[TowerTalk] concrete bases for freestanding towers

[TexasRF at aol.com]

Ken and all,
 
We frequently overlook the wind load generated by the tower structure, not  
including any antennas at all.
 
In the case of the BX style towers (compared to a round leg/z braced  
style), the legs are quite wide, the braces are not only quite wide but are also  
doubled in their X shape. The tower alone could have a wind load of 40 or 
50  square feet, making the antenna load a small part of the total load 
applied to  the base.
 
A casual look at the forces generated by a 64 ft tower like this at 90 mph  
might be on the order of 40 sf X 30 lbs/sq ft X 25 ft (tower  is wider on 
bottom than top) = 30,000 ft pounds. Not a trivial  number!
 
The antenna load of 6 sq ft would be 6 sq ft X 30 lbs X 64 ft = 11,520 ft  
lbs, also not trivial but only about 25% of the total load.
 
30 years ago typical designs were based on 70 mph winds; today many if not  
most locations, use designs based on 90 mph winds. So, yes, things have 
changed  over the years.
 
Add some high rainfall with softened soil and a freak wind storm and you  
have a tower with less than adequate base, disaster is in waiting.
 
73,
Gerald K5GW
 
 
 
 
 
 
 
 
In a message dated 5/13/2011 7:42:33 A.M. Central Daylight Time,  
wa8jxm at gmail.com writes:

On May  13, 2011, at 8:26 AM, Fernando EC1CT wrote:

> What have grown over  the years are the antenna´s size and their wind 
loads...The TA33 has nothing  in common with the actual trapless antennas Force 
12, SteppIRs,  Obtibeams...


I can certainly understand that for a heavy duty  tower, but the Rohn BX-64 
is only rated for a 6 sq ft antenna (probably less  than a TA33 and 
rotator) yet Rohn specs a 5 cu yard base.  

Ken
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