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.
"did not appear to functioning"
s/b
"did not appear to be functioning"
My mistake in manual transcription.
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.No doubt it was only in the CNN report. :') Thanks E.
Very nice job. I had read the online report earlier but your analysis was great.
A question: They are talking gusset plates now. The ones on the bridge were 1/2 in thick. Thick enough and big enough in your opinion?
The conclusions sounds reasonable. Sheer bureacraric inertia kept the report from being taken seriously. Of course, the thing could have stood for another ten years. Maybe the weight of construction materials was the straw that broke the camel’s back.
They were undoubtedly thick enough when new, but the bolted gussset plates (also called splice joints) were badly corroded on this bridge. They say in the report that there was pack rust up to 1/2” think on one of them. That does not leave much material left.
I did not see it in these photos, but I have seen badly rusted splice joints that actually looked like an old fashioned quilt. In other words, the steel plate was actually bowed out between the bolts (or rivets). That has to put a lot of force on the bolts without any additional loads. Actually the splice joints could easily have been a source of the failure. Especially since one was in the high tensile stress area in the bottom chord at mid-span.
Possible, but I doubt that it would have stood 10 more years. The report was really, really bad.
After reading the report, I have absolutely no doubt that terrorism was NOT involved. The mystery is: “how did it stay up that long”, not “why did it fall”.
As far as “what kind of knowledge and equipment would be needed to take out a bridge like this”: A fair amount of both. Any structural engineer could pinpoint weak points in any bridge. This one, being fracture-critical and non-redundant would have been particularly easy — just about anywhere you look. Redundant and NON-fracture-critical bridges would be a lot harder.
Even on this one, the amount of equipment needed would have been quite a bit and anyone doing something would have to have been out where anyone could see them (the best place would have been the lower chords between the main bridge piers). Of course, explosives don’t take up much space, but there is no sign of explosives being used here. Others have suggested a cutting torch. Those box beams that made up the truss were large enough for a person to get inside. You would have to cut through at least half the entire beam to guarantee a failure. That would take some time.
Thank you for the review! Very interesting. Although I wish I knew half the bridge terms you used!
I got an e-mail asking some additional questions and sent the following reply. If anyone else has the same questions, here it is:
Y”ou may be interested in the article I wrote in FreeRepublic.com after I completed my last refresher course in bridge inspection. It covers some of your questions. It is at:
http://www.freerepublic.com/focus/f-bloggers/1598099/posts
Since it lasted as long as it did, I do not believe that there was any “bad” steel or “bad” concrete used. I think the primary reason that it did not last longer was the design. This is not to say that the original designers were incompetent or evil. They were going into unknown territory at the time.
There were a huge number of new bridges constructed as a part of the Interstate system that got started in the late 1950’s, through the 1960’s, and into the 1970’s. This was designed in the early-1960’s, started construction in the mid-1960’s and was finished in 1967. Because of the large number of bridges needed, they had to be built differently from the past. Less cost, less material, and more speed. This led to the adoption of many things that should have been tested out more thoroughly before use. I touched on several examples, but here is more detail.
The problem of fatigue was unknown in bridges before this. Fatigue was known in WWII, but was thought to be limited to airplanes. Unlike airplanes, bridges were big, heavy, used low strength steel with lots of ductility and plenty of excess material for corrosion. Also, the earliest bridges (stone arch) were totally in compression where you cannot have fatigue. However, steel allowed the use of tension. By using both tension and compression, bridges became lighter. However, bridge designers did not consider fatigue problems when designing welding details. They just used whatever was the quickest and easiest connection details.
Some of those welding details have directly caused fatigue cracks in this bridge. Things like tack welds, skip welds, plug welds, back-up bars that are not removed after final welding, are now known to be BAD, BAD, BAD for welded structures. However, usable design standards for fatigue were not widely known in the industry until the mid to late 1980’s, long after this bridge was designed. Other fatigue problems were with design. Diaphragms without without stiffeners on the other side of the web or stiffeners that just ended in the middle of nowhere are bad fatigue design details.
The mere fact that welded box beams were used is also a fatigue problem. It looks like large parts of this bridge were welded somewhere else and trucked to the jobsite, where they were joined by bolted connections. This speeded up construction and was a lot cheaper, but a welded structure, once a fatigue crack starts, will crack completely through. In earlier times, when a riveted box was used, a crack would only go through ONE of the four parts of the box and would stop at the riveted joint.
