A325 vs. A325 TC 4

[SKIAK]

I'm holding in my hands an A325 3/4" diam heavy hex bolt 1-3/4" long, and also an A325TC 3/4" diam 1-3/4" long. So far as I can tell from AISC, the length these bolts are equal. Looking at them from the side it would appear that the TC bolt only has complete threads out to approx 1-1/2" (before it tapers off) where the heavy hex bolt has about 2 more threads.

This is something that came up in the field. The contractor felt after installing the specified TC bolts that they weren't long enough and ordered longer ones. They are asking me to justify that the detailer specified the wrong length of bolt. AISC says the end of the bolt only needs to be flush with the end of the nut, which it was. I only bring up the heavy hex to compare with the TC, with the differences in threads I find it difficult to say the effectiveness of these two bolts is equal.

The bolt manufacturer stated that only 1 to 3 threads in the nut actually end up taking load and that is all that is required to fully engage the bolt. I can't find anything to back this up.

In reality, it seems to me that AISC says its OK since it does not mention that all threads need to be engaged. However I find it strange that one bolt would have less threads and still be considered equal from a specifying standpoint. Thoughts?

 

[Ron]

If you check a couple of different bolt manufacturers, you'll probably find a slight difference in thread count, given a minimum number to engage the appropriate nut (heavy hex or standard). The 325TC is a completely different bolting system that only compares to other A325 bolts in mechanical properties. It is engaged and tightened differently.

As for the 1 to 3 thread engagement being all you need...that's not true. Full thread engagement is necessary to develop full tensioning with less creep for long term application, and you might not be able to develop the full tension required with only slight engagement. Stick with having at least a flush condition (bolt to nut).

 

[paddingtongreen]

If there is money involved, the only way is to check the drawings to see to see if the bolt is long enough to hold a washer, the "grip" (the connected parts) and the nut and still have "full engagement". If it does, then the detailer was correct. Full engagement is the legal requirement, but the contractor is required to "snug" (close the gaps between plies) the joint before breaking the TC bolt.

You should only check the bolt against the ASTM/RCSC dimensions. The other manufacturer's bolt means nothing in this argument.

Normally, the detailer calculates the "grip", the washer and the nut plus 1/8" and then rounds up to the next standard length. This usually leaves a short piece of thread showing, that may be what is throwing the contractor, but if the bolt end was flush, he is wrong.

Check the RCSC Specification for Structural Joints Using ASTM A325 or A490 Bolts for dimensions, it's in the AISC Manual, or check the ASTM A325 itself. There is a slight difference between rolled and cut threads.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

Michael...I'm not suggesting that he check the bolt against another manufacturer for any reason other than to check thread "run on". It could simply be a manufacturing anomaly. My point was not to simply accept the condition of one bolt as the de facto standard for all bolts.

 

[paddingtongreen]

Ron, I understood that. I just wanted to make it clear that SKIAK's decision had to be only against code requirements. I never have a problem with someone looking all around the subject and learning what there is to learn. I know there is a big difference in the appearance of rolled and cut threads.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SKIAK]

Thanks for the replies. Just going off of some information that I found on the net, I find that I need about 6 threads to be engaged to make sure the failure occurs in the bolt and not the nut. If the TC bolt is flush with the end of the nut, like AISC says, then I get only about 4 threads engaged in the nut. I would seem to think that it needs more than that.

 

[paddingtongreen]

SKIAK, I don't know where you got the 6 thread info, but the AISC is the governing code here in the USA, and TC bolts are well accepted as an alternative installation system. If the detailer provided bolts that met the AISC rules, they are fireproof.

I worked on a high importance job before there was much experience with the TC bolt, so we busted a percentage of each heat, in a Skidmore, for statistical analysis for the 12,000 ton project but none failed the criteria.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[SKIAK]

I'm planning to follow what AISC says, I feel fairly confident that they put some thought into what they say, but I would like to find a way to back this up. The information I have found would lead me to believe AISC's recommendation that the end of the bolt is flush with the end of the nut doesn't ensure that the bolt will fail first, maybe it doesn't matter that much... but then again I'm just pulling info of the net.

There are a few sites online that have ways to calculate the required thread engagement, they all seem fairly similar, but there is one here:

http://www.engineersedge.com/thread_strength/thread_minimum_length_engagement.htm

I have also attached a photo of the bolt I'm talking about FYI. The detailer also told me that they do not use AISC's Table C-2.2 (pg 16.2-12) to determine required length.

 

[nutte]

You're second-guessing AISC and RCSC on bolt installation and looking for information to back up their recommendations? Because of some information you found on the web? Seems like a waste of effort to me.

 

[SKIAK]

Nutte:
"Seems like a waste of effort to me." -You're probably right.
"Because of some information you found on the web?" -No.

If you specify an A325 bolt using AISC, regardless if it is a TC or Heavy Hex, you get the same required length. If you look at a TC and a Heavy Hex next to each other you do not get the same amount of threads. I would appear to me that AISC is not very concerned with the amount of threads you have. Maybe it is overkill to begin with, I just wanted to verify that.

 

[Ron]

skiak...as long as you have full engagement of the nut and you have considered whether or not you have threads in the shear plane, it doesn't matter.

 

[SKIAK]

Thats what I have issue with. What does it take to have full engagement of the nut? I would assume all threads in the nut need to be snug against threads on the bolt. Following AISC (or RCSC) you do not get this in a TC bolt because of the tapered threads.

I see it this way for a 3/4" diam bolt:

Heavy Hex: the nut is about .7", with 10 threads per inch minus a little for the start and end threads, so say 6 full threads in the nut.

TC: the end of the bolt tapers about 1/4" so I say we lose about 2.5 threads. 6 - 2.5 = 3.5

The way I see it, AISC only requires you to have about 3.5, or about half, of the nut engaged for an A325TC bolt which has identical mechanical properties to an A325 heavy hex. I realize they are engaged differently but they should behave the same once fully engaged. I have seen the same taper in every TC bolt I have run across, and from different manufacturers.

 

[paddingtongreen]

SKIAK, You say the TC bolts are all alike, that should be a clue; they have been used in important structures for about thirty years that I know of, maybe more. Perhaps it is the thread requirement that you found that is bogus.

Perhaps, if you got a copy of this document, you would be reassured. I don't have one so I don't know if it covers the dimensions that worry you:
ASTM F1852 - 08 Standard Specification for “Twist Off” Type Tension Control Structural Bolt/Nut/Washer Assemblies, Steel, Heat Treated, 120/105 ksi Minimum Tensile Strength

http://www.astm.org/Standards/F1852.htm

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

A325TC bolts should be matched assemblies...threads in nut match bolt. You should not be losing thread engagement.

 

[SKIAK]

paddingtongreen:
We currently don't have a copy of F1852 but I brought it up to the higher ups and it should be getting ordered. I don't have much confidence that this will cover what I'm looking for, but I guess you don't know until you know.

Ron:
I see that they all come in assemblies, but if the nut is flush with the end of the bolt you lose several threads.

Thanks for the replies. My supervisor wanted me to hash this out incase the contractor comes back looking for compensation. I'll keep looking.