TORQUE VALUE ON GALVANIZED/BLUE FINISH A 325 BOLTS 2

[LJBARCOS]

Good evening, please;

Is there any difference in torque value to be applied on a permanent steel structure joint for the same bolt ( e.g. A 325 . Diam 3/4")?, thanks

 

[ajh1]

Not sure exactly what your question is. Structural joints, assuming a requirement for pretensioned bolts, are based on a tension requirement. The only way to get the equivalent torque is by using a testing machine, generally a Skidmore, with a sample of the actual bolts being used. When the machine shows the proper tension, you establish the torque it took to get there and use that torque on the actual installation. The torque/tension relationship is variable depending upon the lubrication level of the bolt and nut, and dirt or other impurities on the threads, etc. so there is no one right answer. For a 3/4" A325 bolt the mandatory pretension force is 29 kips in the preinstallation testing.
There are many instances where a simple snug-tightened bolt is acceptable in a structural joint. For these joints the only requirement is that the plies of the joint are pulled into firm contact and the nut is installed with a few impacts of an impact wrench or the full effort of an ironworker with an ordinary spud wrench.

 

[LJBARCOS]

ajh1, thanks for your answer. The question is only what the differences are relating to how the surface finish affects to the torque value, particularly: galvanized finish vs blueing finish, thanks

 

[ajh1]

Galvanized bolts are required to be lubricated for installation. The lubrication is going to affect the torque values as will any other surface condition effects. Depending upon the lubrication type used there could be a wide variety of results.

 

[LJBARCOS]

ajh1 thaks for your answer.

Our situation, as erectors, is: we have received drawings specifying the torque value to be applied to all joints.
¿Do we need a pre-installation verification procedure? or ¿ is it just acceptable to apply the specified torque value to te bolts being installed?, thanks

 

[ajh1]

It depends upon the purpose of the torque. If the engineers are trying to mandate a pre-tensioned bolt, they need to specify a tension level, not a torque level, and you would need a pre-installation procedure to insure that you do in fact get the tension required with whatever torque you ultimately put on. If they simply want the bolts installed such that they have no fear of them loosening, i.e., basically a snug-tight bolt or slightly tighter then a simple torque might get where you need to be. As stated earlier, torque will vary depending upon lubrication and other factors to achieve identical tension results. The required pre-installation procedures to achieve fully pre-tensioned bolts is spelled out in Section 7 of the RCSC Specification. That is a free download from

http://www.boltcouncil.org.

 

[Duwe6]

In my experience, to achieve the 28kips a 'blue' 325 torques at about 60% of the value needed for unlubed galv bolts. If you use the 'usual' 300-350 ft-lbs on a 'blue' teflon lubricated bolt they will be overstressed to the point of having some break during installation.

 

[LJBARCOS]

Gracias a todos, desde España