[Towertalk] 'Toes' of a tower concrete base

[kr7x@attbi.com]

Gary:

IF you make the foundation straight sided at the plan 
base dimension (largest size at bottom) you should have 
no problems as far as your installation is concerned.

I also wondered why they called out a truncated pyramid 
shape for a foundation in soil. It is very hard to shape 
without forming and does very little to increase the 
capacity for the extra effort. (my opinion only)

73
Hank Lonberg, S.E.,P.E.
KR7X
> In preparing for the installation of my Rohn HDBX-48 tower I noticed that
> the concrete base diagram showed what I refer to as a 'toe'.  That is, the
> hole for the base is rectangular -- something like 5' x 5' x5'.  However,
> the bottom 12" flared outward creating an "L-shape" when viewed
> cross-sectionally --  similar to   _| |_  .  Being quick to draw
> conclusions, I figured the 'toe section' called for in the diagram provided
> additional leverage or strength.  I compared this effect to that of toggle
> bolts (AKA 'hollow wall anchors').  Once pushed through the small hole, the
> 'wings' spread out and provide strength against tipping or leaning.
> 
> Now that plans have changed to use two AN Wireless towers I've been asking
> lots of questions about installation.  I have been told that you can
> eliminate this "L-shaped" area by simply making the hole rectangular.  (The
> dimensions, of course, would have to be increased in order to make the
> vertical sides match what would have been the very outside edge of the
> 'toe'.)  The reason for the 'toe section', I was told, is NOT mechanical
> advantage or anything like that.  It was to save on concrete costs.  Less
> concrete means less cost.
> 
> My question:  Is this 'L-shape' essential to tower strength?  If I increase
> the horizontal dimension of the hole (to span the distance from left toe
> edge to right toe edge), do I weaken the structure?  Of course, this change
> WILL increase the mass of the concrete used in the base.  It would be
> easier, though, to dig the hole if all I need is a simple rectangular
> volume.
> 
> gary b
> k3gb
> RVN BTO
> 
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