[TowerTalk] More friction needed

[JVarney]

Sorry my Gmail messed up the line breaks and made it
unreadable.  Reposting for anyone interested in calculating
U-bolt torsion resistance for masts.

I'm fairly confident this method is a good approximation
because the TIA committee, composed of experienced engineers
in the tower industry, wouldn't write this into the
building code unless it had good support.


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Dave AB7E wrote>> Personally, I think the idea of using multiple
U-bolts is the best way to go. <<

I agree.  And instead of shooting from the hip and guessing,
with U-bolts you can calculate and engineer the number and size
of U-bolts needed to resist the mast torque. This is a simplified
version of the procedure in TIA-222-H

Nominal torsional strength of U-bolt assembly = Tr
Tr = 0.075 (D)(Tp)
Tp = assumed U-bolt leg tension = (20 ksi)(Ag of leg)
   where D = mast diameter, Ag = gross area of U-bolt leg.

U-bolts should not be tightened past yield and should be limited
to 0.85 Fy Ag.

===

Example:  3 3/8 U-bolts and a 2-in mast.

Ag leg = 0.19 sq in Tp = 20 ksi x 0.19 = 3800 lbs
Tr = 0.075 ( 2 in )(3800) = 570 in-lbs per leg
System Tr = (3 U-bolts)(2 legs each)(570 in-lbs) = 3,420 in-lbs
   torque resistance

===

Considering a Yaesu G-800 has a brake resistance 0f 4,000 in-lbs,
the 3 U-bolts is in the ballpark for a medium tribander.
If you ignore the 20k yield assumption and use 42k yield
for stainless and tighten to 0.85FyAg, that gets you 1,000
in-lbs per leg, equal to 6,000 in-lbs total torque resistance.

This simplified estimate ignores vertical slippage forces due
to the weight of the mast and antenna acting on the U-bolts,
which reduces the available torque resistance somewhat.

73 Jim K6OK