Pete Smith wrote:
> When I put up my 97-foot Rohn 25 tower on a pier pin base (my first and
> only experience of this kind), the experienced tower builders who were
> helping me said absolutely nothing about plumbing the tower with any kind
> of instrument. When it was up and all three guy sets were installed, one
> guy stood at the base looking up the tower while 3 others adjusted the
> turnbuckles at the guy anchors to get it approximately straight and
> vertical. Once it looked OK, each one tightened his turnbuckle by the same
> number of turns until the guy tension (measured with a Loos Gauge) was
> correct.
>
> I am wondering how important all this emphasis on precision really is?
>
> 73, Pete N4ZR
It's probably important only if you're looking to get the design
performance out of the system. If you're overdesigned or, alternately,
running on the ragged edge and willing to tolerate failure, it doesn't
make a heck of a lot of difference. 2 feet out of plumb in 100ft tower
would be about a degree. The asymmetry of forces would still be pretty
small.
On the other hand, wouldn't you say that the tower has to be perfectly
vertical in order that the plastic owl can be properly aligned. There
has been much talk on the list of aligning to true north, but that's
only in one plane, and we've totally neglected the need to align the owl
in the other plane.
I think the question is whether vertical means with reference to the
local gravitational field or with reference to the center of the earth
or with reference to the normal to the standard ellipsoid at that
point... these things ARE important, after all.. and they're in that
same fraction of a degree magnitude. Earth is flattened about 1 part in
300, and the difference in "vertical" would be on the order of a bit
more than a tenth of a degree at 40-45 degrees latitude. That's 3" out
of plumb on a 100 foot tower.
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