Subject: [TowerTalk] Calculating Antenna Wind Load
Date: Tue Feb 11 15:07:12 2003
The answer to the question about whether you need to
multiply your Force 12 numbers by 1.5 or not is contained in the following
information:
First, you must remember that both antenna makers and
tower makers are trying to make their product look as good as possible to
potential buyers. Anything an antenna maker can do to make his wind load
look as small as possible will be to his benefit in his advertising
literature. Also, anything a tower maker can do to make his tower look as
strong as possible will be to his benefit in HIS advertising literature.
It has been pointed out that round members (most antennas)
have less wind resistance than flat members by a factor of about 2/3. This
concept and figure is well accepted among structural engineers who are the
experts in this field.
Antenna manufacturers, in order that their antennas will
appear to have less wind loading in their literature, usually take this
fact into consideration when specifying the square footage of their
antennas, ie: if you add the areas of all the elements (or the boom) that
will be seen by the wind when blowing directly at them, and take 2/3 of
this number, you will have the lowest figure for antenna area for that
antenna. (Of course it will be a higher number if you don't take 2/3 of it
. . . )
Tower makers, on the other hand, wanting to make their
towers appear as strong as possible, will calculate the acceptable load on
their towers for a flat plate at the top with its maximum exposed area
catching the most wind force. They will then figure how much MORE area
they can safely take if the load were a round (tubular) member rather than
a flat plate and the original calculation for a flat plate can be
multiplied by 1.5 (same as dividing by 2/3). This is the figure the tower
maker will generally publish in HIS literature. (Rohn gives you BOTH
numbers.)
The problem is that in any overall tower/antenna design,
you can only use this 2/3 (antenna maker) or 1.5 (tower maker) calculation
ONCE . . . either by the antenna maker OR by the tower maker . . . but NOT
by both of them. So in order to know if the antenna maker's square footage
number for applied load compares properly with the tower makers square
footage number allowed load, you have to know if either of them used the
2/3 or 1.5 figure in their calculations. If they BOTH used it, then you
have to take it out of one of the numbers before you can compare the
antenna load to the tower's acceptable load. Since neither tower makers
nor antenna makers generally TELL you if they used the 2/3 or 1.5 factor
in their calculations, you have to ASK them their calculation methods.
There are exceptions to this. Rohn is one, since they DO tell you and I
have heard there are antenna makers who will also share this information
with you. An extremely common mistake is to take Rohn's allowable load
figure for round members (1.5 times the allowable figure for flat members)
and use it to determine how much antenna load you can put on it using the
antenna maker's load ALSO figured for round members (2/3 of flat members).
This results in a dangerously overloaded tower.
Stan W7NI