The next thing was the move to higher strength steel. That means that the material used is thinner, has less ductility (which means less fatigue resistance), and corrodes just as easily. If you lose 1/4” of a 1” thick member it has lost at least 25% of its original strength. However, if a higher strength material only needs a 1/2” thick member to begin with, the same amount of rust will reduce the strength at least 50%. However, high-strength steel was looked upon as a godsend for reducing cost at the time. There is now something called “corrosion resistant steel” (A588 and the like), but it is not good for places that move (such as bolted joints and expansion joints) where a lot of the problems with bridges are.
I also mentioned that the old computers (IBM 1620 was the first one I used) first came into being about that time, along with calculators, which allowed engineers to convince themselves that they could use thinner material. Again, thinner material is also more susceptible to corrosion and fatigue.
Keep in mind that once fatigue cracks are seen at least 90% of the life of the structure (at that point) is gone. A structure with extensive fatigue cannot be repaired to be “good as new” no matter what is done to it. At the very best, you can extend the life SLIGHTLY. They tried to extend the life of this one too much.
In other words, it was the perfect storm – welding, higher strength steel, lack of knowledge about fatigue, and engineers with computers. I don’t believe that anything was done deliberately that caused the collapse other than the refusal by someone to believe that it was as bad as the inspectors said and the subsequent refusal to speed up its replacement.”
I can’t see how that would be very important. As I understand (and I’m not a CIVIL engineer), the concrete road sections are non-structural; they’re only for supporting traffic. I think it is common to remove such sections both for simple resurfacing and more intensive repairs. I don’t think they could afford to do this kind of removal regularly if it were a structural piece. Besides, surely they would know that removing the concrete sections would weaken the whole structure if such were true, and wouldn’t do it. Or, even if it does, they would have to prepare the road/bridge to re-distribute forces before removing a section.
This looks like an even more interesting report than the inspection report. I will study it further.
The first page or so says that there were a total of 52 members in the bridge that would bring down the entire birdge if any one of them failed. That is a LOT of places for disaster to strike. Your theory is certainly possible.
I don’t think you would want to bother using simple-support rollers on both ends. They all have to be restricted in both horizontal directions or else the truss will simply fall off its supports. It’s common practice because it works.
I don’t see how a “sudden quick movement” would necessarily cause a catastrophic failure. I’d be more concerned that the debris held the roller supports in place to act almost like a 2nd fixed joint, building up stress within the structure and possibly being the breaking point for those weak points in the structure.
As someone (some structural person, I think) said on a TV broadcast, it’s probably several factors that melded together that caused this, anyway.
I really appreciate your response to my question. I too, saw the pics of the rusted pieces. Thanks again.
Ben
Thanks for you analysis. It appears to fit the pattern discussed in a book on engineering disasters I read recently.
The book asserted that bridge building follows a pattern — after a status of using known materials and methods to yield safe bridges, new materials and processes become available. The first builders to adopt these new materials and processes grossly overdesign the first bridges, and they are safe. Then as time goes on, they gradually remove the safety factors in the interest of lower cost and/or building time (e.g., using calculators to convice themselves that the thinner high strength steel will carry the load). Then a disaster occurs, and engineers figure out the problems, and change something to deal with that problem so that the bridges are safe again. Then .. new materials and processes pop up, ... and the cycle repeats.
It would be facinating if it didn’t result in death.
Thank you, very interesting reading! I too think it was MNDOT who failed to either recognize the danger signs or act on them expediently.
I also thought maybe we need to change how we do bridge inspections. But if you can see the danger signs in the report, then others should have too. I think the exclamation points were from a different (perhaps earlier) inspection report.
Like people said about 9/11, it seems we suffered from a lack of imagination...we couldn’t imagine this bridge collapsing!
Current practice is to have one fixed point that never moves. What I would suggest would be instead to have one point held in position by a positioning mechanism which could periodically (e.g. once a day) move the entire bridge back and forth a few inches, stopping at a slightly different position each day. For 23.9+ hours/day, a brake would hold the mechanism in place. But each day the brake would unlock and a motor would drive the bridge.
I don't think 'sudden quick movements' are themselves apt to cause damage. On the other hand, if a bearing is getting sufficiently worn that it tends to stick for awhile and then yield, that suggests that it's nearing the point where it's going to stick and not yield.
On a bridge with a single fixed point, a bearing that has frozen up may not be detected until the next major thermal event (very hot day, etc.). Even if there is instrumentation to measure movement, the fact that the bearing doesn't move when the temperature remains constant would hardly be alarming. The seized bearing may not be noticeable until it starts causing major stresses on the bridge, and those stress levels would be likely to increase before there's any opportunity to deal with the seized bearing.
