Some of the comments in this thread seem to illustrate that fighting ignorance is no longer a priority in some corners of the SDMB. I do appreciate the actual information some have given.
For those of you who think closing the river to barges is not a big deal, you are mistaken. Barges are (excuse the technology here) really, really big. The notion of using semis to provide for the transportation of, say, corn normally carried in barges gave me a good laugh. If the river is going to be closed for very long the effects will be felt very widely.
Agreed. If either section of bridge fell it’s going be be unsurvivably violent right near that spot. He’d be a red greasy smear on some hunk of light green steel 15 feet down in the muck at the bottom of the Mississippi before he knew what hit him.
I was just reminded of innumerable Warner Brothers cartoons where the protagonist ties himself to the other half of whatever he’s sitting on and sawing apart. Slapstick ensues. Fortunately 'toons bounce and shapeshift as needed. Men not so much.
Here’s something pretty odd–on my blog I posted a photo that I took crossing the I-40 bridge (looking through the windshield at the “Welcome To Arkansas” sign about halfway across) and mentioned that I crossed the bridge shortly before authorities closed it.
Something about that post caught somebody’s attention and my blog was flagged by google and taken offline. Google’s blogging team deleted the post, telling me that it violated community standards. After a few emails back and forth with no real explanation of what my actual infraction was, they said they had made an error and restored my post and but my blog back online. Very strange.
I’m not sure that beam was ever under significant tension. The arches were build before the tie beams were added: the tie beams take tension only if the foot of the arch moves outwards after construction.
My theory is that the tie beams actually exist to stabilize the structure, handle thermal changes, and to hold it together if something else fails. That box-girder beam is designed to be stiff, not strong in tension.
Also … very odd the way it’s torn, like a fatigue corrosion failure in torsion, or a cracked weld.
I wonder how the crewmen trapped on the towboats were faring before the river’s constipation was relieved? A darn sight better than the folks trapped on various Suez craft over the years have fared no doubt.
IANA Structural or Materials Engineer, but I noticed the same torsion and wondered about that.
If there was an unplanned long-term torsion load on the member it would not be too surprising to have it fail that way. Finding out more about whether there were, or are, unplanned torsion loads should probably be on the agenda.
OTOH, once the member failed, it’s not too surprising that the rest of the structure twisted slightly as that load path became inoperative. Which would pull the broken ends, at the end of a fairly long lever arm, into a position reflecting the new load distribution, not the former undamaged load distribution.
The thread on the engineering experts’ site cited above had none of those folks commenting on the torsion, or at least not that I noticed. Which makes me suspect it’s more the latter scenario.
I look forward to somebody who has real skill in this area commenting on my WAGs.
What I was looking at was the nature of the cut: it looks like it might have been torn or cracked, not bent or stretched. My theory about the structure is that those beams primarily stiffen the road bed (which is just suspended by strings from the arch). The other site has people thinking that they are tie rods, but — I don’t believe it.
That’s beyond my expertise. I recognize the terminology but that’s about where I run out of knowledge. It will make for fascinating reading when the final full report does come out.
“The Arkansas Department of Transportation announced today it had fired an unidentified bridge inspector who missed structural failures in the Interstate 40 bridge in 2019 and 2020.”
My best guess at this point is that it is a fatigue crack that initiated in bending due to alternating stresses caused by movement of the other end of the beam. This end, being fixed, created the stress concentration.
In addition, I suspect the alternating stresses are considerably higher than the fatigue limit of the steel. With alternating stresses barely above the fatigue limit, you tend to get several cracks forming. As only one crack seems to have formed and progressed in two years (and progressed quickly), this likely suggests that this is the only fatigue crack in this member. Other members may have other cracks. A full Magnetic Particle inspection will be needed to discover any smaller cracks.
There is another image on the Eng-Tips board (Discourse won’t let me link to it directly) that appears to show the weld joining the top plate to the inboard plate of the beam has failed as well, probably due to twisting of the beam as only a small ligament of the bottom plate remained.
The 45 degree line is what you get when a material that is weakest in tension or compression fails in shear. It’s also not what typically happens with a box girder (where you get buckling rather than a tension failure). I was positing tearing, which does not imply a shear failure.
Interesting. There also appears to be a buckling near the top front edge to left of the eyelet-coupling-thingy, maybe twice as far to the left as the crack is to the right.
Also, what’s going on with the bottom edge on the left? Although I realize it’s not diagnostic, especially in such a low res image, but that contrast looks very like the crack. I wonder if that thing has been rusting out from the inside? Surely the non-criminally-incompetent inspectors check wall thickness every so often?
I saw that, had a double look, thought !!! – then I thought naw, it must be just pixelisation, if that was real, somebody else would have said something about it –
It’s also got a picture of what the old bearings looked like (pointy instead of flat)
As I read it, they jacked up the bridge to swap out the bearings.
Some of the theories that have been advanced:
The strings that support the roadbed aren’t the same length and don’t support the same weights: this caused the road bed to cycle up and down with temperature changes.
There’s been a lot of heavy trucks over the bridge over the years. This caused the roadbed to vibrate up and down quite a lot.
