<< First, they use a flat bottom plate on the concrete. I doubt that these
rotate at all to relieve the moment at the bottom.>>
Kurt's webpage says:
"The wide footprint on the pier may allow the tower to rotate about the
vertical axis, but resists the ability to lean over without developing
significant bending loads"
Common sense (admittedly not always valid) says the flat-plate, common,
pier-pin base WILL rotate if the tower is rotating. What a flat plate does NOT
do (that the tapered type does) is allow the tower to lean.
Buried base: Leans? No. Rotates? No.
Flat pier base: Leans? No. Rotates? Yes.
Tapered pier base: Leans? Yes. Rotates? Yes.
<< Second, I see no reference to the complicated rebar cage that one has to put
in the hole to make up for the missing tower section. >>
This is immaterial. If you feel tower sections are good enough, why not just
use another tower section as "rebar" for the pir-pin design? Hell, the "rebar"
section could tolerate being really hosed up in a pier-pin application - in a
buried base, at least the last few inches/feet need to be straight.
The main drawback (and it's not unimportant) to any pier-pin design is the need
for temporary guys. But even the flat plate design alleviates the twisting of
the very bottom sections' legs. Also, with a pier-pin design, the drain holes
are above grade, no condensation/drainage problems. My ground is wet clay and
I believe (maybe I'm wrong) that the oft-mentioned "buried base sitting on
gravel for drainage" would, in my case, simply result in the buried sections'
legs staying full of water. (I currently have 8 holes dug for 2 towers and
they are totally full of water).
73 Mike N2MG
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