Ice adds wind area and weight to a tower. Towers with smaller members are
more affected than towers with larger members from a percentage stand
point. Ice is generally not good for the health of any tower.
w/o ice w/ 1/2" radial ice
Rohn 25
+ 3/4 hardline on each face
Effective Area 1.61 3.70 ft^2/section
Weight 41.20 97.44 lb. / section
Rohn 55
+ 3/4 hardline on each face
Effective Area 2.10 4.40 ft^2/section
Weight 95.0 192.40 lb. / section
Other than the weight of the tower section, 45 looks like 55.
These are per EIA-222 Rev E. from some calcs I did for a guy trying to make a
choice. The standard specifies that the wind pressure can be reduced to 75% of
the normal wind pressure.
Some local meterological data may indicate that a lower percentage may be
applicable in your area. Check with your state climatologist or meterological
center.
I usually use 25 pounds per square foot to keep the math simple and most towers
I deal with for amateurs are in an 80 MPH wind district and under a 100 ft so it
gives me a good ball park number when asked for an informal opinion.
Anyway the ice load on Rohn 25 increases the wind area by (3.70/1.61) = 2.29.
I generally find ice loading to be the most severe case when evaluating a tower.
Of course if you live in Florida, then ice is not much of a concern, just
hurricanes.
de n0yvy steve
______________________________ Reply Separator _________________________________
Subject: Towers and ice.
Author: Gene Smith <Gene.Smith@mailhost.bellhow.com> at ccmgw1
To: <towertalk@contesting.com>
Date: 8/19/96 2:39 PM
I have been trying to determine the effect of ice on a tower such as Rohn 25G.
Rohn gives antenna wind loading sizes with and
without ice for their freestanding 25G but not for
bracketed or guyed.
I emailed Rohn about obtaining the max antenna
size with ice for their 25G and 45G towers for
a bracketed installation.
The next day I received a phone call from a
gentleman from Rohn explaining that ice is not
really a problem with high wind speed because
the wind blows the ice off the tower. He went
on to say that Rohn installed a tower in
my area (Cleveland/Medina, OH) for a utility
company and that they used the non-ice wind loading
numbers for the installation. He said that the
reason the ice numbers do exist in the
documentation for the freestanding tower
versions is because marketing wanted it.
Is this really true? What are the real
effects of ice on a tower?
Gene
WD8OZL
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