August 3, 2007

Bridge Was In Bottom 100 Heavy-Use Bridges In US

The bridge that collapsed on the Mississippi ranked in the worst 100 heavy-use bridges in the US -- but still was deemed fit enough for full service The Pioneer Press reports on the apparent contradiction, which calls into question the assumptions on which bridge inspections are based:

The Interstate 35W bridge that collapsed Wednesday was not just "structurally deficient," it was among the 100 worst heavily used bridges in the country.

It was also deemed perfectly safe.

How this bridge could have both labels at the same time seems to defy logic. But the contradiction goes to the heart of a complicated national bridge inspection program meant to ensure we can safely drive across the country's 709,000 bridges every day. ...

The I-35W bridge was in particular need of attention. In a comparison with about 4,000 similar bridges nationwide - those carrying more than 100,000 cars per day for a major highway, regardless of age - the I-35W bridge, as well as the Interstate 35E bridge over Cayuga Street in St. Paul, are in the bottom 3 percent in terms of their sufficiency rating, according to the U.S. DOT's National Bridge Inventory. ...

Dan Dorgan, state bridge engineer for the Minnesota Department of Transportation, said Thursday that the I-35W bridge's low sufficiency rating was mainly because of corrosion in the bearings, corrosion of the steel end joints and fatigue cracks. More recent inspections in 2005 and 2006 showed no evidence the cracks had grown or that more cracks had formed, Dorgan said. At that time, engineers characterized the bridge as "fit for service," Dorgan said.

This contributes to the sense of mystery as to why the bridge failed so completely. Engineers had last inspected the bridge on May 2nd to look at "minor concerns" about weld joints. No problems were found in that inspection, and MnDOT planned more inspections in the fall. Given the close looks in 2005 and 2006, it doesn't seem like MnDOT took its eyes off the ball, but they must have missed something.

An outside consulting firm, URS, apparently bolstered MnDOT's decision to opt for more aggressive inspections rather than repairs. Last summer it offered that as one of three options; the others were to install reinforcing plates or to combine inspections with the retrofits. The Pioneer Press indicates that aesthetics may have played a role in that decision, although it gives no actual source quotes for it.

Redundancy has also come up as a focal point in the debate. In 1967, when this span was finished, bridge design did not always include redundancy, and this bridge was no exception. Since its design was fairly unique -- with no center support in the river -- the inspections became all the more critical. Newer bridges do have redundancies in their design to prevent catastrophic failure from a single breach. This indicates that design may have more to do with the collapse than first thought.

The news that the bridge was among the 100 worst heavy-use bridges in America will shock Minnesotans, and it won't be limited to us, either. That list should be made public immediately so that action can be taken to prevent another collapse.


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I just called in to Garage Logic where Joe was reading off a list of MN bridges that have been graded insufficient by MnDOT. I asked him what the hell good the report was if buried somewhere in the middle [Read More]

Comments (26)

Posted by Porkopolis [TypeKey Profile Page] | August 3, 2007 11:55 AM

Harry Reid is reflexively complaining about our nation's crumbling infrastructure.

With that in mind, the argument can be made the Pork Barrel Politics Kill.

Posted by La Mano | August 3, 2007 11:59 AM

But, at least you have a billion$+ trolley. Never mind the hundreds of millions for pro-sport complexes.

No problem here.

Posted by martin morgan | August 3, 2007 12:00 PM

All the levees in New Orleans passed inspection too. Until they failed.

Posted by n | August 3, 2007 12:02 PM

More than unfortunately, downright saddening, is the fact that these events are going to start happening with more frequency. I suspect it will become an election issue--repairing the infrastructure. But I'm on board for that. In an ironic way, it can be a litmus test for any politician. We know that they are going to spend and pass poorly conceived legislation--that's what they do. But now they have the opportunity to spend on things that need to be done and make laws--well, one out of two ain't bad in this case. Meaning that now in order to be an incompetent politician someone will have to work twice as hard. The carefully produced fog around their intellect and intentions will evaporate.

