Wednesday, May 12, 2021

I-40 Bridge over Mississippi River, Cracked member


First, let me lead off by saying that I don't have any inside information.

Second, me assure you that I have done more stress and durability analysis than the usual Internet, Armchair Expert.

Had the cracks occurred at the ends of the doubler plate it would have been understandable. Cracks occur at the end of reinforcing plates every day and twice on Sunday. But this crack was not at the end of the reinforcing plate and was more than a "crack". It was a total fracture.

What is baffling about this fracture is that there are no visible, abrupt changes in geometry that say "stress riser". Nor, looking at the location in the overall structure, is there any reason to suspect high stresses at this particular point.

Perhaps there are internal gussets and reinforcements that are not visible but that would not be an economical place to put them from a fabrication standpoint.

While it is remotely possible that the beam was too short and a bit was welded onto the end to make it longer, the fracture is not quite perpendicular to the section. That makes the fractured weld hypothesis very unlikely.

What no responsible engineer wants to suggest is the possibility that steel beams with very-low, cold temperature impact toughness were built into critical, US infrastructure like bridges.


Low-carbon steel is a marvelous material. It is inexpensive, tough, amenable to common fabrication techniques and equally strong in all directions.

During WWII, steel became critical to the war effort. War runs on nitrates, oil, steel and blood.

Somebody had the bright idea adding a little bit of this and a little bit of that to common, low-carbon steel and increasing the strength by 50%. The thinking was that if it was 50% stronger than applications like ships would use 30% less.

Contributing to the soon-to-be-problem, production science suggested that welding was faster than riveting. Ships designed to transport US war materials to Europe were made of High-Strength-Low-Alloy (HSLA) steel and were primarily of welded construction.

U-boat predations of merchant shipping made more northerly routes advisable. The high seas of northern latitudes made it more difficult for Nazi U-boats to get a good periscope fix on targets to launch torpedoes.

Toilet paper

Toilet paper comes in rolls. The strip of paper is scored with a row of tiny holes about every four inches. I trust that most of my readers have seen pictures of toilet paper if they have not personally used it.

The user unrolls the desired amount and initiates a tear at the closest row of perforations.

Almost always, the paper separates at the desired location. It happens with such reliability that those of us who use toilet paper rarely give it a thought.

Some materials are supremely insensitive to suggestions like the row of perforations. Bubblegum is a prime example.

Other materials are supremely sensitive to suggestions. Glass for instance. The slightest scoring of the surface will cause a pane of glass to fracture in the indicated location.

Back to WWII

It was well understood that higher strength steels were often more brittle than lower strength steels.

Alloys were prepared. Tests were run. Ships were built.

What nobody had anticipated was that the first generation of HSLA steel used to build the Liberty Ships were temperature sensitive. Alloys that performed well at 60F could fail miserably at 20F.

North Atlantic. Cold. Stormy. High, humped waves.

All welded construction. 

Square-cornered access to holds, designed to facilitate rapid production.

Documented cases of captains looking for U-boat periscopes seeing six Liberty ship in one direction and by the time they swept back in that direction there were only five. The sixth ship had snapped in half and sunk just that quickly.


Why did ship-designers choose the first HSLA alloys?

The entire point of choosing them was that they didn't use rare, costly alloying element nor did they require exotic thermal processing at the mill. After all, mill capacity was a limiting factor.

Those economics have not changed. First generation HSLA steels are still less expensive. There is economic advantage to slipping a few (or more than a few) into the mix. After all, when the Interstate system was first built the ability to give every steel beam a unique bar-code label was a pipe-dream. It would not be that hard. And if they were sent to a hot state like Tennessee, what are the chances of them ever getting cold?

The crappy thing is...

The only good way to test for low-temperature notch sensitivity is to cut a sample, notch it, cool it and test it.

There might be twenty million steel beams built into bridges in the US.

Furthermore, those beams might be just fine until somebody comes along and cuts a sample out of them. Then the beam will be like the pane of glass with a line scored in the surface.

Like I wrote earlier, no responsible engineer wants this to be the case.

They really, really, really want to find some other "smoking gun". Otherwise it will be at total shit-storm.


  1. Looking at the overhead pic I see what looks like corrosion, rust and paint loss on the flat upper surface of the beam. It looks white in the photo. Is this a sign of deterioration of the beam?
    There is a second bridge nearby (I-55) to take the load. However, all NB and SB river traffic is stopped and will be for an indeterminate time. That is a significant supply disruption. Could be lots more economic disruption.
    Please keep an eye on the investigation.

