[TowerTalk] TowerTalk Digest, Vol 120, Issue 10

[Kirby Hallenbeck]

Message: 8
Date: Sun, 09 Dec 2012 08:53:15 -0600
From: Richard Thorne <rthorne at rthorne.net>
To: Towertalk <towertalk at contesting.com>
Subject: [TowerTalk] Antenna Loads on Rohn Tower (45g in this case)
Message-ID: <50C4A5DB.10702 at rthorne.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

I have 80' of Rohn 45g installed per 90 mhp specs ( I live in a county 
with a wind speed rating of 85mph).  The Rohn catalog states I can have 
14.5 square feet of antenna at the top, which I do in the form of a F12 
C31 tribander and F12 D240 2 2 element 40m beam.

I'd like to stack another C31 at 40' just above the first set of guys.

Can the tower handle that?  It seems like lots of stations do this with 
no failures.  Short of an engineer looking at this, what is the groups 
thoughts on this?

Thanks

Rich ZC

Rich, 

I would stick with the engineered limits personally. Go through the
equations the structural engineer put on the permit and understand them,
assuming this is a permitted tower. The equations will show the vertical and
horizontal components of force on the guys anchors transmitted to them by
the wind forces on the tower and antennas. The weight of you guy anchor has
to resist the vertical forces and for the horizontal forces the face area of
the guy acts as a plow to resist the horizontal forces which is soil
dependent, and that came from a table the engineer had. 

Most hams get away with overloading their towers until the spec wind load
occurs, and then it fails. This is a probability exercise, so overloading is
essentially gambling that you won't see the 90mph event. Not a bet I would
take, and I have been given a lot of crap from "experienced hams" for being
so conservative, but they're just ignorant of the principles of statics
analysis taught in 1st year mechanical engineering. The math is very clear
on this. 

Kirby - AF6OP