Failure of anchor rod via stripped threads 4

[Ingenuity]

I recently repaired a 60 ton center-hole hydraulic ram (for post-tensioning bar stressing), and to verify that the new seal kit was indeed holding I did a recent load test. I grabbed a 1" dia x 105 ksi F1554 galv anchor rod with heavy hex nuts and washers (I usually use a 150 ksi PT bar but not on this occasion).

Anyway, got to about 3000 psi of gauge pressure (about 36 kip of load) and the threads stripped, pressure dropped back to less than 2,000 psi then I increased it again, and every increment to about 3,000 psi I continued to strip the threads.

So got me thinking that what I thought was 105 ksi materials appears to be only 36 ksi (1" dia tensile capacity is min 35 kips). Opps - and why I usually use 150 ksi PT bar with non-UNC threads.

But I was surprised that the it did not fail at the root of the threads. I have always assumed that the mechanical threads were stronger than the tensile failure of the thread-included section.

The separation you see in the photo is a grinder cut (not section failure) because I had to remove the bar from the test fixture.

Load was applied essentially monotonically, except for the stepping increment after first thread failures.

I learnt something today, besides keeping better account of anchor rod grades.

 

[dik]

I don't HDG Grade 105; I seem to recall that it shouldn't be HDG.

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

-Dik

 

[desertfox]

Hi XR250

Nuts are retapped after galvanising see this link:-

https://galvanizeit.org/hot-dip-gal...ar/connection-concerns/overtapping-guidelines

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dauwerda]

Thanks for the update, Ingenuity. Great information!

I wonder if manufacturers actually chase the threads after galvanizing or just make the nuts larger to begin with? I certainly have had my fair share of these nuts feeling loose.

If the nut is supposed to conform with ASTM A563, it is required to be over tapped after galvanizing.

7.4.1 Nuts to be used on bolts with Class 2A threads before hot-dip zinc coating, and then hot-dip zinc coated in accordance with Specification F2329/F2329M, shall be overtapped after coating, to the minimum and maximum thread dimensions in Table 5. The major and minor diameters shall also be increased by the allowance to provide the corresponding minimum and maximum major and minor diameters. When specified by the purchaser, lower overtap values are permitted as long as it is sufficient to permit free assembly with hot-dip zinc coated bolts

If mechanically deposited zinc coating is used, they are overtapped prior to coating, and not after.

 

[dik]

Just found out... can be... I've not done it and it may have been some earlier (or possibly wrong) info.

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

-Dik

 

[XR250]

Nuts are recapped after galvanising see this link:-

I imagine reputable manufacturers do this. I wonder if "offshore" suppliers would?

 

[dauwerda]

I don't HDG Grade 105; I seem to recall that it shouldn't be HDG.

For higher strength steels hydrogen embrittlement does start to become a concern during the galvanizing process (specifically when being dipped in the pickling acid). This is why ASTM standards currently do not allow A490 bolts to be HDG. F1554 Grade 105 is allowed to be HDG however (without detrimental effects).

 

[dik]

thanks dauwerda... I just looked up on Portland Bolt (my goto fastener site) and it appears that Grade 105 can be HDG... I posted the note above. I still don't think I will as I haven't done so in past.

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

-Dik

 

[r6155]

In my opinion, the procedure is:
1) One ungalvanized and one galvanized anchor stud shall be tensile tested. Both with the same batch and material test report. I will be present at the test.
2) I will be present during the Galvanizing.
3) The dimensional control will be done with calibrated instruments.

I don't trust anyone.

Regards

 

[SandwichEngine]

I'll do you one better. Had a building get partially blown down during erection during a straight line wind event. Everyone is safe. The column tipped over from the bottom of the photo to the top. Some anchors fractured as expected. Some striped. Some pulled out of the concrete partially and then stripped. One partially stripped. My best guess is that the stripped anchors are on the high side of the baseplate as the column fell and somehow that loaded them at an angle or also induced bending and somehow that caused those to strip. Really have no idea. Thoughts welcome.

 

[desertfox]

Hi SandwichEngine

When the column toppled were all the anchor bolts fully tightened, or were some, or all of them at some intermediate tensile loading?
The threads that stripped could that be from the side of the hole in column baseplate coming into contact with the anchor block as it fell over?
From the picture it appears all the anchor bolts failed across the section of the bolt.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[dik]

Is that ever neat... I would have thought they would have 'torn' the anchor rods at the U/S of the nut.

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

-Dik

 

[Tomfh]

This is all very depressing. I always trusted the threads to yield the bolt.

 

[dik]

That could be, Tom... I've always thought the threads would be intact and the threaded part would fail... It makes more sense for a ductile failure if the threads fail. Maybe failure mode was correct.

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

-Dik

 

[desertfox]

Hi all

Here is an interesting site about bolt failures:-

https://www.bulten.com/en/Services/Academy/Failure-Modes

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[Brad805]

The damage to the threads could have been the reality of the rotation during the collapse? A picture of the mangled base plate would be interesting.

Disregard image. Misread.

 

[dik]

Excellent link, Fox...

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

-Dik

 

[SandwichEngine]

Brad805,

The column tipped the opposite way from what you show.

Desertfox,

It's very possible that some of the bolts weren't fully tightened. The building had yet to be racked square. I don't know if that's a standard erection procedure but several other anchor bolts in parts of the structure that didn't fall had some bolts not all the way down.

 

[dik]

A question: Do you want the failure to occur by thread stripping (for ductility) or through the bolt itself (less ductile)? I've always thought that failure should occur through the tensile stress area... now I'm not so sure. Also, I would think that the nut threads would be slightly stronger than the bolt threads.

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

-Dik

 

[Brad805]

I see I misread. Too early I guess. Was there a grout allowance for these base plates? If so, I would expect the columns would have been bearing on typical steel shim stacks. If that were the case I would expect the loading to the anchors would be very different than design during the collapse.

 

[desertfox]

Hi all

dik in my opinion bolted joints should fail through the tensile region of the bolts and not through thread stripping; however if the all anchor bolts were not fully tightened down then that would mean the anchor bolts had different tensions just prior to failure. That would then mean that some of those bolts might well see tensions that far exceed what they were designed for and that might explain some of the thread stripping. Normally the female thread shear area is larger than the thread shear area of the anchor bolt so you are correct in saying that the female threads are stronger but the material grades of the nuts and bolts would need to be considered.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein