[TowerTalk] Calculating Forces for Tilting tower

[Dave Tipton, W5DMT]

So, first off, thanks everyone for the repliers.

I intend to use a 3000 lb winch, and I did like the idea about sistering in
some extra 2x4's.   (I was already looking at doing that, and potentially
cross guying that with a piece of EHS to the other side of the garage.)  

I saw one post about 2 sets of guys, and that information is patently false.
Per ROHN's catalog, only one set of guy sis required at 40 feet.   2 at 50.
A call to Rohn, confirmed this.

Otherwise, Thanks for the good information.

Dave

-----Original Message-----
From: TowerTalk <towertalk-bounces at contesting.com> On Behalf Of JVarney
Sent: Wednesday, May 29, 2019 4:39 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Calculating Forces for Tilting tower

The use of simple geometry and statics assumes the tower is perfectly rigid.
It isn't. With the lifting point at 25% of height and 50 pounds cantilevered
at the end, the tower will bend somewhat at lift off.

Simple statics only holds when the strain (bending) is less than 3%. When
the strain is greater than that, it becomes a "geometric nonlinearity" and
cannot be solved analytically. You have to use finite element analysis to
model it numerically.

So what will happen is that when the estimated
740 pounds is applied (as estimated by Jim L. and Dave H.), some of that
energy will be diverted into bending the tower instead of lifting it.
It will take a greater pull to overcome that before liftoff occurs.

Simple statics is fine for back of the envelope guestimating but you should
use a large fudge factor.  Perhaps size everything with the assumption that
it could take 1200 pounds of lifting.

Besides all that, a general rule of thumb is the lifting point should be no
lower than one third of the tower height.  If you could raise that guy
anchor up to say the 15 foot level, that would reduce stress on both the
lifting cable and the tower itself.

73 Jim K6OK
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