[TowerTalk] Rohn base specs

[TexasRF at aol.com]

Just a bit of pencil scratching on the back of an envelope shows a base  
like this to be far from overkill, especially considering it is based on  
"normal" soil. When I drive a medium sized tractor around my place this time of  
year and see 12 inch deep ruts happening I know my soil can't be "normal".
 
If one is fortunate to purchase really good concrete, it and associated  
rebar will weigh in at about 4,000 pounds per yard or about 120,000 pounds in  
this case.
 
On flat ground, no soil resistance, it will take 60,000 pounds of force to  
just begin lifting one edge of the foundation. A tower planted in the 
middle  creates a two to one lever requiring 120,000 pounds of force to begin 
lifting  one edge of the foundation.
 
Assuming 90 mph winds, the tower alone, with no antennas could exert  
upwards of 10 pounds per lineal foot and SSV tower may well be more than this.  
But, for the back of envelope calcs we can use 10 pounds. At an average 
height  of 140/2 or 70 ft, this totals 1400 X 70 = 98,000 ft lbs of force.
 
That leaves 22,000 ft lbs for the antennas, rotator, mast and coax. Using  
the same 90 mph wind speed which will impart a load of about 30 pounds per  
square foot load, this allows 22,000 / 140 / 30 = 5.2 sq ft for the antennas 
 system.
 
This is all simple enough but only a trained engineer can do the exact and  
proper calculations to take into account your soil conditions, wind speeds 
in  your area and then apply appropriate safety margins to keep you and 
himself out  of legal troubles in the event of a tower failure. 
 
Reinforcement of the concrete is a critical issue. While all this 120,000  
pounds of force is happening, the foundation has to remain one solid chunk 
of  material with no cracks. This is serious stuff with really bad 
consequences if  done improperly.
 
Overkill? I don't think so!
 
73,
Gerald K5GW
 
 
 
 
In a message dated 1/18/2011 7:18:36 A.M. Central Standard Time,  
K7LXC at aol.com writes:


In a  message dated 1/15/2011 7:22:06 P.M. Pacific Standard Time,   
towertalk-request at contesting.com writes:

>  Looking at the  Rohn base specs, I am quite surprised at  the  
requirements,
assuming I read it right.  For a base, without   piers, the specs call for a
14'-3" X 14'-3" X 4' concrete base ( 30.1  cubic  yards).

>  To me that seems like considerable  over  kill.


Umm, so you're an engineer who has  calculated the  relative forces to 
arrive at that conclusion? I  suggest you follow the LXC  Prime Directive 
to 
"DO what the  manufacturer says" in order to construct a  reliable and safe 
 
installation. 

Cheers,
Steve    K7LXC
TOWER TECH  -
Professional tower services for  hams




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