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)
Hank Lonberg, S.E.,P.E.
> 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
> gary b
> RVN BTO
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