[TowerTalk] tower bases and weight versus surrounding ground

[kr7x at comcast.net]

Jeff, etal:

A freestand tower's (aka cantilever tower) foundation strength is developed in one of two ways. It always depends on the soils strength not the weight(mass) of the base. The type that is utilized depends on the design decision by the engineer and project local conditions. Both work it just depends on what is the best way to construct the specific foundation.

The first method is to resist the vertical, horizontal and bending moment is a post-type foundation. These are the ones that are relatively small in plan but significantly large in the vertical (they go deeper in the ground). The strength is developed by the soil's allowable lateral bearing pressure. The size and depth is a direct function of that value, the weight of the foundation is secondary. Drilled piers is another type used quite a bit in the electrical distribution industry but not too often in an amateur installation.

The second method is the resist the vertical, horizontal and bending moment by using a spread footing type of foundation. These types are in relative terms larger in plan and shallower than post-type foundations. The local soil's allowable vertical bearing pressure is the key factor in determing the size of the foundation in plan, and the depth is determined by concrete strength. Again the weight (mass) of the foundation is secondary and is determined by the size required by the above.

Quoting a foundation by amount of concrete or mass is meaningless. This value is determined as the result to the loads, soil and foundation type. It means nothing other than how much concrete is used.

Regards
Lonberg Design Group, Ltd.

H.S. Lonberg, P.E., S.E. / KR7X
President

-------------- Original message -------------- 
From: "Jeff Kinzli" <kinzli at kinzlicoils.com> 

> 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. 
> 
> Thanks, 
> 
> -J 
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