Tom (et al)--
In general I agree with you on the EARTH holding up the tower
in typical self supporting tower installations. BUT I have just
been involved with installing four 80 ft SSVs with lots of
log periodics on each for a commercial installation.
Per Rohn specs these towers (about 9 ft on a side at the
base) were installed with a single concrete pad that is
only about 4 ft in the ground. It is 14 x14 ft by about 4.5 ft.
In this case the sheer mass of the concrete seems to be
the main support since typical installations like this (in the
past) were done with smaller concrete piers that went much
deeper into the ground.
It is my understanding that the cost of excavating in some
very rocky situations can exceed the cost of concrete. So
it is cheaper to build a very large, but somewhat shallow,
concrete base.
For a typical foundation mix this pad turns out to be about
114,000 pounds--a pretty significant change in the C of G.
I haven't done the moments calculations--assume that Rohn
did a good job of CYA.
Just some input to stir the pot, so to speak! ;-)
73--John W0UN
>N4KG: Ideally, the Guys LIMIT movements of the tower :-).
>
>Maybe, in the case of a mast or a tower with no guys
> > the base offers a change in centre of gravity.
>
>Yes, but that is NOT the primary contributor to keeping a self supporting
>tower up. Picture a tower attached to a block of concrete ABOVE ground.
>How much bigger would that block of concrete have to be to keep the
>tower from blowing over. THAT would relate better to your idea of the
>contribution of a low center of gravity. And even then, the SIZE and
>SHAPE of the BASE are significant factors in the load carrying capability
>
>of the pole / mast / tower. N4KG
>
>As I said before, the EARTH holds up the tower. The BASE only
>provides an adequate coupling to the earth. That coupling requirement
>is quite different for self supporting vs guyed towers. Center of
>gravity
>is NOT the critical item. N4KG
>
>I recommend you read up on the subject of STATICS in a college
>Physics or Mechanical Engineering Text. Then perhaps something
>on the holding capability of soils in a Civil Engineering Text. Then
>perhaps you will have a better perspective on the contributions of
>each component to the whole picture. I guarantee you, center of
>gravity is virtually insignificant compared with the others. N4KG
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