Jim:
Maybe I'm missing something....but I see 4500 lbs of bending
moment on the mast, where it passes through the house, and
about 1100 lbs of resisting force where the mast sits on the
floor of the attic.
15 sq ft x 30 lbs/sq.ft.@100mph = 450 lbs
450 lbs x 10 feet (ht of mast above roof) = 4500 lbs
This is the force which will bend the mast.
4500/4ft = 1125 lbs
This is the lateral force produced at the lower end of
the mast, which has to be withstood by the floor joist
system.
I have no idea about the strength of your mast material.
I have some concerns about your roof truss system, in that
it's probably not designed to handle 2.5 tons of lateral
load. You need to translate this load into the vertical
walls with some structure, and possibly spread it over more
than one bay.
AND, I would be similarly concerned about the design of
the attic floor joist system. It may need reinforcement,
to spread the load over a few beams.
Somebody needs to check my figure for windload @ 100mph. I
didn't have time to look it up.
Jim, N2EA
From: "WB9UWA Jim Shaffer" <wb9uwa@gte.net>
To: <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Sun, 17 Mar 2002 22:19:11 -0600
Subject: [Towertalk] Mast guying and hazer?
Hi All,
I am planning a new EME array that will overload my 2 1/2" mast. This mast
is fastened to the
attic floor, 4 feet below the roof. It is obvious that I need to either
strengthen the mast or add
guy wires. I will need to hold something on the order of 15 sq ft of
windload at a height of
10 feet above the roof. This works out to something like 5000 lbs at the
base of the mast
even though only something much less then 600 lbs of lateral force will be
applied to the mast
at 10 feet above the roof. Guy wires seem to be the logical solution even if
they are to be steep
so as not to interfere with the array. I am thinking about something like 3
feet out in four directions.
I am thinking that 3/16" Rohn guy wire should handle this with its 4000 lb
rating and add substantially
to the strength of the mast. The guys would be attached about 6 feet up.
Above this there is a rotating
mast.
Jim Jarvis
Keithley Instruments
Essex Vermont
802 872 5830 voice
802 872 5831 fax
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