I am in the process of putting up a tower and I have some concerns that I
hope the group can help me with.
The tower is currently 55' of Rohn 45. 5 straight sections and 1 top
section. The bottom section in buried in 4' of concrete as per Rohn's specs
for a bracketed tower. It is bracket to house at 15 feet and will be
unguyed. I can not do so as the tower is less then 10' from the property
line.
For the mast I have 21' of Schedule 40 1 1/2 galvanized steel pipe. I plan
on having 8' of this in the tower and 13' above the top of the tower.
Antennas will consist of the following from the top down:
Diamond X510NA (1.2 SF) (17.2 feet long)
Cushcraft 719B (1.2 SF)
Cushcraft 13B2 (1.8 SF)
One of the following HF beams:
MA5B (3.22 SF)
A3S (4.36 SF)
A4S (5.5 SF)
The wind rating for my county (Sarasota, FL) is 110 MPH.
I am getting a little concerned about the 55' height and am considering
taking one section out leaving me at 45'. I would really rather not do this
but will consider it for safety sake. I have been told they have gone much
higher then this with 25G and no guides. Rohn does not have any charts that
fit my configuration. The closest I can come is to look at their 100'
bracketed tower (5.5 SF at 70 MPH, 2.0 SF at 80). It has two brackets one
at 33 feet and one at 66 feet. Table may be found at:
http://www.rohn.net/CommPro/Towers/Bracketed/Bracketed.htm. In this case
the tower extends 34 feet above the top bracket. I am pushing it a bit with
40' above the top bracket but the cement is only 15' below the top bracket
instead of only another bracket 33' below the top bracket. My installation
should be stronger then the 100' bracket tower example Rohn uses.
If I look at the self supporting Rohn 45 towers
http://www.rohn.net/CommPro/Towers/Bracketed/SStowers.htm it shows 5.1 SF at
70 MPH and 1.4 SF at 80 MPH for a 40' self supporting tower and 2.3 SF at 70
MPH for a 45' tower.
I know I am pushing the Rohn figures a bit but I also understand that they
have some room for error in their figures. With lawsuits today who can
blame them. As I stated earlier my county has a 110 MPH wind rating but
none of Rohn's towers examples have charts for this. Commercial towers in
this county must be rated to 105 MPH per zoning ordinances. My total wind
load is going to be somewhere between 7.42 SF and 9.7 SF. I am only 10'
from my property line making it impossible to do any sort of reasonable
guides.
I am also concerned a little about the mast. The Diamond X510NA (1.2 SF) is
17.2 feet long and rated for 90 MPH. The wind load at 90 MPH is 29.87
pounds at 21.6 feet or 645 foot pounds. Assuming a 25kps rating for the
schedule 40 it should hold up and logic tells you the 17.2' antenna would
give way before the 13' of 2" steel pipe will. The small beam at 5.5 SF is
only a load of 137 foot pounds.
Do I really need to take some of the tower down or do I need to reduce my
already low antenna loads?
73--
Dave Armbrust - AE4MR
ARRL WCF Section Manager
(941)378-1701 Fax: (941)929-0040
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