You don't mention your mast size or wall thickness. With 2" O.D. 1/4" wall
at 70MPH, I calculate ~20,000 PSI yield strength required. For 1/8" wall I
get ~32,500. At 90MPH the requirements go to ~32,000 and ~53,000 PSI
respectively. I calculated this with a spreadsheet I wrote based on a NCJ
article a few years back. It has NOT been confirmed by a professional
engineer and I don't guarantee the accuracy of the figures I have quoted.
Stainless is a rather soft material and I would avoid it for structural
stresses. If you are able to locate stainless structural tube, the same
source must also have access to other types of tubing?
Another point. T2X rotors will not fit into a 25G tower section without
cutting a brace.
Mike, W1NR
-----Original Message-----
From: owner-towertalk@contesting.com
[mailto:owner-towertalk@contesting.com]On Behalf Of Sylvan Katz
Sent: Thursday, January 18, 2001 11:51 AM
To: towertalk
Subject: [TowerTalk] Reality Check
Hello TTs
Before I place my order for a pair of Force 12 antennas I thought I would
put my design before the TT participants for a reality check and comments.
First, let me thank those of you who have made comments and suggestion off
the reflector.
My plan is to put a Force 12 C31XR and a EF-240s on top of a 58 foot
foldover 25G. The hinged which is at the 28 ft point will be PINNED and
LOCKED. I will NOT be winching the antenna using the tower winch but rather
I will use a crane with a bucket that can reach to 80 ft from just outside
my parameter fence. The foldover will come in handy for dressing the tower
before I erect the antennas.
Currently the tower has 2 sets of four 3/16" EHS guys attached the 28' and
58' points. I am using large diameter screw anchors that have been embedded
in 1.5' x 1.5' x 6' concrete at a distance of 37' from the base of the tower
(my fence line on a residential lot). The portion of the screw anchor above
ground has been tarred to prevent corrosion. The base of the tower sits on a
concrete block 3' x 3' x 3.5'. In the past 30 years we have rarely had
extreme winds gusts of 90 mph and an extreme hourly speed of 60 mph. I am
using 70 mph as my design speed.
The Rohn table indicates that 60' 25G tower with two sets of three guys
attached at the 28' and 55' points and a guying radius of 48' can a handle
a top round load of 16.8 sq ft at 70 mph - no ice (we rarely get icing
here - it is too cold :-). The total area of the Force 12 antennas is 13.6
sq ft. My calculations indicate that if I place the C31XR at the top of the
tower and the EF-240s 10 ft above that I will add about 1 sq ft of mast area
and I will need a mast with a yield strength of about 40,000 psi. Good mast
materials are hard to find in this area but I might be able to locate some
42,000 psi stainless steel. Barring that I will have to reduce the
separation between the two antennas to 9 ft and use 6061 schedule 40
aluminum with a yield strength of 35,000 psi.
My rotator is a T2X that will be mounted as far down the tower as possible -
likely within 10 ft of the base of the tower. I have a thrust bearing at the
top of the tower and I am going to place another bearing about 10 ft below
the top of the tower. I have not quite decided how to handle the coupling
between the 18 - 20' top mast section and the lower section which will run
down the center of the tower to the rotator. Also, I have not decided what
type of material I need for lower section of the mast.
That is about if folks. Your comments and suggestions are welcome. I would
rather error on the side of safety and if need be I will forego the 40m
yagi.
cu .... sylvan
----------------
Sylvan Katz
Saskatoon, Sask
VE5ZX & G0TZX
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