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.
Interesting! Thanks for posting.
Great stuff, Jim. Your input here on this disaster has been excellent!
Been watching this one with interest, since I used to be involved in large industrial accident/fire investigations many years ago.
Rust + connection failures + stress cracks = very bad medicine.
I hadn't thought of the implications of going to thinner steel, but of course that would mean a narrower margin for error (and rust).
This sounds like the way the Kennedy/Johnson administration ran the Viet Nam war from the White House basement.
It's actually not symptomatic of the age ~ rather of the Democrat dominated political structure in this country.
This same sort of approach is where we get the Global Warming panic. Government weathermen and computer programmers create climate models that satisfy the demands of their politician bosses.
These problems are a demonstration of why every function of government must eventually be tied into the economic and totally privatized.
Read you with great interest as I am not knowledgable about briges. You seem to have offered fine commentary with an expert eye. Canadas’ road and bridge structures, I have read, are worse than ours. This tragic episode is a bridge ‘9/11’..I do not know if anything will be done nationally in the forseeable future. Anyway, thanks for your effort in producing a thoughtful opinion and evaluation. It was a good read. I hope your evaluation passes to a higher level.
Jim, that’s fascinating. Thanks for taking the time to write it up. Very interesting.
Very interesting....thanks.
Your prediction is now part of the public record. At least one set of honest eyes has been cast on this disaster. We’ll see what the “official” whitewash ... er, I mean ... conclusion ... is.
Check here.
Also there is a very interesting inspection report dated June 0f 2006.
Check here .
I wonder if there are more bridges like this.
Thanks for the report. Very interesting.
A FReeper's point of view is usually much more valuable to me than the usual press release.
*Sigh*
This appears to have been completely avoidable.
This is what I love about FR! We have a wealth of experts in so many fields.
Thanks for the insight on this tragedy
Bet there are a lot of private inspection firms that are glad they didn’t do the last few inspections. Just think of the liability. With a report like this, the state is going to pay out alot in civil lawsuits.
If the bridge was being inspected by state employees, who knows what the didn’t find.
I’m sending this to my engineering son. Thanks
Wait a minute, I thought it was Bush’s fault!
“I wonder if there are more bridges like this.”
That was rhetorical, right?
There were thousands of bridges built about the same time using the same materials, and designed by engineers of the same generation of background and training.
Luckally, not all of them are subject to the same corrosive environment, but some are.
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