[TowerTalk] tower bases and weight versus surrounding ground

W5LT W5LT at tx.rr.com
Tue Feb 13 08:22:15 EST 2007

It is possible to design a foundation that can just sit on the ground and
resist the over turning moment (OTM) of the tower/antennas. The soil
underneath must support the compression forces that can exist however, hence
you see the terms "soil resistance" or similar in the tower foundation
drawings. The foundation must be designed to be very strong (lots of re-bar)
so as not to flex or crack, since there is no surrounding support. 
Large area, not very thick slabs are used frequently to support earth
station antennas for example. 
The most common style tower foundation is the small area, very deep
foundation, usually because it uses the least amount of concrete and is
therefore theoretically cheaper. These do depend significantly on the soil
resistance to prevent 'rotation' due to the OTM. A traffic light stanchion
foundation is a typical example. The optimum foundation design would depend
on the soil characteristics at the site. The drawings supplied by the tower
manufacturer are only 'typical' and specify a soil resistance assumption,
but are usually conservative. 
Bob, W5LT

-----Original Message-----
From: Jeff Kinzli [mailto:kinzli at kinzlicoils.com] 
Sent: Monday, February 12, 2007 10:11 PM
To: towertalk at contesting.com
Subject: [TowerTalk] tower bases and weight versus surrounding ground

Hey there,

I've been wondering this one for a while, and wonder what your thoughts are.

For a self-supporting tower, how much of the base depends on the mass
of the concrete, and how much of the base depends on the rigidity and
surrounding soil to "hold" the base in place?

To take it to an extreme, if it were *only* about weight, one could
pour the foundation on top of the ground and have it be as useful as
being in the ground. I'm pretty sure this is not the case, however.

The reason I ask is for placement of my tower. The ideal location for
the tower is on a slope, and the ground is not as hard-packed as other
locations, and I wonder how much this will affect the outcome, and how
much deeper I should go to accomodate.

I guess I'll end up enlisting a civil engineer to get it right, but
I'm just curious if anyone has any ballpark estimates.



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