Posted by athingortwo | August 3, 2007 12:32 PM

We can appreciate that Captain Ed is trying to be informative with these posts of his on the Minnesota bridge collapse, but the fact is that any list of "100 worst high use bridges" is completely useless in figuring out what happened and how to prevent it from happening again. Just as useless as are media reports that the bridge was found "deficient" as long ago as 1990. Nearly all bridges are deficient to one degree or another ... and even if a particular bridge is perfect today, it's only a matter of time before imperfection sets in due to normal weathering processes, wear, and tear. These terms and lists are meaningless to the average layperson, not to mention the average AP or local newswriter who breathlessly reports on such information.

We do in fact have experts available, both within the responsible agency and in numerous outside institutions as well as the NTSB. The experts are going to study the collapse in great detail, and they are going to come up with recommendations as what we can do to prevent a repeat failure. Until then, all other discussion is uninformed twaddle. It's time for a "time-out" on all these wannabe bridge experts to shut up and stop spreading half-baked stories that amount to little more than rumor and innuendo and ignorant speculation and judgmentalism. I think Captain Ed is starting to vere into that realm.

Obviously, it is not acceptable for bridges to fall down and kill people in America.

Equally obviously, no system, nor public infrastructure is or ever will be perfect, even if we spent every penny that every working American ever took in as personal income and forced them to spend it all on bridges, railroads, airports, and highways. So it is by no means time to wail about underspending on infrastructure. Nobody ever has enough money for everything they ever wanted to spend it on - be it government, private enterprise, special interest groups, or individuals.

Note all the special interest groups yakking about how we have to spend double or triple on infrastructrure ... and how many of those groups' members include roadbuilding contractors, steel and concrete manufacturers, construction equipment manufacturers, etc. etc. Of course they demand we open our pocketbooks even more than we already do.

We don't know yet what was the cause of the collapse, but we will know soon enough. The experts will figure it out. Certainly we ought to be able to feel safe crossing bridges in America. But let's keep it real too, folks.

This is the first major bridge collapse in America since 1987 (i.e., the NY State Thruway). This failure killed maybe a dozen unfortunate people, more or less. About 3,700 times more than that number (i.e., about 45,000) are killed every single year on America's highways from accidents, most of which accidents are easily preventable by us drivers (i.e., don't drink and drive; don't drive aggressively; don't drive when sleepy; don't fix your hair or fiddle with the satellite radio when driving; etc. etc.). You're also far more likely to be killed by colliding with a deer or a moose than from a falling bridge in America.

In other words, this accident doesn't justify a political stampede to grossly ramp up government taxation and boondoggle spending.

Cool it, in other words.

Posted by Tim | August 3, 2007 12:40 PM

Being ranked in the 100 worst condition heavy use bridges does not contradict the claim that the bridge was "safe for use." The former is a comparison while the latter is a definition. They are as apples and oranges.

The bridge failed because of something someone missed. A failure in carrying out the inspection procedures or a failure to include everything that needed to be included in those procedures. The failures could have been due to malice, incompetence, or "unknown unknowns."

Engineers apply safety factors in their designs to account for these issues, but sometimes these factors are not enough.

When failures occur, some grieve, clean up, and try to do better the next time.

Others do otherwise.

Regardless, despite all of our efforts, the world is not a completely safe place, and no amount of private or governmental whinging will ever make it so.

My condolences to the unfortunate people involved in this latest tragedy.

Posted by I R A Darth Aggie | August 3, 2007 12:53 PM

This indicates that design may have more to do with the collapse than first thought.

As is usually the case, no one thing, of and by itself, is the root cause of failure. It will be found that a series issues, working together lead to the failure.

All the levees in New Orleans passed inspection too. Until they failed.

Keep in mind that some of the levees in the NOLA area weren't built to specification in the first place.

It does require one to ask are current inspection methods up to the task?

Posted by Mark Hagerman | August 3, 2007 1:06 PM

Being in the bottom 100 doesn't really mean much. By sufficient hair-splitting, we can RANK 4000 bridges by overall safety, but that doesn't tell us by how much the worst bridge is worse than the best.

