[TowerTalk] tower base concrete or not

[Tower (K8RI)]

>
> I dunno?  My "intuition" sez:
>
> ----- Original Message -----
> From: "Tower (K8RI)" <tower at rogerhalstead.com>
>
>> The concrete only does three things at the base of the tower.
>
>> It keeps it  from sinking,
>  Then why do the manufacturers specify the hole at  about twice as deep
> as it is wide?

Phil already got that one and it's been a real problem here in central 
Michigan.
That sounds strange as our winters have been getting milder over the last 50 
years, but with less snow the ground freezes more deeply.  (We still get 
close to 3 months where it never gets above freezing in the daytime)  Get a 
lot of snow and the ground will thaw.
>
>>It adds area to keep the base from shifting, or moving,
>   Do you mean sidewall area? Lateral shifting and moving--or tipping?
>
It offers resistance to all three, but the predominant is resistance to 
lateral movement.

>> it > protects the tower base (if properly done). It doesn't take a
> lot.
>  ok
>
>>  (It does  not prevent the tower from tipping over). To do that takes
> a pretty good > chunk of concrete and a very strong tower.)
>  It was said during construction of Seattle's space needle's that
> enough concrete was used at the base to place the center of gravity of
> the   needle (and concrete) below ground.  I think that the concrete
> specified by tower manufacturers may (?) also place the center of
> gravity below ground.  At least having the hole deeper than wide helps
> in that respect.
>
> Granted, "intuition is no substitute for stress analysis.  Only one of
> my four tower stress analysis calculations includes foundation
> calculations. As a retired electronics engineer, I cannot follow the
> analysis which uses so many undefined symbols.  However the analysis
> does include the following note "Foundation has been designed to
> accommodate the following loads; Overturning Moment, Base Shear, and
> Structure Weight".
>
There is a resistance to the overturning moment, but not enough to prevent 
it, but with guyed towers it doesn't need to.

> Does the overturning moment mean tipping?

Yup.  It's just for self supporting it takes a lot more concrete than it 
does for a guyed tower.  It's true the normal base resists tipping, but it's 
not enough to prevent it.  OTOH most guyed towers are not strong enough to 
go self supporting except as a relatively short installation.

I believe the 45G is only rated for 40 or 50 feet self supporting with a 
small tribander.


Roger Halstead (K8RI, EN73 & ARRL Life Member)
N833R, World's Oldest Debonair (S# CD-2)
www.rogerhalstead.com
>
> k7puc
>
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