Mike Gilmer - N2MG wrote:
> K3KO wrote:
> << 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.
As Hank pointed out, both bases are pinned in a strict sense.
The moment I was referring to is not developed by any fixivity to the
footing, but by not having all three legs sharing the axial load. If one or
two legs are carrying significantly more load than the other one or two,
then a moment is developed by the offset of the varying loads on the legs
from the section centroid.
The concern I would have is the failure of a leg carrying more that its
share of the load, instead of what the moment is doing in the whole
section. I think I did this to a detailed tower section model once and saw
the problem, that was a few years ago, so I'd have to go find it again.
A thick base plate that is stiff enough keep the legs sharing the load
would eliminate the problem. K8LX asked about it. It seems to have been
figured out for the base plates on the rotating towers, but they have a
single centralized contact location with what is below. I'd start there.
I think the tapered base is better when the tower deflects, and is not
unlike what we do to get rid of the same problem at the base of a yacht
mast, most have a rocker plate to allow misalignment and keep the loads in
the section stable.
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