What we'd like to see, if possible, is the probability per day of each one collapsing.

Posted by mikey | August 3, 2007 1:15 PM

Design? The Tacoma Narrows bridge was a design problem. Everything exposed to the elements has a finite lifetime.

This heavily used interstate bridge was 40 years old in a corrosive and extreme temperature environment. It would have been overkill, but if the engineers really had their way, this bridge would have been replaced at least twice already--unless it was maintained like the Golden Gate. Obviously, inspections aren't enough.

Posted by cirby | August 3, 2007 1:47 PM

The 100 worst professional basketball players could probably kick you to the curb in a one-on-one game, but that doesn't mean they can't play basketball.

Posted by Paul | August 3, 2007 1:49 PM

The whole flap about "structural deficiency" is a prime example of what happens when journalists and laymen encounter unfamiliar professional jargon. The "deficiency" in this case is akin to getting a 94% on a spelling test. Sure, my spelling is "deficient" in one way or another because I missed a few words, but the overall grade is acceptable when viewed in context.

There is rarely a single cause to an engineering failure. The space shuttle Challenger explosion is a good example. A design flaw coupled with abnormally low temperatures caused the o-rings to fail. Not only that, but concerns and warnings never made it up the food chain due to institutional failings. I suspect we'll find a similar chain of design problems, misjudgments and possibly even a convergence of abnormal conditions (like high temperatures + construction work + assymmetrical loading due to closed lanes) lay behind this failure.

Posted by TW | August 3, 2007 1:49 PM

Budget issues always weigh heavily in public works decisions. The decision they were trying to avoid was of replacing the bridge. They even said so in the report:

The authors said their research helped determine that "fatigue cracking of the deck truss is not likely." They added that the bridge shouldn't have any problems with fatigue cracking "in the forseeable future" and that there was no need to "prematurely replace this bridge because of fatigue cracking, avoiding the high costs associated with such a large project."

That they considered reinforcing the steel is a good indication of how concerned they were. But drilling all the holes to do that would weaken it too much so that's a non-starter, which put them back at replacement.

Going out on a limb here, it appears to me the bridge was already in failure and it took a perfect storm of events to push it over the edge. The roadway deck is most likely a composite, meaning that it acts to stiffen the steel underneath and also accepts most of the compression load across the top of the structure as a whole. But the bridge has sagged over the years, evidenced by the non-vertical alignment of the steel mentioned in the report, placing the deck into high compression.

Although the wearing surface should not be included in the ability of the deck to carry loads, it probably got included by the forces at work, and was temporarily holding the deck together. When the current work removed some of the wearing surface, the compression load in the remaining concrete became too great and it fractured. At that point it's no longer part of the structure, but is just more dead load. The steel below is no longer stiffened and also takes on the additional compression and weight, down it goes.

If my conjecture is right, the investigation would turn to whether or not there were recommendations on the amount of wearing surface that could be safely removed at any one time. Or, whether the portion of deck that comprised the composite was compromised, either before or during construction.

Posted by lcooker2 | August 3, 2007 2:22 PM


Wikipedia describes the bridge as an arch-truss style bridge. I am not an engineer but have done some
static truss analysis in early physics. The purpose of
the arch is direct the primary force vectors down the
arch, rather than vertically. If you look at other examples of arch-trusses, you will see that the arch is
rather steep, to direct the force vector angle down the
arch, with a smaller vertical component. Now, take a look at the arch on the bridge that crashed. There
is almost no arch. This means that, absent a
mid-river support column, the force vector is almost entirely vertical, down to the river, rather
than the support columns on either side of the river.

