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.
Very interesting, Jim. Thanks for your report.
Sehr wertvolles. Danke!
See post 11. That is the same thing online that I was given a hardcopy of.
Apparently Quebec had a similiar bridge failure recently.
I met and talked with the Quebec Transport Engineer who was in charge of many of those inspections. He voluntarily changed jobs to get out of the political situation. He said that the politicians want to spend the money elsewhere and he couldn’t continue to report the way they wanted.
I just glanced through the pictures. That just looks like pure neglect in quite a few of them.
We're now prepared to deal with the reporting...
> Bet there are a lot of private inspection firms that are glad they didn’t do the last few inspections.
Include mine. I have produced (or supervised the production of) a number of bridge reports, although none were this bad. I am happy to say that the worst ones were recognized as such and set in a schedule to be replaced. Maybe not as fast as I would like, but they at least recognize the problem. I have wondered what we would do if a bad report was totally and repeatedly blown off by the state DOT.
Naaah...didn't you hear? It was global warming.
I would tell my friends and family to take another bridge. Well, maybe not all the relatives. ;-)
ping
A review of the bearing measurement data indicates a large and erratic discrepancy between the measured and theoretical bearing movements for temperature changes. It appears that each bearing behaves in its own way in terms of overcoming the friction for longitudinal movement. ... There is enough frictional resistance due to corrosion and debris to keep the bearing from moving until there is enough thermal force built up in the system to cause a drastic and quick movement of the bearing to relieve the force.
My feeling is that just such a "drastic and quick movement" was the trigger of the collapse, and I'm even inclined to believe that a bearing on the South pier may have actually failed due to such a movement. This would neatly explain everything.
Maybe I'm cynical, but which makes more economic sense for a state: (1) spend state tax dollars to properly maintain a bridge, or (2) spend those state tax dollars on something else; when the bridge fails as a result, let the federal government use other people's tax dollars to rebuild it.
Welders make good welds and bad welds.
From my understanding, common practice is to build bridges with a fixed support point, and with movable bearings at other support points. Would it be practical to replace the fixed point with another movable bearing that was attached to a motor that would move it every day or so? I would think that would help prevent the other bearings from seizing up, detect if one of them sizes up anyway (assuming instrumentation at the motor point), and provide an alternate relief point until such time as the seized bearing is repaired. I'd think that would be an all-around win.
Naaah...didn't you hear? It was global warming.
But global warming is Bush's fault, isn't it?
I knew this was going to have to be a factor. I suspect also that when they begin to break up the wreckage, they will find the embedded "fabric" and rebar in the surface to be disintegrated from repeated salt applications.
Ping
At pier 6, there was significant surface corrosion, debris and dirt packed into various areas of the bearing. All the roller bearings seem to have thick coatings of paint and did not appear to functioning as intended under the live load.
"Thick coatings of paint" dear friends! Remember those ads for Caterpillar in Sci Am ? - "The Ideal ... and the Reality."
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