If the 'fixed' bearing were replaced with a motorized one, then in the event that the bridge started to experience stresses as a result of a seized bearing elsewhere, the motorized bearing could be moved so as to relieve such stresses (it wouldn't need much power, since the motor would be trying to move the bearing in the direction it already wanted to go). The only danger would be if two bearings seized up simultaneously; that risk should be reduced by the fact that an alarm would sound when the first bearing seized up.
What the heck, I’ll toss my two cents in here too.
I don’t disagree with anything Jim has posted, but because I’m not a licensed engineer, I’m willing to stick my neck out a bit further.
Between cracking, and frozen bearings, I like frozen bearings for collapse initiation better, for two reasons.
1. The cracks were inspected regularly, records kept, trends noted, and repairs made, in at least some cases. To fail the bridge, either a new crack would have had to appear since the last inspection, or else a known crack would have had to change behavior since the last inspection. Frozen bearings, however, could have failed the bridge by inducing greater than design loads, either by remaining frozen, or by popping loose at the wrong time. It’s a fine line, but in my opinion, frozen bearings are more likely to fail the bridge just by being frozen, than cracks are likely to fail the bridge, as long as they crack at historical rates. Frozen bearings do not require a change in recorded behavior to fail the bridge.
2. The most significant problem in this bridge’s history were cracks in the east end of the crossbeam/endbeam/rocker bearing assemblies, both north and south, directly caused by a frozen rocker bearing at the east end of the south crossbeam, in 1986. The bridge was closed, jacked up, the rocker bearing was replaced, crackes were drilled out, and plates were bolted onto the cracks caused by the frozen bearing.
The 2006 inspection report indicates the exact same (structurally speaking) bearing froze, except this time on the west end of the south crossbeam, but the bridge was not closed, the superstructure was not jacked up, the frozen rocker bearing was not replaced, cracks were not bolted or plated over or drilled out, in short, no corrective action was taken or recommended.
I’m also willing to localize the initial point of failure. Like Jim, I believe a tension member failed first, and that member could have resided in the first, second or third panel of the east truss north of pier six, in the main span of the bridge, as Jim suggests. It also could, however, have resided in the sway bracing between the east and west trusses at pier 6.
Failure in the west truss, one or two panels north of pier six is also possible, but less likely as a triggering event.
Failure at the pier 5 crossbeam, west end, from 2006 damage due to the frozen west rocker bearing, or east end, from the 1986 east truss damage, is also possible, but third in liklihood behind the other two.
The 2006 report also noted that the roller nest under the pier 6 east truss was possibly frozen, and this area appears to have been critical in the failure sequence.
Guesswork, speculation and supposition, but if I had to call it, I’d say that greater than design loads were imposed throughout the trussed spans, by frozen expansion bearings at multiple points, and that tension members at one of three points:
...in the sway bracing between the east and west trusses at pier 6,
...or the top chord of the east truss between pier 6 and the second strut north of pier 6,
...or the diagonal truss member in the east truss, running down towards midspan (span 7), from the pier 6 kingpost or just north of the second strut north of pier 6,
...were the triggering failures in the collapse sequence.
A gusset plate failure under tension at any of the above namjed points is included in the likely failure mechanisms.
Obviously, a crack at any of these points could have failed, but in my opinion, the triggering event was the excessive stress imposed by the frozen bearings that caused these cracks, not cracks due to rust, corrosion, or other reasons.
Am I positive?
No.
Am I positive I believe one of these three were the most likely initial trigger points?
Yes.
Does Jim agree?
It’s up to Jim to say yes or no, but reading between the lines seems to indicate that he does.
Am I positive that I believe the most likely direct cause of failure was frozen bearings, and/or resultant cracks, rather than cracks due to other causes?
Yes.
Does Jim agree?
Let’s ask Jim.
?
Keep in mind though, when reading his answer, that a licensed professional engineer has a lot more to lose in risking a guess, than a retired construction stiff who quit school before earning an engineer’s degree due to financial reasons.
If a working engineer guesses wrong, friends and/or clients get crushed.
If an old salt guesses wrong, sitting around the cracker barrel in idle speculation, he might look silly when NTSF issues a final result.
That’s a significant difference.
If Jim refuses to speculate, it should be taken as professional concern, and judicious restraint.
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