Posted by Kurmudge | August 3, 2007 2:30 PM

My mechanics expert (prof at a major international university; I sent the Captain a link to a very good primer on this stuff by this person) disagrees with TW above:

"Nah, I don't buy it and my bullshit detector is what makes me a good engineer, far better than most of my peers. The simple truth is: cracks only propagate in tension. Compressive loading only causes failure when it becomes extreme and then tensile loads develop. The structural elements of that bridge were what failed, and that's steel beams in tension. Cracking in the deck could happen without a catastrophic bridge failure. "

She (who has walked across and past that bridge many times) says that there had to be a fundamental design flaw and lack of redundancy.

Posted by Labamigo | August 3, 2007 2:38 PM

"The news that the bridge was among the 100 worst heavy-use bridges in America will shock Minnesotans, and it won't be limited to us, either. That list should be made public immediately so that action can be taken to prevent another collapse." Capt. Ed

Nope, I disagree.

With today's personal injury lawyers and politicians like Hillary and the Breck Girl, a city would be irresponsible if it did not immediately close such a bridge. And then the bridge sits idle for 5 or 10 years.

Posted by Paul | August 3, 2007 2:52 PM

With today's personal injury lawyers and politicians like Hillary and the Breck Girl, a city would be irresponsible if it did not immediately close such a bridge. And then the bridge sits idle for 5 or 10 years.

IANAL, much less a member of the Minnesota bar, but in most cases the doctrine of sovereign immunity would apply. In any case, most engineers I've known are extremely conservative in the conduct of their profession. If there were any evidence of imminent or likely collapse this bridge would have been closed immediately, out of concern for life and limb, not the threat of the plaintiff's bar besieging the courthouse. There could be some exposure for private entities, like private engineers retained by MinnDOT, however.

Posted by hap | August 3, 2007 3:19 PM

I keep seeing people talking about inspectors missing something. Everyone should keep in mind that inspections on any structure are imperfect because you can't take the thing apart and look at all the bolts, rivets, welds, rebar, steel...what-have-you. You can only inspect visible portions of the structure and infer what may be hidden. It's as much art and experience as it is science and engineering.

Posted by athingortwo | August 3, 2007 3:20 PM

Mikey - you say the Tacoma Narrows bridge failure was due to a "design problem" ... that's true to the extent that engineers had a lack of knowledge and applicable design codes at the time (1930s) that bridge was designed and built. Until the Tacoma Narrows bridge failed, structural engineers did not understand the interplay of how two different, yet related physics phenomena (aerodynamic forces and the effects of dynamic resonance) worked together to amplify stress. The Tacoma Narrows bridge failure eventually led to directed academic research, analytical modeling and wind tunnel testing, and thence to changes in the design codes - none of which design tools were available to the bridge's original designers. Now, of course, aerodynamics and dynamic structural analysis (including seismic as well as aerodynamic effects) are used for the design of virtually all safety-rated structures, inclusive of regional effects for wind-loading, seismic risk, flood and scour effects. etc.. Engineers simply know a lot more now about structural design than was known or even considered 60-70 years ago..

Posted by cathyf | August 3, 2007 3:33 PM

There could be some exposure for private entities, like private engineers retained by MinnDOT, however.
You say that so casually... I have a friend who is a retired civil engineer who had a successful design company. He bid on a project to design a bridge. He was not the lowest bidder, so he was not awarded the contract. The winning bidder went on to design the bridge, which was built. There was some defect found with the bridge, and the friend, along with the other two losing bidders, was sued. The legal theory was that they were partially liable for the problem because if they had bid a lower price on the project then they would have won, and so the bridge would have been designed differently...

Posted by TW | August 3, 2007 3:35 PM

Kurmudge -

Cool, we'll find out soon enough. I'd be interested to know if there is data that reports the amount of sagging over time, as well as the number and location of the cracks over that same time. They knew it was failing, just not at what rate.

Testing labs should do pretty well over the next few years as this investigation widens.

Posted by cirby | August 3, 2007 4:19 PM

Does anyone have reliable figures for how many major bridges get retired or demolished each year?

Seems like that should be the criteria for whether things are "safe" or not.

