here are the Portland Bolt torque specs for grade 5 bolts which I found
is the grade Rohn supplies for R25
size-tpi proof load tw tg tp
1/4-20 2700 2025 4 11 8
5/16-18 4450 3338 9 22 17
where load is the maximum clamping force before the yield point is
reached (85,000 lbs/sq in for grade 5), the required bolt torque ft-lbs
tw for galvanized and waxed, tp for plain, and tg for galvanized.
The "snug plus 1/2 turn" guide is commonly used for tightening bolts in
shear connections. All of the red iron bolts in two steel buildings
I've put up were torqued with a torque wrench to about 25% of the rated
load, which is a better way. Note if graded bolts exceed the load
rating, then they are stretched and shouldn't be used again.
My bet is the Rohn "slightly flattened" guideline is to keep enough
tension so the nut doesn't fall off but not to permanently deform the
leg. It is pretty easy to torque a R25 tower bolt to 11 or 22 ft-lbs
and last week the R25 stick I made a hinged baseplate for was
substantially flattened before I got it. If the R25 bolts are tightened
to the load ratings, then 5363 lbs of force is flattening the leg, which
intuitively, seems excessive on the relatively thin wall tubing.
Nuts that don't loosen are a much better solution and then snug tight is
good enough and the joint is definitely held together by bolt shear. It
seems that Rohn is now providing such nuts from what others have
posted. So, IMO R25 tower sections are held together with shear
connections. A back of the envelope calculation I did indicates a 1/4"
plus 5/16" grade 5 bolt have just enough cross section (ignoring that
the threads might be in the shear plane on one end) to withstand the
maximum 25,000 lb downforce spec for R25 mentioned in another post if
all 3 legs share the load equally. I can't calculate if the holes yield
or tear out before the bolts break.
A terrific guide to bolts is on the Fastenal web site,
http://www.fastenal.com/content/documents/FastenalTechnicalReferenceGuide.pdf
Grant KZ1W
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|