Many folks have answered the "how far from the dirt" and "how do I get the
rebar off the ground" questions - but no one seems to have tackled the "do I
really need a rebar cage" question.
Coincidentally, I just poured the base for my new HG-70HD (6.5 yards of 3000
PSI with 1 1/8" anchor bolts and a full rebar cage), and as part of the
process I spent about 30 minutes chatting with a good friend who happens to
be an expert on reinforced concrete structures (he used to engineer, design
and build dams for a living).
To make a long story short, the whole game is making sure that if your tower
tries to fall over, it has to move the entire block of concrete that makes
up the base. To accomplish that, you need to do two things:
1) Insure that the anchor bolts (or base legs, for some towers) have good
adhesion with the concrete. This is typically accomplished with some kind
of device (either a plate, a nut, or a 90 degree turn) at the base of the
bolt. Concrete doesn't adhere very well to smooth steel - that is why rebar
has deformations in it.
2) Tie the vertical members to horizontal members, so that the vertical
lifting force generated by the tension on the leg is transferred to the
entire block. You basically don't care about the legs that are in
compression, since concrete is very very strong in compression. The problem
is the leg or legs that are in tension. You need to insure that you have
plenty of horizontal steel (to transfer the load away from the leg in
question, and into the rest of the block), and that the horizontal steel is
securely tied to the vertical leg. This is usually accomplished by laying
the horizontal rebar _on_top_of_ either the plate or J at the bottom of the
anchor bolt, so that lifting the bottom of the bolt also lifts the
horizontal steel.
So the bottom line is yes, you do need a rebar cage. Not as much for
traditional reasons having to do with shrinkage of the block, but rather to
insure that legs in tension don't just pull out of the block - but rather
have to move the entire block.
NB: I am not a PE, and nothing said here should be taken as anything other
than the thoughts of an educated lay person.
Hope this helps.
All the best,
***dan
Dan Levin, N6BZA
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