack of) proper maintenance and perhaps even temporary removal of critical structural elements.
I was waiting for you to ping in.
Have you seen the surveillance video of the collapse?
It's never too early to speculate. That's what makes it speculation.
Only half-joking. I don't see any harm in raising the question, as long as you make clear and keep firmly in mind that it's a loose hypothesis at most. Since the collapse coincided with the resurfacing, an unusual event, that's certainly something to look at. Not as the rood cause but as something that added jsut enough stress to trigger an existing flaw.
Wow. Bridges suck.
“If they did, they are idiots.”
I like Mythbusters because they’re funny (and Kari is hot), but they’re not to be mistaken for any real in-depth experimentation.
It’s the “Hollywood Principle” - the act of turning anything serious into entertainment destroys it.
A truss bridge is a perfect problem for Finite Element Analysis since each element in the truss can be modeled directly. I have seen many examples of FEA which I would not trust however. I say that because an unstated assumption is made that materials are homogeneous and isentropic. Such materials as cast iron and wood fall wide of this mark and I would regard any such analysis as questionable at best.
Regards,
GtG
Well said.
Tesla—an underappreciated scientist.
The mythbusters are hollywood special effects guys. I have observed them make fundamental mistakes that lead them to false conclusions. They need a physicist on their team and should not be cited.
I have an idea, you go start a heavy construction company, put your heart and soul into it, build an organization of talented people to bid and manage the work, pledge everything you own to finance this enterprise and then you explain to me how you will risk it all to build something not meeting the plans and specifications you contracted for...what are you going to gain?
Don’t give me the feather bedding union workmen bullshit, who do you think builds our roads, bridges, tunnels and sewers...elves.
mnehrling : I thought the mythbusters debunked that?
Sorta kinda. There were two experiments of theirs that focuses on resonance on bridges. One of them was a direct attempt to take on this particular myth: Adam built a scale bridge, and Jamie built an "army" to march in step using pneumatic actuators.
The scale bridge did break, but apparently from the sheer pounding of the boots; they did not record any wave motion, any sign that a pattern or resonance existed. It was one of their more inconclusive and disappointing experiments, and I think it's because they were never really equipped to set up a valid test. The problem did not scale well.
The second experiment was as described below:
Abathar: Did you see the episode when they hooked a small little linear motor on a huge bridge and got it to resonate when they hit the right frequency?
They were trying to duplicate an experiment by Tesla, and it shocked the crap out of them when they felt that tiny weight shaking the bridge from about 100 ft. away.
They got a discernible vibration over limited distances, but not nearly enough to cause damage. IIRC, they concluded that you could get a good resonant wave across one beam, or one cable or set of cables, but in something as complex as a bridge, different materials would resonate at different frequencies, and they would tend to dampen each other.
MplsSteve: She [Mom] said that whenever they [marching band] crossed a bridge, they broke any kind of marching cadence they had.
Westlander: The military has known for years that troops marching in step can create enough vertical force to destroy a bridge. It is standard practice for soldiers to break step at every bridge crossing.
Neither of those is evidence that it's scientifically valid; only that some folks believe it.
In the case of the military, a lot of training is there to form habits, habits that will stick with you on a near-instinctive level when you're under extreme stress and haven't slept. Training soldiers to break step for a bridge makes a lot of sense if they might one day be asked to cross a rickety wooden bridge, a rope bridge or a floating bridge.
LibFreeOrDie: Resonance caused by pedestrians:
Resonance apparently resulting from wind and poor design: “Galloping Gertie” Tacoma Narrows Bridge, 1940:
http://www.youtube.com/watch?v=HxTZ446tbzE
Both of those were resonances caused by lateral forces -- in the case of the Millennium Bridge, pedestrians swaying to keep their balance and then feeding the problem, and in the Tacoma Narrows, a steady gale force wind that whistled through the open truss work underneath. Both were the result of unanticipated lateral forces, not the vertical forces the bridge was designed from the beginning to weather.
Thanks for letting me know you posted this. There are two factors in the collapse. One set would involve the actual condition of the bridge including any cracks, missing bolts or whatever. Any defects would make it possible for the bridge to collapse. The second set of factors would involve events that affected an existing defect(s) and caused it to fall at that particular time. The second set essential would be the triggering mechanism for the collapse.
Interesting too...
Thanks for the link. I hadn’t heard that yet. That situation would indicate the concrete may actually have been holding the bridge together and keeping it stablized which I believe is what reinforced concrete is supposed to do.
According to an NTSB official one of those assigned to the investigation will be a man who helped develop the computer program that will be used when he was a graduate student at the University of Minnesota.
the answer is no. The bridge is designed for wall to wall end to end traffic probably X 3 or 4
You raise a valid point that it's an entertainment show, not a peer-reviewed engineering study, and should be treated as such. Another point is that their brief is to test urban legends, not to establish engineering standards. So their goals are limited; if the myth is that a penny dropped from the Empire State could kill someone on the sidewalk below, if they can disprove that, they're done; they do not need to prove that no object from that height could ever do harm.
To their credit, the Mythbusters are good about listening to the complaints from their fans, and revisiting myths when there's a problem with their methodology. In fact, they went to alt.folk.ore.urban before they rolled the first camera to ask the urban folklore aficionados what legends they would like to see tested and how. They took advice from that group, which like this one, included a wide range of experts.
One example is their "bullets fired up" test. In scale tests, wind-tunnel tests, and full-scale practical tests, they concluded that it is not true -- it is a myth -- that bullets fired straight up into the air can fall straight down with enough velocity to kill.
But here's the thing, which they explained and illustrated on the show. It is well-documented fact that people who shoot guns into the air are often drunk, stupid or both. They are not using a plumb bob. And even a few degrees off vertical, you've got a spinning projectile on a stable arc, moving much faster than its terminal velocity under gravity alone. There are well-documented cases of lethal stray shots from as far as two miles away. But they weren't "straight up" and "straight down."
The one that bugs me is their investigation of the "sword slicing a sword" Hollywood myth. First, they actually managed to snap one sword in half with another, at a reasonable analogue of the speed a human could achieve, but the high-speed camera revealed that the struck sword flexed almost in half and snapped as it whiplashed back, which they said didn't count as "cutting." I thought it was a lame dodge.
More importantly, they tested similar swords against each other. My understanding, and I don't think I'm assuming too much here, is that samurai or knights or other elite troops had precision-crafted weapons made of the best materials known, and ordinary foot-soldiers had stamped-out crap because they were cannon-fodder who weren't expected to live long anyway. And if they managed to survive, they could pick up a better weapon along the way.
But I digress.
Mythbisters is not the be-all end-all of questions like these; but ti's a baseline at least, something accessible to a layman. Not to mention that it's fun, and life should be fun. If they get something wrong, it's easier to start from what they got wrong than to start from zero.
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