Bolt Pretension in Thin Connection Plies

[ToadJones]

I am working on some projects where we'd like to specify turn-of-the-nut method for per-tensioning bolts on some connections between cold form and hot rolled steel.

The cold-form specification is pretty clear that the standard nut rotations from RCSC are not valid due to the small bolt grips due to the thin plies within the connection.
The nut rotations are based on bolt lengths = 4d and 8d and have only been tested on typical hot rolled connection grips (not sure what the typical grips are ???).

My intuition tells me that the issue is that the amount of strain is much higher per a given nut rotation for a short bolt grip vs. a longer bolt grip.

1). Is this the reason the specified rotations in RCSC are not valid?

2). Can testing using a Skidmore Wilhelm still be used to verify the bolt preload with the thinner plies?

My guess is that the required nut rotation for the smaller grip will be less

 

[ATSE]

If there is hot-rolled (i.e. thick) sections in the grip, I would treat the cold formed sections (say, at least 16 gauge) as a washer or shim.
Two 18 gauge studs would be a different consideration because of what your intuition is telling you. The strain difference between snug tight and fully pretensioned depends on the the stiffness and irregularity of the plies.

 

[MikeHalloran]

It's difficult to control preload of a short stud in a short stack.
Aircraft engine builders get away with it on cylinder base studs because of the short maintenance intervals.
The solution they can't use on aircraft because of the weight is longer studs or bolts, with cylindrical spacers to artificially increase the stack, or in the case of engines, using long studs to sandwich the cylinder between the head and the crankcase.


Mike Halloran
Pembroke Pines, FL, USA