I agree. Calculating equivalences (round versus square) almost for sure
requires a competent engineer, and it might even (maybe even probably)
be significantly different depending upon location. For example, I'd
bet that all of the factors Jim mentioned are relevant, but possibly in
different weightings depending upon soil conditions. Since soil is, I
believe, treated as a viscous liquid, the resistance to overturning may
be treated differently in Florida than it is in Colorado. And in any
case, I'd bet you have the same 2/3 factor between round and square (for
viscous resistance) that you have for surface area of an antenna element
in the wind.
By the way, it isn't that as hams we can't (usually) come up with
(mostly) workable solutions on our own, but when we're talking permit
approvals and financial liability issues, it just doesn't make sense to
me to guess. I'm all for a deep skinny round hole instead of a blocky
rectangular one, but I don't think it would be trivial to convince
anyone I had arrived at the proper dimensions on my own.
Jim Lux wrote:
> Not necessarily. If the mass is being used as a "big weight in the
> ground", then that's what's important. If it's for a free standing
> tower, then it's mostly resisting the overturning moment, so long and
> skinny is probably better than short and wide, for a given amount of
> concrete, just because the lever arm is longer. (one deep piling
> provides the same resistance as several horizontal pilings of the same
> size, because it's direction independent).
> If the base is basically to "float" the downforce from the guys and
> resist the shear force from wind on the tower, then big and flat might
> work better... I don't know off hand.
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