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.
Taken from here: http://www.startribune.com/10204/story/1343624.html
McLucus needs to stick to his day job. What does he think the load is in bumper-to-bumper traffic?
A truss bridge is a complex of triangles, the most stable connective load distribution arrangement around. Consider three popsicle sticks pinned in a triangle. Push (load it) at any point at it won’t budge, (A four-sided arrangement will collapse when you push nearly anywhere on it.) But remove just one of those pins in the triangle and it collapses.
Mr. McLucus, stick with long division.
Shifting the weight of the traffic around was probably just the straw that broke the camel’s back.
The bridge probably fell because its “weakest link” somewhere broke and the rest followed.
The bridge probably fell because its “weakest link” somewhere broke and the rest followed.
Oh, yeah, welcome to the debate Perdogg ~ we've had quite a few engineers, traffic analysts and public infrastructure planners come onto the various threads.
So, here's what one/several said: The weight of the maintenance contractor's equipment on that bridge was miniscule. It was designed and built to accommodate peak traffic which would be wall-to-wall cars and trucks PLUS ice and snow load PLUS 100% of that as a hedge.
The problem seems to be that the bridge design (which hasn't been used since 1973 when it the standards for bridges of this size and type were changed) can result in a total bridge failure if only one structural member fails.
It's called a "non redundant" bridge design.
Ever since the Tacoma Narrows Bridge < http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge >disaster in 1940, bridge designs are studied and built to take on all the worst-case possible scenarios. This includes a packed bridge, a bridge with harmonic driven loads traveling at resonant frequencies, severe wind, ice load, pavement crumbling, corrosion, etc. After that, the standard is to make it atleast 50% stronger, and today more like 100% stonger.
The one thing that can be and is often overlooked is the processes and decisions made during the original construction. Poor steel quality, poor workmanship, poor welds, poor joints, poor maintenance, mis-assembly, poor concrete etc.
An example of one of these that occurred was in a Kansas city hotel where a set of stacked catwalks were designed with an inherently difficult build specification, and the builder asked for a design change. A Licensed Professional Engineer signed off on the proposed change, which was the seed fault in the collapse. Instead of having each of the two decks of catwalk hand independently on the cables (This would be like you hanging on a rope by your hands, and the person below you hanging separately on the same rope), the cables were daisy chained. (This would be like you hanging on a rope by your hands, and the person below you hanging by your legs instead of the rope. At some point you are going to lose your grip when the load gets to be too big),
Many failures have happened due to “overbuilding” where a structure has adders built on, beyond the capability of either the design or the construction to hold the additional load.
I would LOVE to be on the investigation team on this accident. However, I’m betting that a PC team will be assigned this duty instead. It will probably be called an “freak accident” so no one’s feelings get hurt.
OK, but what does that have to do with the collapse of a bridge where construction started 43 years ago?
Something that I thought about, is that they were laying asphalt topping. I don't know what they were using to flatten the surface, but if they were using one of these 25 ton machines:
The vibrations could have been a contributing factor. When the city repaved or street about three weeks ago, this is what they used to smooth the surface. My entire house was shaking and the windows rattled. No telling how many were on the bridge at any given time.
Physics is so cool, the complexity at this level when it takes into consideration all the anticipatory problem solving and what ifs of loading bearing etc ahead of time are just too much for my imaginative, tangential oriented mind to handle.
Kudos to you Structural Engineers and Physicists out there. Glad God made us all differently. :-).
‘many officers were in spots that didn’t allow radio communication’
Shall I assume Big Brother has jamming equipment to negate cell phone detonation of IED’s ? Just wondering....
From the video I saw showing the collapse, it appears a cord failed...like breaking a string of tightly stretched beads at one end...the bridge appeared to fail from west to east, not in the center.
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Funny you should mention that.
I was having dinner with the Parental Unit (aka my Mom) last night and she was in a marching band back in the late 50’s.
She said that whenever they crossed a bridge, they broke any kind of marching cadence they had.
Best bet is shabby construction in the first place and a lot of rake off by the contractors.
It would be simpler and cheaper to use more steel and increase the strength of the members in the present design, or make suspension or stayed bridges as commonly done. The idea of adding many more "components" in a network are counter productive. They add connections which are expensive and a maintenace problem.
The arch of the roadway flattened out!
To see how visible the flattening was link to the full size at flikcr. (click "all sizes")
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