Posted on 08/09/2007 6:32:29 PM PDT by jim_trent
I just received a copy of the last bridge inspection report on the bridge that collapsed in Minnesota last week. While there is no smoking gun, it points to MANY possible failure points. Where I am coming from is this: I am a Certified FHWA bridge inspector and have additional training in fracture critical bridges (which this bridge was). I am mainly concentrating on the center section, since that is where the failure started.
The report was dated June 2006. It is 50 pages long. Interestingly, it was NOT done by a private engineering firm (like mine) while under contract to the MNDOT. MNDOT did their own inspection for their own people to review. The bridge had been inspected yearly back to 1996 and every two years before that to 1988. There was no Federal requirement for bridge inspections before that.
Although I have read elsewhere that the engineers supposedly had used “exclamation points” in their report to emphasize the importance of what they were saying, I found none in this report. It recommended yearly inspections, some small repairs, and nothing else. At most, there was a weak recommendation that “the eventual replacement of the entire structure would be preferable” (to the repairs listed). The word “eventual” does not denote any particular urgency to me.
It does list some things that should have alerted engineers to the problems, but nothing was evidently passed on higher (to the politicians that control the purse strings). For example, it says about the Main Truss Members, “The truss members have numerous poor weld details.” Then it lists numerous cracks at the ends of tack welds, at internal diaphragms that did not have outer stiffeners along the web, welding tabs left in place, plug welds, etc. These are all VERY bad when it comes to fatigue cracking.
But the worst problem was rust. There are about 20 pages of color photos, mostly of badly corroded details. There were some places that there were actually holes rusted through the metal. A combination of fatigue cracks and corrosion is death for any structure. Some of the statements are as follows: “Pack rust is forming between the connection plates.” “The floor beam trusses below stringer joints have section loss, severe flaking rust.” “Truss bottom chord gusset plate has section loss, flaking & pack rust.” “Sway bracing has severe pitting and a 3” x 8” hole due to rust.“ “Some areas (of the trusses) have section loss with holes due to rust.“ No use repeating any more. From the pictures, this is worse than any bridge I have personally inspected. The deicing system was installed in 1999, which could have only made the situation worse. Some of the floor drains dropped directly onto the truss and the corrosion is even worse there.
In addition, there was vertical and horizontal movement at several support locations. Some of the gusset plates were bent. Some had shed bolts (there were empty holes where bolts had been originally), probably from a combination of rust and force from shifting. Just a few inches shift, but that can induce large, unplanned forces into the bridge before a single vehicle drives over it. About half of the expansion joints were non-functional, too. This alone would not cause failure, but it cannot help.
They say that they remove the “plastic pigeon screens” every other year to check the inside of the trusses. They were put on because of the buildup of bird crap inside the box trusses several years ago. There was nothing said about cleaning it out so a thorough inspection could be made. A quick look-see into an uncleaned box could hide a lot. Also, this means that the yearly reports could not be as thorough as they should have been, considering the condition of the bridge.
In the back of the report are several drawings of the truss with the type of stress in each member. About 1/3 of the lower chords were always in compression. About 1/3 were in tension all the time. And about 1/3 reversed stress (went from compression to tension as a vehicle traveled over the bridge). At least that part of the bridge was well designed. The top chords were about 1/4 in compression. About 1/2 in tension. The remaining 1/4 reversed stress. The members between the top and bottom chords were alternately compression and tension.
My guess is that the failure was in probably in a member that reversed stress. That could be either top or bottom chord, but I am guessing bottom. It could have also been in a tension member. That does not narrow it down much. However, it looks like this bridge was an accident waiting to happen. If it did not fail in the spot that they finally decide it failed, it would have failed somewhere else -- and soon.
The fault was not totally with the inspectors. They accurately portrayed the bridge as a piece of crap (although I think they downplayed urgency more than they should have). I believe the fault is the people within MNDOT who got the report and sat on their hands.
