Turn of the nut method with an impact gun?

[psychedomination]

I am currently working on a bridge that has slip critical connections. The bolts need to be pre-tensioned to about 170kN. The method chosen to pretension the bolts was the turn of the nut method, using a 1/3 turn. I am having some issues with the contractor in achieving this third of a turn. It was originally thought that the contractor would snug tight the bolts and then extend the lever arm of a wrench with a piece of steel pipe or something to turn the nut a 1/3 of the way. The contractor is stating that there will not be enough space under the bridge to use an ~ 4' lever arm. Fair enough... due to the lack of space I told the contractor to use an impact gun to snug tight the bolt and also use an impact gun to turn the bolt a third of a turn. They originally used a 3/4 impact gun, however, it only turned the nut about 1/4 turn.

They used this gun :

http://www.ingersollrandproducts.co...wer-tools/products/impactools/261-series.html

I told them to use a more powerful 1" air impact gun. I was under the impression that impact guns can actually break bolts. So will a more powerful 1" air impact gun get at least a 1/3 turn?

BTW they are using 20mm diameter A325 bolts that are 80mm long if that makes any difference.

Any help or advice with methodology would be greatly appreciated.

 

[dik]

You can chalk the nut and bolt and check that the movement of the nut is 1/3 turn. Never heard of a bar added to an impact wrench. With the chalk, you can still use the impact wrench.

Dik

 

[psychedomination]

@dik,

Thanks for the fast response. Yea they are chalking the nut and bolt to check for the 1/3 turn. It is just that the impact gun they were using was not getting the 1/3 turn. The methodology was as follows :

- Give a hit of the impact gun to snug tight the bolt.
- Mark the bolt and nut with chalk.
- Use the impact gun to turn the bolt to the required 1/3 turn.

With regard to adding a bar to increase the lever arm, that wasn't to be added to the impact wrench, it would've been added to a normal spud wrench or torque wrench if they were going to turn the nut by hand.

 

[jdgengineer]

I'm not familiar with AASHTO code which I'm assuming you are designing to, but is it possible the bolts were tightened to much initially in the "snug tight" configuration?

AISC Specification for High Strength Bolts (2009) defines snug tight as "Snug-Tightened Joints
All bolt holes shall be aligned to permit insertion of the bolts without undue
damage to the threads. Bolts shall be placed in all holes with washers positioned
as required in Section 6.1 and nuts threaded to complete the assembly.
Compacting the joint to the snug-tight condition shall progress systematically
from the most rigid part of the joint. Snug tight is the condition that exists when
all of the plies in a connection have been pulled into firm contact by the bolts in
the joint and all of the bolts in the joint have been tightened sufficiently to
prevent the removal of the nuts without the use of a wrench"

Perhaps the impact wrench tightened more than this on the initial install?

Also keep in mind AISC requires more or less turns of but to achieve pre tensioned limit based on length of bolt. I'm not sure if AASHTO has similar requirements or not. See Table 8.2 of document referenced. The turns vary between 1/3 to 1 full rotation depending on bolt length.

 

[MikeHalloran]

On this site, I have read a multitude of definitions of 'snug tight', mostly conflicting with each other, all the way up to, roughly, 'the biggest guy you have, hanging on the longest wrench he can carry'.

The quote provided by jdgengineer above suggests that a compliant procedure would involve spinning the nut on manually until the resisting torque starts to rise, then chalking the nut and using the impact gun. ... but nobody wants to do anything manually these days.

My own experience says using the impact gun to spin the nut on will give somewhat inconsistent results. Perhaps better to spin it on with a battery powered drill or screwdriver, especially one that has a torque limiting clutch, then switching to the impact gun.



Mike Halloran
Pembroke Pines, FL, USA

 

[TehMightyEngineer]

Just want to point out that it's always an option to switch method to DTIs or other verification methods. At the very least trying an alternative method on a select number of bolts might help determine if your "snug-tight" is actually closer to "fully pre-tensioned".

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[Ron]

I agree with jdgengineer that the impact wrench is probably taking the tension past snug tight. Snug tight is defined by the Research Council on Structural Connections (RCSC) as [highlight #FCE94F]"the condition that exists when all of the plies in a connection have been pulled into firm contact by the bolts in the joint and all of the bolts in the joint have been tightened sufficiently to prevent the removal of the nuts without the use of a wrench." (See Section 8 of the "Specification for Structural Joints Using High-Strength Bolts", prepared by RCSC. [/highlight]

Further, the commentary in that document states [highlight #8AE234]"The snug tightened condition is typically achieved with a few impacts of an impact wrench, application of an electric torque wrench until the wrench begins to slow or the full effort of a worker on an ordinary spud wrench."[/highlight]

This is all subjective and depends a lot on field variables. I'm much more a fan of quantitative measurements using a torque wrench calibrated to bolts in a Skidmore-Wilhelm device or direct tension indicators whether load indicating washers or Tension Controlled (TC) bolts.

