[TowerTalk] Rohn Base Plate and Pad

[Wilson]

My plan is to erect 40 feet and place a Force 12  C-3 atop it ( I think 7
SQ Ft or so)
and attach it to the gable end of my roof with a house bracket at the 16'
level.


VAST OVERKILL
Rohn only approves things that are expensive.
You have a few hundred pounds of vertical load and a few hundred of shear at the bottom, as the wind tries to pivot the tower at the bracket.
Assuming the bracket is just a hinge, the shear at the base is 2.5 times the horizontal load, assuming the load is all at the top, very conservative.
A pad the size of your plate and 18” deep will be way more than needed!
Rather than the three big bolts, one 1/2” pin will be fine.  In the center is nice, but it really doesn’t matter.
The bracket will take out any twisting tendency and the base shear can be taken out anywhere.
I have 40’ of 25G on a tilt plate, bracket at 22’, and I tilt it down with a line from the bracket.
If you do this, your base will see some uplift, so you’ll need a couple of anchored bolts.  Two 1/2 bolts will be fine, if you seat the anchors properly.
I have run double 2X4s across four studs to help with the bracket loads.  Some hurricane clips at the top of the studs, to be sure they are tied to the top plate, is a good idea.  Depending on the quality of your framing, clips on surrounding joints would be good.   
If these studs are broken by a gable end or window, then their lower end connection is important.  Some structure ti tie the bracket point back to a couple of ceiling joists or rafters can help a lot.
WL