Estimating Bolt Torque from Snug Tight Specification

[POPEYES]

I understand that the RCS specification defines snug tight as "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."

For analysis purposes is there a generally acceptable method of converting this to a torque? I've seen a value of 150 ft*lbs be used in some places.

 

[SwinnyGG]

I would argue no.

There is no way to know. Assuming any level of bolt preload is dangerous.

 

[POPEYES]

I get what you're saying jgKRI, since accuracy would be low, but I would also argue that assuming 0 pre-load could be dangerous in certain situations.

 

[JStephen]

"This may be attained by a few impacts of an impact wrench or the full effort of a man using an ordinary spud wrench", per the ASTM A325 spec in the 9th edition ASD. I expect that second half is where the 150 ft-lb value comes from, obviously depends on the person involved, the "fullness" of the effort, the angles involved, etc.

 

[POPEYES]

JStephen I believe that is the old definition, but you probably know that as you referenced the 9th ed. spec. I agree that is where it's coming from though. 150 ft-lb does seem reasonable.

 

[BridgeSmith]

We still use that "full effort of a man" criteria to define snug-tight, but we go a step further and give the length of the wrench to be used, based on the size of the bolt.

 

[dauwerda]

Popeyes, it may be an old definition but it is still a current definition as it is in the 15th edition of the steel manual as well. (Section 16.2 8.1).

 

[ajh1]

The snug-tight definition has bounced around for a few years, but the most current is the "full effort of an ironworker" version. The first one cited above would be fine for a condition that was intended to simply be snug-tight, but was found to be in conflict with the turn-of-nut guidelines for torqued connections. It didn't give sufficient tension during the snugging operation to achieve the full required tension after the turn-of-nut application. This perhaps should have been obvious as "the ironworker" version was the snug-tight definition in place at the time the turn-of-nut rules were developed.
The challenge with any torque-based target is that it is totally dependent upon the lubrication condition of the bolt and the nut. For the snug-tight condition it is also dependent on the flatness of the plates or parts being joined which then drives how much effort is required to bring the pieces into full contact.
Now if one was to define an amount of turn required after full contact was made to establish snug, then you would have a somewhat constant measure you could apply.
A reminder: The target is some amount of preload (which RCSC intentionally does not define), not some amount of torque.