Posted by Kurmudge | August 3, 2007 5:35 PM

TW- wasn't taking a shot at you, just passing on information! I had actually suggested the same idea to her yesterday, and she shot me down fast. Her entire doctoral thesis was built on microfracture testing and crack propagation.

The reason compressive loads have to be extreme is that the tensile load on the opposite side is only a third of the compressive load- the full degree just doesn't get translated across. So for compressive loads to generate enough tensile stress to be the proximate cause of that kind of failure, the concrete would almost have to be compressively crushed into powder, not just show some potholes.

Most likely there was one hidden latent failure mode in an out-of-sight element, where the loads were then shifted to other underdesigned elements and triggered the cascade as each segment gave way and added to the stress on the next piece. This is exactly why the WTC fell, explosion conspiratorialists notwithstanding.

They saved a few bucks in 1967, though. The MnDOT designers probably got an award.

Posted by Del Dolemonte | August 3, 2007 5:49 PM

athingortwo said:

"Until the Tacoma Narrows bridge failed, structural engineers did not understand the interplay of how two different, yet related physics phenomena (aerodynamic forces and the effects of dynamic resonance) worked together to amplify stress. The Tacoma Narrows bridge failure eventually led to directed academic research, analytical modeling "

There was a lesser-known incident in the 1960s, although it wasn't a bridge. One of the US Coast Guard's LORAN C towers in the Pacific collapsed, and it turned out the cause was harmonic vibrations (I don't recall the exact details but it had something to do with the guy wires I think). In a nutshell, the tower literally shook itself to pieces.

Posted by TW | August 3, 2007 6:24 PM

Kurmudge - Thanks, I didn't take it badly at all, we're still guessing. I just wanted to think out loud. My thought was that the top chords of the steel truss went into catastrophic failure if the concrete deck did crush itself into powder and break the composite bond.

Yeah, I think a localized failure that cascaded is likely. But that -could- still point back to the retopping construction if they made some big deep cuts in just the wrong place.

Apparently the postbot here throws any post into the moderated bin if it includes a hotlink. But if you go to Wikipedia and search on 'bridge disaster' you'll get a list that includes some discussion of cause.

Posted by Steve Peacock | August 3, 2007 7:46 PM

What I find astounding is that, while U.S. bridges and roads crumble, our tax dollars are being used to help upgrade and modernize Mexico's highways, ports, and other mdoes of transportation. My research has uncovered such U.S.-financed Mexican-infrastructure projects, but has largely fallen on deaf ears, despite having supporting documentation to back up my claims. See the embedded link in this post for more info.

Posted by Conrad Wareham | August 4, 2007 7:18 PM

I am not a college grad engineer but have worked in the field many years around steel support structures, even in their fabrication and maintenance. When I first viewed the tragedy in the news with the I-35W bridge collapse I was in awe but also not surprized. Why? Because I am no stranger to dealing with the lack of concern for maintenance on assets by country and state government. The first thought that came to my mind was negligence. My interest in this tragedy caused me to do some research on this bridge and confirmed my suspician. It seems to me that this bridge was an accident waiting to happen. What is this cop out by some the "structural deficincy" doesn't mean anything? How about the bearings that needed replacement? What was the purpose of the bearings in the first place? Are not the bearings a maintenance concern? How about the rust on the bridge spans? Was there any ultra-sonic testing done to consider the amount of good metal left? How about vibration on the bridge caused by trucks & cars, or outside sources like the trains going under the bridge? there is so much technology out there now that has been developed to test metals for cracks, defects, and the result of stress, so why couldn't the engineers foresee what was going to happen with this bridge? Are the engineers with the MnDot operating in the stone age? Or were they influenced by higher ups that were more interested in the budget. We have a problem in our society of making capital expenditures for new equipment and infrastructure but not budgeting adequately for the maintenance of these assets, and worst not budgeting for their eventual replacement. So I see this tragedy as having been avoidable, but through apathy, ignorance, and neglect by the people we trust to take care and maintain the public assets, we were let down. I think when the people get over their shock and find out that this could have been avoided, heads are going to roll from the top down, and this will be a good thing.

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