Actually the vertical load is indeed passed thru the cables and is
transferred to the next lower section by the pulley connection to its top
thru the stiffening band around the section. This distributes the load to
all 3 legs.
Bending moment does not necessarily create axial load in the legs. It
creates axial stress in the leg sections and this is additive with the axial
stress caused by the vertical load in the legs to create the total stress in
the legs. The leg sections act like little columns between the bracing
points. When the stress value gets large enough the leg can fail thru
lateral buckling between the bracing points.
The overlap sections act like a ferrule in a fishing rod they transfer the
lateral load between them by the upper section trying to move the lower
section in one direction at the top of the overlap and the upper section
trying to move the lower section in the opposite direction at the bottom of
the overlap section. This creates equal and opposite forces which transfer
the lateral load to the lower section. It also creates a moment due to the
distance between the top and bottom of the overlap which transfers the
moment from the upper to the lower section and so on.
I have written a program that calculates this load, force and stress
transfer between the sections of crank-up towers in order to analyze them
for loading conditions for my clients.
It is tedious but not that complicated.
Hope this helps your understanding.
Lonberg Design Group, Ltd.
Hank Lonberg, P.E.,S.E. / KR7X
From: Pat Barthelow [mailto:email@example.com]
Sent: Saturday, January 29, 2005 10:14 AM
To: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
Subject: Rohn 25/Crankup loading
>From: "Hank Lonberg" email@example.com Said:
>If you notice the sections get larger as you go downward from the top. The
>tower sections get stronger and can handle more moment as you go downward.
>Just like the moment diagram for a cantilever beam, which they are albeit
>vertical. The maximum moment is at the bottom and reduces as you go towards
>the top. Rohn 25,45,55,65 are constant in section size.
>Lonberg Design Group, Ltd.
I notice something different also, in how/where the load is carried on each
section of a crankup. Assuming no blocks installed, and the lift cable
carrying the total load of the elevated tower sections, the load is not
carried down the corners of the tower, at least not directly. It is
applied at single point sources in each tower section, namely the cable
pulley axle bolts, in my tower, located on the side of the tower trellis
work. The corner joints of the tower 'float'. That is, there is no direct
downward transfer of stress though the three corner tubes of the tower.
Does this single point source of loading change any basic dynamics of load
transfer down the tower structure?
73, DX, de Pat Barthelow AA6EG
(831) 646-0388 firstname.lastname@example.org
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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