    1. I would expect red-rust. I think that might be sand/cement dust that filtered down.

      It is notable that there are no rust stains around the crack. That suggests that the fracture is recent (i.e. less than five years ago)

  2. So what you are saying is that Uncle Joe and the Ho's "infrastructure" plan should actually make some much-needed repairs and replacements to our "infrastructure" such as bridges and highways instead of just sending billions and trillions to their friends and to foreign countries ? FAT FREAKIN' CHANCE !

    1. 'Iron'y hah. As least the bridge repairers will have excellent child care infrastructure available.

      When I spec drill pipe, one of the many many items considered is material toughness. Testing of drillpipe steel, as it is being manufactured, is exhaustive, and it includes the Charpy impact testing you've mentioned. But material toughness has always been, to me, one of the most important aspects - considering the comprehensive battering that drillpipe is subjected to over its design life.

  3. Is it possible that a not visible external stress caused the fracture? Such as part of a bridge piling settling or shifting just a fraction of an inch, perhaps due to instability of the bedrock under the riverbed, or perhaps due to a small earthquake that barely registered.

    I am sure this is the kind of cause that engineers are hoping to find, as it might then be concluded that this occurrence was unique to this bridge. The alternative is ominously costly and scary.

    I can guarantee they are working around the clock to repair this. Just the daily cost of stopping all barge traffic under the bridge in the meantime is astronomical.

    1. If this is a .gov project, are they really working around the clock to get this done asap? Having grown up around NYC, I saw decades-long construction that should have taken just a few years, but the real purpose of the project was the redistribution of tax dollars.

      Is this a foreshadowing of the last chapter of Atlas Shrugged?

  4. What surprises me, is the point of shear (as you said, no stress riser visible) given that the long section of beam leading up to it did not flex over it's length.
    Cold, or not, it's not like the Edmund Fitzgerald riding the waves.
    I'm really curious as to what the repair technique will be.
    Unless they discover many other fractures elsewhere.

  5. My bet is internal corrosion or a shifted foundation.

  6. I would have expected a break at the end of the visible section; breaks don't happen where there is no change in load. it also looks VERY even and in a relatively convenient place - could this have been sabotage of some sort?

  7. This is going to open the proverbial can of worms on that bridge... And I'm betting on months of closure as the re-examine the entire structure to see what 'else' is going to be found.

  8. Having spent years investigating structural deterioration of different sorts, this one is a mystery. From what I see, corrosion is not an issue. Not a lot of horizontal or vertical displacement. Off the top of my head, maybe a material flaw of some sort from the original beam production? Engineers will have fun investigating this one.

  9. Cold theory? Hmmm
    What was it that happened just a couple of months ago? A record breaking Deep pool of Very Cold air that broke all kinds of records for length of time below freezing even as far south as the Texas coast. If your supposition is correct on the cold causing a fracture in a susceptible HSLA part, then it could be a tip of a very large Titanic shaped ship. That girder could be cut free and analyzed to see if it is in fact sensitive to cold. While the rest of the bridge may take longer to investigate, this smoking gun should be able to shine some light on whether this is one-off or part of a possibly very large problem.

    MSG Grumpy

  10. High volume bridges are subject to a lot of vibration. Does bridge steel suffer from the same weakness as aluminum wherein vibration causes scratches to become cracks?

  11. Gee.....speaking of cracks & 'members', it's been a long time for this old fart, I wish my member could find a friendly crack

  12. Gee.....speaking of cracks & 'members', it's been a long time for this old fart, I wish my member could find a friendly crack

  13. @Ms. Anne Thrope- are you mixing your metaphors ? Or are you one of those new-fangled she-males or something ? Plus- you stutter.

    1. Misanthrope definition: a person who hates or distrusts humankind.

      Example in a sentence: "A misanthrope would tell a city slicker that a conibear is a countryboy sex-toy"

  14. I was commenting on the NAME that he or she chose. "Ms." is generally used to denote a female of the species. But who knows these days...

  15. Ain't the interwebz great! You can pick and choose your level of anonymity, humor and lack thereof and even your very gender (as well as any other facts you may be uncomfortable with...)
    But facts is facts fellows and fellowettes. That bridge is broke. The government that is responsible for maintaining/repairing that piece of infrastructure is, and has been, notoriously incompetent and corrupt for a looong time...
    Whatever is done about it is apt to be not enough, overpriced and barely delivered.
    'Let no opportunity go ungrifted' has been the unspoken agenda of the political class. I fully expect this to blow up into a multi trillion dollar infrastructure boondoggle at all possible legislative speed.


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