BTW, the bridges built when this one was built had a combination of bad factors that made them “wear out” much sooner than planned. Three things came into being that all made fatigue a problem -- something that bridge designers never had to deal with before. One was the introduction of computers and hand calculators, which allowed more loads to be checked and the use of thinner material. Higher strength steel became widely available. A7 (30ksi) and A36 (36ksi) steel were used before that -- very ductile and low strength (thick), so that rust would not affect it as badly. 50ksi to 100ksi steel became readily available at about that time. That meant thinner material, again, more susceptible to rust. In addition, welding substantially replaced bolts and rivets. Along with bad welding details, fatigue cracks were inevitable. Although it came along a few years later, the adoption of deicing (either on trucks going over the bridge or mounted directly on the bridge) was also bad. “Stress-corrosion” cracking is what did this bridge in.
The collapse of that bridge is an indictment of the entire liberal government culture of MN. That includes the DOT and the University of MN. But not a thing will change. They don’t even realize what a failure their culture is.
The public, of both sides of the spectrum, has to shoulder part of the blame as they don't get interested in infastructure until the body count gets noticable.
You might want to read my “All About” page, particularly the first part. In my experience, liberal politicians and “conservative” politicians are just alike. They both want to spend our tax money on what they want to spend it on, not what the other party wants to spend it on. Unfortunately, NEITHER party wants to spend it on maintenance.
Thank you.
Thanks for the comments, Jeffers—
Can you post the link to the photo you think you see the east kingpost on?
Kwuntongchai
I agree, Thank you both for all the info you have posted and responded to. I will let you know how things end up with our bridges. Hopefully we can stop a catastrophic event from happening.
I’d rather not post any pictures yet.
I found the SW kingpost, the member referred to earlier isn’t it.
I’m reconstructing the east truss pier 6 panel and the next adjacent panel north from the east truss, and so far have the kingpost, the top chord, bottom chord, 1st tension diagonal and second diagonal, originally in compression.
Still missing the first vertical strut north of pier 6, but I know where to look. Some other angles may be helpful, I need to search through some imagery I acquired earlier.
I also have a significant gusset assembly, and several other major components, but I need time to sort out exactly what they are, and to make sure they’re not part of the west truss.
It’s an involved process, requiring several different images to piece it all together, and my schedule today is very tight.
I’m essentially offline for the weekend, but I may get a few hours of analysis in.
Early to mid next week, I’ll post what I have found, along with whatever enlightenment it brings into the failure sequence, if any.
Sorry for the delay, but I’d rather follow this to see where it leads than post partial data, in comparison with what I’ve found so far, on the fly and hard to follow.
It’s complex, but it can be simplified once I understand it better, and it’s better to wait till then to to even open the door.
New story from the Star-Trib—MnDOTs discussions from Dec, 2006 of if and when the bridge might collapse.
http://www.startribune.com/10204/story/1370130.html
and
http://www.startribune.com/10072/rich_media/1370367.html
One wonders why
1) they didn’t put a weight restriction on bridge traffic, and
2) why they didn’t pour additional pilings to support the landside cantilever arms. Rather than risk compromising the entire structure to drill in reinforcing plates, simply shorten it into 3 or 4 spans so it can’t possibly collapse.
Kwuntongchai
Are there more current bridge collapse threads out there? I’m not familiar with the freerepublic setup, can anyone steer me towards an index, search function, or active thread?
Also, what other discussions have people found elsewhere on the web?
Thanks in advance,
Kwuntongchai
I am having a Press Conference today at 2 pm.
The Thruway Authority inspected these bridges and were done in 4 days? I was told a good inspection of these bridges would take 2 months.
Oh well, time to expose their failure to properly maintain these bridges with our toll money.
You two have been a great help along with a couple others here locally. Thank you very much.
It doesn’t take two months to inspect a bridge. The time depends on a lot of factors, but I have never heard of a bridge taking that long.
That is what a bridge guy told me.. I thought it sounded excessive.
All went well today and I really focused on the concrete and bearings..
They have sprayed colors on certain concrete columns, red, green, orange... Do you know what the colors mean? Is red a serious condition? I would hope so.
OK, Where ya been, anyone know???
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