[TowerTalk] Is A Tower Weaker in Some Directions?

[Jim Thomson]

Date: Sun, 05 Oct 2014 17:02:10 -0400
From: "Gene Smar" <ersmar at verizon.net>
To: "'Patrick Greenlee'" <patrick_g at windstream.net>,
<towertalk-bounces at contesting.com>, "'Cox, Norman R.'" <nrc at mst.edu>,
<towertalk at contesting.com>
Subject: Re: [TowerTalk] Is A Tower Weaker in Some Directions?

     If the wind were to blow normal to one face of a tower, the opposite
single leg would be the weakest.  Assuming the cross-bracing holds (does not
fail) upon exposure to high winds, the windward legs of the tower will be in
tension, i.e., the wind will attempt to elongate them when they bend away
from the wind.  The remaining leg on the opposite side of the tower will be
in compression, i.e., the wind will attempt to force it downward.  This sole
leg must withstand this downward, compressive force by itself.  It will
deform (bend) when sufficient downward force has been placed upon it, much
as a human's knees would buckle if that human were required to support
heavier and heavier loads on his shoulders.

     Now consider the case when the wind is from the single leg side of the
tower and towards the opposite face.  The single windward leg will be in
tension and the two opposite legs will be in compression.  Unlike the first
example with the wind applied on the face, the two leeward legs of the tower
resist the downward force from the wind with twice the counteracting force
of a single leg.  To get this tower's two legs to buckle will require twice
the wind force on the opposite leg vs the single leg example above.

     A properly designed and constructed guyed tower will not impose
downward force on the legs.  The windward guy(s) will counteract the wind
force and keep the tower erect.  The guys should not deflect sufficiently to
allow a downward force to be applied to the leeward leg(s).  This is why the
foundation of a guyed tower need not be massive: it will merely resist the
dead weight of the tower and its antenna loads.  There will be no
wind-caused overturning moment applied to the base as is the case with a
self-supporting tower and its massive concrete base.  

     This is also why one must not guy a tapered, self-supporting tower.
Guys impact unnecessary downward vertical force on the structure.  If you
believe you must guy a SS structure, then re-check your design and select a
"beefier" tower.  And ditch the guys.


73 de
Gene Smar  AD3F

##  he has a freestanding crank up tower.. US tower corp  HDX-555.... not a guyed 25/45 G type tower.   He cant guy  the HDX-555, even if he wanted to. 
Your structural analysis theory is flawed.   You wont see any difference if any,  regardless of  base orientation. 

##  as far as guying a freestanding, tapered tower, its done all the time.  It’s the strongest structure there is.   The legs will easily handle the downward force.

Jim  VE7RF