It seems to me (I haven't done the math) that you'd be better off
taking the time and trouble to dig the deeper hole and pour concrete in a
cubish form, rather than use long, cross-type footers to support a tower.
If you constructed the cross and fixed the tower to its center, any
upturning moment (the wind trying to push the tower over) would exert a
significant force at ground level. This in turn would be absorbed by the
cross-concrete almost exclusively. There is, in your proposed design,
insufficient earth (dirt) surrounding the concrete to aid this resisting
force. This is the main reason that SS towers with appropriately designed
foundations don't fall over - the dirt helps keep it upright.
So the forces that are trying to turn the tower over are taken up by
the concrete. However, the <long lever> effect applies also to the concrete
<levers> that form the cross. The wind force will essentially break the
concrete cross(es) at the point they attach to the center. And the longer
you make these cross arms, the more the lever effect will concentrate the
force on the center attachment point of the downwind cross arm.
I've recounted on this forum previously about my experiences with
constructing towers in Haiti. These folks are resourceful enough to dig the
SS towers' foundation holes (four per tower, one per leg!), and not have
problems with cave-ins. (Although I believe they used machinery to dig the
holes.) To get your proposed solution to work, one would have to provide
considerable reinforcing bar and increased concrete tensile strength (I
haven't done the math, as I said). The extra co$t of this approach seems
not worth the trouble. If absolutely necessary not to dig, one might be
able to design a concrete cube, as others here have suggested, to sit on the
top of the dirt. But this concrete structure would indeed have to be
massive. (At some point the laws of economics supercede the laws of
Gene Smar AD3F
From: Tim Makins, EI8IC <email@example.com>
To: TowerTalk <firstname.lastname@example.org>
Date: Thursday, February 27, 2003 3:02 AM
Subject: [TowerTalk] Tower Bases
>Many years ago, at school, I remember seeing the illustration of how you
>could move the world if you had a long enough lever. It occured to me
>recently that the same idea could be used to construct self-supporting
>bases, but, as I'm not an engineer, I thought I'd come to Towertalk to be
>The idea is: Instead of the usual cube of concrete, how about using a
>horizontal cross. Maybe 2 feet deep, and 2-3 feet wide, with an internal
>rebar cage to prevent snapping. The length of each arm of the cross to be
>determined by whatever formulas you guys use - I was thinking along the 12
>foot mark, as a discussion-opener.
>You will note that I haven't mentioned tower height, as obviously all the
>parameters inter-relate. If you must have a figure, lets say 40-60 feet.
>The reason for this ? Well, I was thinking 'outback', and 'developing
>here, where a mechanical digger isn't always available. Easier to dig slit
>trenches than a big pit. Less problems from cave-in too.
>Sit down, take one of these beers, and discuss.
>73s, Tim EI8IC
>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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