[Towertalk] 'Toes' of a tower concrete base

[EUGENE SMAR]

Gary:

     I agree with Randy AC7NJ:  Don't abitrarily change the engineer's
design for the foundation.  If I understand your suggestion, you are asking
if it would be OK simply to dig out the foundation to the outside dimension
of the toe.  I wouldn't recommend this.  You might be pouring more concrete
into the hole than can be supported by the underlying earth!  After some
period of time (probably less than your mortgage), the concrete foundation
(and the tower, the Yagi, the mast, the rotator, etc.) might start to sink
or tip into the earth.  Wouldn't that be embarassing?

     I believe the L-shaped foundation you mentioned is called a pier and
column design.  We used these all the time when I was designing substations
for a power company (I designed the electrical stuff that sat on top of the
concrete.)  The construction crews would dig the initial hole to the
dimensions of the toe (usually square) down to the required depth (kind of
like you want to do), install reinforcing bars, both horizontally for the
toe (called the pier) and for the smaller vertical column.  Then they'd pour
concrete to the required depth of the pier (usually one or two feet) and let
that mess set up for a day.  The next day the'd come in and form up the
column on top of the pier, add the anchor bolts and pour more concrete into
the form, thus making the smaller vertical column part of the design.  When
this mess was set up (couple of days) the crews would remove the form,
backfill the hole around the column and compact it properly.  Backfilling is
the key to building this type of foundation safely, not undercutting at the
bottom of a smaller hole.

     One more thing - the pier and column type of design actually DOES
provide mechanical advantage while at the same time minimizing the amount
(and cost) of concrete used.  The trick is that the toe actually pushes
against the overlying soil and the soil, if it's the right kind and
compacted properly, will resist this overturning moment.  Thus the overlying
soil is used to keep the foundation (and the tower, etc.) upright in spite
of heavy winds (up to the design limits of the entire system, of course.)
And the soil is cheap (free!) because it's already in the ground.  Just add
concrete and steel.

GL es 73 de
Gene Smar  AD3F

-----Original Message-----
From: ac7nj@bentonrea.com <ac7nj@bentonrea.com>
To: blueis@sprintmail.com <blueis@sprintmail.com>
Cc: TOWERTALK@contesting.com <TOWERTALK@contesting.com>
Date: Friday, June 28, 2002 9:12 AM
Subject: RE: [Towertalk] 'Toes' of a tower concrete base


>Gary/K3GB
>The design of the tower "toes" was carefully engineered by the tower
>manufacture and shouldn't be arbitrarily changed. Structural changes
>need to be made by a engineer or otherwise qualified individual.
>
>Randy
>AC7NJ
>
>
>-----Original Message-----
>From: towertalk-admin@contesting.com
>[mailto:towertalk-admin@contesting.com] On Behalf Of
>blueis@sprintmail.com
>Sent: Friday, June 28, 2002 4:33 AM
>To: Towertalk@contesting.com
>Subject: [Towertalk] 'Toes' of a tower concrete base
>
>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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