Hello Tim:
Well you are close to what can be done. Take your cross arms and fill in
the empty spaces in between each arm with concrete and reinforcing and
you have what is called in various circles a spread footing, a mat
footing or sometimes even a raft footing. It is a large square or even
rectangle in plan. Square shape works better for freestanding or
cantilevered towers.
What happens is that the overturning moment is resisted by the strength
of of the soil on the underside of the footing. The distribution of the
soil pressure is determined by the geometric shape which defines the
geometric moment of inertia of the cross section. The up shot is the
larger the plan dimensions the larger the I of the section and the lower
the soil pressure for a given overturning moment and vertical load. What
you do is to increase the plan dimensions until the soil pressure is
equal to or lower than the allowable soil strength.
Since this discussion is about towers and wind load the wind can blow
from any direction and there fore the foundation needs to accommodate
that fact. A cross may be ok for four directions but not all directions.
The other common cantilever tower foundation is the post type which in
usually square but deep. I acts more like a embedded post to resist the
overturning moment. Both of these depend on the characteristic allowable
soil strength. Rock is a different matter and can be handled
differently.
>From memory a 80 foot freestanding tower with 30 square feet of antenna
at the top typically, in very poor soil, would take say a 6'x6'x 8' deep
post type foundation or a 8'x8' by 2.5' plus/minus spread footing to
accommodate the same moment. It is a matter of what works better in a
given situation. The spread footing requires more concrete but it is
easier to dig 2.5' down than 8' down. On the other hand that is a big
area for the spread and you may not have that available. Also the bottom
of the spread must be below the frost line so it may be thicker just for
that.
Anyway you were close Tim and your idea is actually used quite commonly
in designing foundations.
73
H.S. Lonberg,P.E.,S.E. / KR7X
Lonberg Design Group, LTD.
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Tim Makins, EI8IC
Sent: Wednesday, February 26, 2003 11:44 PM
To: TowerTalk
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
tower
bases, but, as I'm not an engineer, I thought I'd come to Towertalk to
be
proved wrong.
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
world'
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
www.qsl.net/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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