On 5/13/11 5:18 AM, WA8JXM wrote:
>
> Is my memory faulty, or have the recommended bases grown over the years?
> Were the old recommendations inadequate, or has everyone grown super
> conservative over the years? "If one yard is adequate, three will be
> better, so let's use five yards"???
>
The other thing that's changed is the design wind load. I suspect
(although haven't gone back to look at old versions) that the wind speed
for which the tower installation is designed has increased over the years.
This is one of those fairly subtle things, and also fits in with the
whole "definition of failure" issue. The mfr may consider failure any
permanent deformation, while the ham may consider anything that doesn't
result in a pile of scrap metal as perfectly fine.
The "failure probability" might have been driven down for some reasons.
Maybe before, the design covered 2/3s of the typical installations (in
terms of material properties, soil bearing strength, etc.) and now the
design covers 99%, so it has more "margin" to cover the edge cases.
One thing driving that is city regulations. A LOT more cities are
requiring "engineered" installations: that is, backed up by calculations
and analysis rather than empirical recommendations from the mfr.
And, of course, there's the economics. If you're buying everything from
scratch, adding $100 for another yard of concrete to a $10,000 tower
project isn't a big percentage change in the project cost. That's the
sort of rationale that would go into a generic engineering design:
what's the penalty for making it a bit bigger and making the design more
robust.
On the other hand, if you're putting up a tower you already have, and
you're doing your own labor, etc., then the $100 is a bigger fraction of
your cash outlay.
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