[TowerTalk] Do Bracketed Towers Have an Overturning Moment?

[Bert Almemo]

WOW!! 10,5 cuyd!! You could have built a house on top of that tower! :-))

73 Bert, VE3OBU



-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Gene Smar
Sent: Thursday, August 28, 2008 7:41 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Do Bracketed Towers Have an Overturning Moment?

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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