[TowerTalk] Do Bracketed Towers Have an Overturning Moment?

[Gene Smar]

Jeff:

     In a self-supporting tower, all the overturning moment is applied at the concrete base.  For my short (64-foot) tower and its modest HF Yagi load I constructed a base of 10.5 cuyd of 3500 PSI concrete with rebar, per Trylon specs.

     For a house-bracketed tower, this overturning moment is applied to the bracket, its mounting hardware and the house structure.  (Because the rigid fastening (bracket) is higher above ground than a SS tower, this force would be lower than on a SS tower.) You can see why using a couple of half-inch lag bolts into the siding and wood sheathing of a typical residential installation may not generate enough holding force to keep from pulling out in high winds.  That's why using a house bracket requires a bit more engineering than we have been discussing here on TT.  

     For more info on the subject of house brackets for towers check out the 8/96 QST article by K1KP.  Good luck with your study/project.


73 de
Gene Smar  AD3F


From: Jeff Stevens <jeff at mossycup.com>
Date: 2008/08/28 Thu PM 06:13:02 EDT
To: towertalk at contesting.com
Subject: [TowerTalk] Do Bracketed Towers Have an Overturning Moment?

I'm fine with calculating the overturning moment of a freestanding
tower.  My questions is about guyed and bracketed installations.

When I think of a force diagram for a bracketed tower, it appears to me
that there is no overturning moment at the base of the tower.  The
bracket has a reaction due to the lateral force of the tower due to wind
loading.  The overturning moment needs to be calculated AT the bracket
and the problem becomes one of calculating the tension and compression
in the tower legs.  Yes?  No?

It seems the base of a bracketed tower needs to be designed so it
doesn't fail due to the weight of the tower.  Overturning moment isn't
an issue.

I'm probably wrong here and would appreciate any corrections.  This is
just a learning exercise.  I'm not looking to design a 150' tower in a
crowded urban area or anything.

-Jeff
W7WWA

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