[TowerTalk] Plumbing a tower

[Jim Lux]

BobK8IA at aol.com wrote:
>  
> In a message dated 1/22/2008 2:53:29 P.M. US Mountain Standard Time,  
> xdavid at 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.