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.

 

[bridgebuster]

Ron - you never disappoint.

 

[TehMightyEngineer]

uncalibrated workers

I like it.

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

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

 

[dozer]

I wouldn't think an impact wrench would be the best for that. Google "Hytorc" for some offerings in hydraulic wrenches that would provide a more controlled method to tighten the bolts.

 

[psychedomination]

Thank you for all of the advice. Just as an update a stronger impact gun did work. I had the iron workers snug tight the bolts with two hits of the impact gun (on a low setting), mark the bolt, nut and steel and then do the 1/3 turn with the impact gun.