This is an EXTREME case John, but I still contend
that CG is NOT the determining factor. For example,
the same mass could be shaped into a LONG NARROW
form with the SAME Center of Gravity but the overturning
forces would be quite different depending on whether the
wind was along the long dimension or perpendicular to it.
Bottom Line: CG is essentially IRRELEVANT and only a
minor result of other more significant factors
such as size, shape, and strength of materials.
Tom N4KG
On Fri, 02 Aug 2002 W0UN--John Brosnahan <shr@ricc.net> writes:
>
> 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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