Turn-of-the-Nut w/ DTIs? 1

[ChuckerD]

An erector is asking whether it is acceptable to use turn-of-the-nut method to achieve full pre-tensioning of the bolts where there is an uncompressed DTI washer installed at the bolt.

My answer was that turn-of-the-nut is not applicable because the uncompressed DTI washer tells us that full pre-tensioning has not been achieved. Therefore tightening must continue beyond the turn-of-the-nut requirements to compress the tabs on the DTI.

Any thoughts?

Thanks,
ChuckerD

 

[HgTX]

If turn-of-the-nut tells them one thing and the washers tell them another, someone's doing something wrong. Is the DTI completely uncompressed, or just not quite achieving the proper gap size?

There is a range of possible acceptable tensions from the minimum required to achieve the proper friction force to the maximum of getting dangerously close to the bolt capacity. The developers of the turn-of-the nut procedure have chosen a particular rotation to give a desireable tension, enough to keep the connection together but not more than is needed so as to stay away from the limit.

If they follow proper turn-of-the-nut procedure, they will get whatever tension the developers of the turn-of-the-nut procedure have decided counts as "full".

The DTI manufacturers have calibrated their washers to compress at an acceptable tension as well. Different types of DTIs are supposed to be calibrated to match different bolts, so theoretically they should compress at the same tension that matches the turn-of-the-nut proper tension.

So the possibilities are:

1. They're not actually following proper turn-of-the-nut but instead using some torsion-based system.
2. They're using the wrong turn-of-the-nut procedure for the bolts at hand.
3. They're using the wrong DTIs.
4. It's not actually off by that much. If you were so inclined, you could find out what tension in the force unit of your choice corresponds to the proper compression of the DTI and to X turns of that thread bolt, and see if they really are the same.

Hg

 

[HgTX]

Also you might want to put a link to this thread over in the welding & fastener forum, forum725.

Hg

 

[Ron]

The methods are "either/or". Don't mix them up. No, it is not proper to do turn-of-nut over a DTI.

While properly done turn-of-nut method will give consistent results, they are not quantified as in the semi-quantified DTI method, so there is no real correlation between the two methods.

Either method can be checked in a Skidmore-Wilhelm tensioning device to see if you are getting the desired result.

There are several reasons the DTI might not be working. The contact face of the nut or bolt might be galling, thus indicating you have some bad fasteners (assuming you are using ASTM A325 or A490 fasteners). The DTI could be upside-down. The fasteners are stretching (again, a poor quality fastener).

 

[HgTX]

They could also have failed to go back and re-snug the earlier bolts after snugging the last bolts, meaning that they were starting from less than snug-tight on some of those bolts. Many ways to mess up turn-of-nut.

In my earlier post I started with the logic that since there is no One True Tension, the two methods might not necessarily give the exact same tension, but then by the end of the post I wound up sounding like it should be exactly the same. Not what I meant, though they should be in the same ballpark.

I assume spec called for DTIs. Out of curiosity, what justification does the contractor present to justify leaving the connections with uncompressed DTIs?

Hg

 

[ChuckerD]

The bolts in question are 1.25" dia A490. The clearances for the bolts were provided according to tabular dimensions in AISC spec. Unbeknownst to us (and not noted in the literature) was the fact that said bolts cannot be torqued enough to fully pretension without using a multiplier - which will not fit in the provided clearance.

The erector was able to compress the tabs on the DTIs using a sledge and knocker-wrench (very difficult 35' in the air in a lift). However, compressing the DTIs took nearly a full turn past the snug-tight condition (as opposed to the 1/2 turn required by turn-of-the-nut for these bolts). The end-owner was willing to accept use of turn-of-the nut in place of DTIs to ease installation of the bolts per the erector's request. Several locations already had the DTIs installed prior to this request, but with very little compression of the tabs.

 

[Ron]

ChuckerD....I see the problem now. It appears that turn-of-nut was done on the DTI's thus the comment "compressing the DTIs took nearly a full turn past the snug-tight condition". You can't do that and get any substantive information. While you are trying to compress the DTI using turn-of-nut procedures, you are not in a "snug-tight" condition...you are trying to deform a compressible material (the DTI nibs).

Check both in a Skidmore-Wilhelm device...you'll see that a properly applied Turn-of-nut method will favorably compare.

Do the turn-of-nut without the DTI's. You will likely find that the 1/2 turn requirement results in sufficient tension (check it with the Skidmore)

 

[Lutfi]

Be careful where the erector places the DTIs. Read the suppliers recommendations.