[TowerTalk] FW: Rebar base for new 18HT installation?

[KM5VI]

Allen,

Over the past year I have had much discussion with some structural engineers
that I work with about this very subject - at least as it pertains to a
couple of self supporting tower installations (also along the Texas Gulf
coast).  It seems that the stress concentrations in the concrete bases for
these applications tend to concentrate towards the top of the base and
especially in the vicinity of the anchor bolts.  If one looks at the history
of design revisions (from at least one tower manufacturer) over the past two
decades regarding reinforcing steel recommendations - there has been a shift
towards more conservative steel reinforcement and treating the bases more
like columns rather than ballasts.  

Don't know what your tube assembly looks line but assuming that it extends
sufficiently into the concrete to adequately transfer the tower's reaction
moments and shears, one would think the resulting stress concentrations
would be similar to an anchor bolt/vertical bar design.  Placing several
closely spaced rebar loops within the top 12-18 inches of the pour might
help to distribute out the high stress concentrations in this area.  Two
sets of loops offset by 45 degrees (looking from the top) can be used for an
even stronger design.  In such an arrangement, one set of loops has its
corners adjacent to the concrete corners and the other has its corners
adjacent to the middle of the concrete sides.  Conventional design for
columns would repeat these loops (at a wider spacing) down to the bottom
where there might be several additional loops in the bottom 12-18 inches.
(Note here that the bottom of the base benefits from reactive soil pressure
whereas the top is exposed!) The corners of the loops would then be tied off
to heavy vertical bars (or combinations of therein).  These verticals
reinforce the concrete against tensile stresses associated with the large
moments that are imparted into the base from the tower under wind load.  But
here, the length of the sides for a "wide" base produces stress
concentrations along the sides at lower values than the shear forces at the
unsupported top (it depends somewhat on geometry).  None the less - some
vertical reinforcement generally has to be there to handle the side of the
base that comes into tensile load under wind load moments.  Again, in your
case the tube steel may well be performing this function.  When visualizing
concrete stresses, it may help to remember that concrete by itself has great
compressive strength, and comparatively little tensile strength.  The steel
is placed to make up for the lack of tensile strength but must bear into the
concrete in a manner that is compressive - or at least relies on pullout
(which is why rebar has those funny little ridges).

If you do place steel loops - bend the bars with adequate radius, overlap
them on the ends and then turn them in towards the center with about 10 bar
diameters protruding away from the bend.  Tying four bars together defeats
the purpose unless they are overlapped and turned in similarly at every
joint.  One can bend 1/2" or smaller bar around a piece of pipe or other
rigid round object by hand pretty easily.  Be sure to keep at least 3" of
concrete cover over the bars (industry standard but also a subject of
controversy).  Don't heat or weld bars unless they are specifically designed
for it.

If in doubt - seek out and get advice from a (local) structural engineer
that knows your soils.

Hope this helps and GL on your project.

73
Matt
KM5VI




-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of Allen
Brier N5XZ
Sent: Monday, January 21, 2013 11:20 AM
To: towertalk at contesting.com
Subject: [TowerTalk] Rebar base for new 18HT installation?

I'm wondering if I need to install a rebar base for my new 18HT tower, or go
with the standard "base tube assemblies" per the installation manual. The
manual does state: "For extra security, steel reinforcing rods can be added
to the base before the concrete is poured."

Tower will be installed in sandy soil (Texas Gulf Coast) and I am not
currently planning to guy it.

Recommendations from the group?

Allen Brier N5XZ
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