Just remember though, the force on the bottom of a tower is not just the
weight of the tower. Assuming 3 directions and 2 levels of hand tightened
guys at about 100# tension leaving the tower at a 45 degree angle(to keep it
simple) you get abt 70# for each guy or another 420# of down force in the
static case. When the wind blows, the upwind side has to supply enough side
force to counter the wind which then adds to the down force (at 45 degrees
its an equal down force) so don't forget that... in any case you can easily
get 800-1000# of down force, mostly from the guys doing their job of keeping
the tower vertical.
David Robbins K1TTT
AR-Cluster node: 145.69MHz or telnet://dxc.k1ttt.net
> -----Original Message-----
> From: firstname.lastname@example.org [mailto:email@example.com]
> Sent: Thursday, December 24, 2009 06:39
> To: firstname.lastname@example.org
> Subject: [TowerTalk] Making an insulated base for 80 meter vertical from
> Rohn 25
> Hello All,
> Thank you for all of your questions and suggestions.
> The fluid characteristics of plastics was what scared me about the
> commercially available tower leg insulators. Like the fellow's mobile
> antenna, a
> tower leg is under compressive and expansive loading. That is why I wanted
> get the insulating material away from the legs and use it on the base in
> purely compressive application.
> To answer some other questions, 6 sections of Rohn 25 weighs about 240
> lbs. A few extra pounds for the phillystrand and stinger. The antenna will
> probably weigh in at 260lbs or so when up.
> The Rohn tilt base is 15" square. That comes out to 225 square inches of
> compressive area onto the 1" thick HDPE (high density polyethylene) base
> insulator. That makes a little over 1 psi static down force onto the
> insulating pad.
> There will be additional forces that remain out of the calculation, the
> amount of tension on the guys, and the amount of torque applied to the
> bolts. I shouldn't have to go crazy torqueing down the base bolts as
> is minimal lift on a tower section.
> The PTFE (teflon) 3" round x 1/2" thick insulating washers will be under
> the force of the torque of the base bolts and any flexing of the steel
> plate from wind loading the upper sections only. It will amount to
> whatever that
> force is against the area of about 6.5 square inches. If it shows to be a
> problem I can fabricate 6 new ones from fiberglass. I'd like to try it
> PTFE first though because of the superior UV and insulation properties.
> I hope this answers some of the questions posted. Looking better or worse
> Marty AB5GU
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