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Tappan Zee Bridge Broken Bolts 4

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Counterfeits?

The problem with sloppy work is that the supply FAR EXCEEDS the demand
 
If they are failing in use, why do the bolts appear to have been twisted apart?
 
Nothing to explain why the Hydrogen Embrittlement... coating looks like Dacromet. Faulty material, were metallurgical tests done at the time of fabrication? Were the bolts fake, from an offshore source? I was involved with a major landmark project in Toronto where offshore A325 bolts ended up on the project. Failure looks like my A490 failed bolt sample under high tension and shear.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
stevenal said:
If they are failing in use, why do the bolts appear to have been twisted apart?

You have to apply torque to bolts when you preload them. That torque remains as internal stress inside the bolt after tightening is complete; there is no path for the internal torsional stress to be relieved unless something moves after preloading.

In the lab on a machine, bolts fail with perfect necked regions normal to the bolt axis consistently; in the real world they often fail in more complicated ways.
 
You don't "have to" apply torque to preload bolts. Systems such as SKF's Hydrocam don't torque the fastener.

This hydrogen embrittlement happened on the San Francisco Bay Bridge as well. No counterfeit was necessary, the bolts were USA made. The engineer specified a galvanized coating. I remember hearing about the galvanizer acid washing the bolts but isn't the galvanizing solution also rich in hydrogen due to electrolysis?
 
I'm not sure that torsion from original torquing load makes a lot of sense to me (as an EE) since I thought most of the applied torque goes to developing axial preload and only a small fraction overcomes torsional friction in a well lubricated thread.

I agree real world is different, the actual load on the bolt can be prying with a moment.

With all that said, I remember hearing about stuff failing in the plane of maximum shear stress... it seems like for pure axial load that would be at a 45 degree angle like I thought I saw in the photo. I'm sure much smarter people than me can give better guesses at the failure mechanism by examining the photos.

=====================================
(2B)+(2B)' ?
 
I'm not an expert by any means, but I think part of the phenomenon that gives the appearance of a torsion failure is from the spiral nature of the threads. The failure initiates in the minor shank and sort of follows that path along the thread curve as it approaches the brittle fracture point. At least that's my best guess.

Btw this is a really morally depraved thing they (allegedly) did here. They knowingly covered up a potentially catastrophic defect then had the gall to argue for the records to remain sealed to prevent terrorists getting ahold of the structural details... No, these people are lower than terrorists -- because they were willing to risk innocent lives for just money.
 
The story also mentions a construction worker was injured by a broken bolt. It would be interesting to learn more about what exactly happened there. Also sounds like if it wasn't for that injury the safety manager may never have caught wind of the cover up scheme...
 
Some failure modes for highly stressed bolts release all of the strain energy sufficiently fast that the bolt and nut can exit the joint (opposite directions) rather fast. That is one possible contributor for the injury report. In the case of hydrogen embrittlement cracking the failure might be delayed as the cracks need some time to grow to the point of sudden failure.
 
Did they ever determine the cause of the hydrogen embrittlement?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
dik said:
Did they ever determine the cause of the hydrogen embrittlement?

Possibly:

galvanizeit.org said:
...steel with ultimate tensile strength greater than 150 ksi may exhibit hydrogen embrittlement due to a tight grain structure trapping hydrogen molecules readily available from the pickling acid used in the galvanizing process

Source: American Galvanizers Association: Link
 
Unless something changed in the last 8 years, they aren't supposed to be hot dipped for that very reason:

Capture_uj5oqg.jpg


[URL unfurl="true"]https://galvanizeit.org/knowledgebase/article/a-490-bolts[/url]
 
faceengrpe said:
Some failure modes for highly stressed bolts release all of the strain energy sufficiently fast that the bolt and nut can exit the joint (opposite directions) rather fast. That is one possible contributor for the injury report. In the case of hydrogen embrittlement cracking the failure might be delayed as the cracks need some time to grow to the point of sudden failure.

Yup, I've done this, the sex screw holding the seatpost in my bike went 'pop' and shot across the room when I overtightened it.
 
electricpete said:
only a small fraction overcomes torsional friction in a well lubricated thread.

That's true, IF the thread is well lubricated. While it's possible the steel crews were lubricating all these bolts, I doubt it.

Even with lubrication- a 1" A490 bolt has a proof load of 95,000 lbs or so; a 'small fraction' of 95,000 lb is still a lot of force.
 
The automotive industry got smart decades ago and started using Magni 565 coatings instead of galvanizing on their high strength chassis fasteners. It seems more recent structural applications are starting to favor high strength fasteners and the experience just hasn't caught up. Eventually they will learn to not galvanize high strength fasteners. Interesting to me, the joint appears to be well coated/sealed and corrosion resistance of the fastener shouldn't have even been a consideration.
 
TugboatEng - I can't speak the decision to galvanize these, but the experience you mention has caught up. See the last line in the post I made earlier. "A490 only allows a type of zinc/aluminum coating..."

Magni 565's data sheet says "Magni 565 is a chrome-free duplex coating that combines an inorganic zinc-rich basecoat with an aluminum-rich organic topcoat."

Of course in this case it sounds like they hot dipped it anyway if hydrogen embrittlement is really the cause. I would speculate, however, that it's substandard bolts. They mentioned that the bolt failures were more-or-less isolated to certain sections of the structure. That bridge was a series of prefab elements that were manufactured at a variety of yards (including one here in Hampton Roads) and shipped to the site by barge. It's possible that one of the yards had a bad supply of bolts. I repeat, this is speculation.
 
I thought everyone was well aware that A490 aren't to be HDG... The bolts appeared to have a Dacromet type of coating system. Were the bolts HDG?

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik
 
And every now and then you find someone electro-galvanizing in place of hot dip, which is even worse for hydrogen.
I am sad Moon, my bike doesn't have any sex screws ....

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed
 
I'm making assumptions because our local bridge, the San Francisco Bay Bridge experienced similar problems. There were two cases of failure and hydrogen embrittlement was a factor in both. The first, the bolts were HDG and had been pickled. The second case they blamed on poor grouting which allowed water to come in contact with the bolt. I didn't know stagnant water to be a source of hydrogen embrittlement but then again I don't know many things.
 
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