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@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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|