BobK8IA@aol.com wrote:
>
> In a message dated 1/22/2008 2:53:29 P.M. US Mountain Standard Time,
> xdavid@cis-broadband.com writes:
>
> Three inches out of 100 feet also requires an accuracy of 0.3 inches for
> a plumb bob string hung from a 10 foot section of tower.
>
> Three inches out of 100 feet calculates out to be a total angle of 0.14
> degrees. By that criteria, the 0.1 degree resolution of the digital
> angle gauge I used doesn't seem like that much overkill. I guess I'm
> just one of the particular ones ...
>
> http://i228.photobucket.com/albums/ee141/xazdave/HD-70_Tower_Leg_Angle_89r1Deg
> rees.jpg
>
> 73,
> Dave AB7E
>
>
>
>
>
> Dave;
>
> Steve was quoting EIA/TIA-222 as it pertains to guyed towers (deviation from
> true vertical no more than 1 part in 400 or 3" in 100'). For self supporting
> towers, like yours, the spec calls for max deviation to be no more than 1
> part in 250, or roughly 4.8"in 100 ft, i.e. 3.36" for your 70 foot AN
> Wireless
> tower. ;-)
>
> 73, Bob K8IA
I note that Kurt Andress's evaluation of guyed towers shows typical
deflections of 16" on a 100 ft tower. Now, that is a guyed tower, but
it shows that the deflection under load could be substantially more than
the TIA222 no load out of vertical number. I would imagine that a free
standing tower would have more deflection, and still be perfectly safe.
One could look at the off-vertical situation by considering that
installing it non vertical puts a static side load on the whole thing.
The load would be sin(theta)*weight. Say the whole thing weighs 1000
pounds (I don't know if this is plausible.. it's just easy to calculate)
and you're 2 degrees off vertical (about 3.5 feet in 100). The side
load is about 35 pounds (distributed along the whole thing). That's
pretty small compared to the wind load (90 mi/hr = about 20 lb/sq ft,
and you know the tower has a lot more than 10 square feet of cross
sectional area)
So, the verticality requirement is probably more of an aesthetic one,
or, in the case of a telescoping tower, limited by the ability to still
extend and retract while not under too much load.
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