 

[psychedomination]

Thanks for the responses so far.

Yes snug tight in the code is quite subjective. To snug tight the bolts I had the contractor do only one hit with the impact wrench. I got one of the iron workers to see if he can turn it after that hit with a normal spud wrench. He wasn't really able to turn it much more by hand. The impact gun was getting it to turn about 1/4 of a turn (Which the iron worker perhaps could've done by hand with some serious effort).

This leads me to think that the impact gun being used had the same strength as an iron worker using a 14" spud wrench. Therefore not heavy duty enough to stretch the threads? Or am I wrong in this logic?

I assume that the worse case scenario would be that the bolt would just break - but would also assume that a heavy duty impact wrench can break a bolt. A 1/3 of a turn is really not that much considering it is the lowest turn used in the turn of the nut method.

 

[Ron]

psyche....probably right about the wrench. Was it air or electric?

 

[psychedomination]

@Ron

They were using a 3/4 air wrench :

http://www.ingersollrandproducts.co...wer-tools/products/impactools/261-series.html

 

[dik]

My understanding is that 'snug tight' is the maximum effort an ironworker can exert with his spud wrench. This usually occurs at the beginning of when an impact wrench first starts to 'impact'. At this point the nut/bolt are chalked and the amount of turn is measured from the chalklines. Turn of nut works really well and is common in most jurisdictions, including international.

Most sources do not recommend using a calibrated torque wrench. Measured torque has too great a variation.

DTI washers work really well.

 

[bridgebuster]

If I may, allow me to throw a wrench in the works. In all my years of bridge construction - mostly in NYC - I've never seen an ironworker mark 1/3 rotation from snug. Typically, they run up the connection bolts them hit the them with a quick burst and that's it. Then everything is checked with a calibrated torque wrench, no click-type wrench. I found that air guns work much better than electric. I'm sure someone will say we've been doing it wrong all these years.

 

[Ron]

BB...nice pun! Sorry to disappoint, but I agree with you! Getting an ironworker to stop, mark and turn is always an effort and has to be forced to make it happen.

 

[Tmoose]

I think automotive turn-of-the-nut / angle torquing starts with tightening the fastener with an ( e

http://wwwwww!

boooo! hiss ! ) torque wrench to some nominal value, like 30 lb-ft or so.

A few trials would help understand how important and variable the initial "snug" is.

 

[XR250]

Then everything is checked with a calibrated torque wrench, no click-type wrench.

I am by no means an expert in this area but I use torque wrenches alot in my shop. They are only accurate when the nut is turning (i.e dynamic friction is involved). How do you check it from a static condition?

 

[SwinnyGG]

I think automotive turn-of-the-nut / angle torquing starts with tightening the fastener with an ( e

http://wwwwww!

boooo! hiss ! ) torque wrench to some nominal value, like 30 lb-ft or so.

This is correct almost all of the time in the automotive world.

The caveat to that is that in the automotive world, you have a very high level of control over the conditions of a joint- parts are typically very flat/parallel, well machined, everything is clean and lubricated consistently, and at a consistent temperature.

I would suspect that in bridge building there is a lot more variation in the condition of parts, fasteners, temperature, etc etc.

How do you check it from a static condition?

The truth is ugly- but you don't. If the bolt head isn't moving you aren't really checking bolt preload.

 

[thaidavid40]

You could always start using twist-off type bolts. That should give you consistent tension development. And they are easy to use with air guns.
Dave

Thaidavid

 

[XR250]

The caveat to that is that in the automotive world, you have a very high level of control over the conditions of a joint- parts are typically very flat/parallel, well machined, everything is clean and lubricated consistently, and at a consistent temperature.

Actually, most torque values are for clean, "dry" threads AFAIK.

 

[darthsoilsguy2]

most "snug-tight" bolts are semi-pretensioned and it is just doing a good job. most production-level ironworkers have never put a bolt in the skidmore and watched when the tension kicks in. when we would bring the skidmore out, the foreman would usually be the one hanging out with us and the crew somewhere else. this results in uncalibrated workers.

another power wrench i've seen allows you to set the amount of turn from 1/4 to 1/2. theoretically you send someone through just to get snug and then the 2nd guy finishes it

but, definitely have done TON method with impact wrench quite a few times, but usually moment connections in simple buildings

 

[SwinnyGG]

Actually, most torque values are for clean, "dry" threads AFAIK.

Stock values on a standard torque chart are, sure. For automotive components, the lubrication is designed in and the torque specified in the manual is specified with that lubrication in mind. Some fasteners are lubricated, and some are not, depending on the application.