Hello, Jim, Jim, and all;
Jim is correct in calculating the loads and questioning the adequacy of the
roof and joist structure. He is also correct in questioning his recollection
of wind force: use 40 lb per sq ft (AIA standard) for 100 mph; 30 lb per sq ft
for 80 mph.
For mast strength, a mechanical engineer's handbook (such as Marks) will
provide formulas for use in calculating strength of various cross sections
(cylindrical, square, solid, tubular,etc.) and for material strengths. Sellers
of masts may provide the maximum recommended bending (moment) load for masts
they sell. Remember to include some safety factor and don't compromise the
strength with holes, cuts, or other stress risers near the point of maximum
bending.
If you guy the mast, a concern is the vertical load imposed on the attic floor
and its structure and buckling the mast. (Guyed masts or towers often fail by
buckling when overloaded!) This vertical load equals the horizontal force
component at the point of attachment of the guys divided by the tangent
(multiplied by the cotangent) of the angle between the guy and the mast. At
30°, the vertical force is 1.7 times the horizontal force! You may play with
the calculations (spread sheets make it easy to vary the dimensions) and arrive
at the lowest position for the guys that does not exceed the maximum bending
moment and also creates the least vertical loading.
Look for information on buckling in engineering handbooks or from the mast
seller .
Another problem with major structures on the house is that they may crack
plaster or wallboard even if they don't damage the structure.
For fastening guys to the roof, lag screw type fasteners may be marginal. A
better plan is to place a substantial member (2X4 or 2X8) across several
rafters to anchor a welded eyebolt, with substantial flat washers under the nut
to spread the load over the wood.
I'm no expert on preventing leaks. Most amateurs will start with some kind of
caulk or sealing compound. Success and longevity probably depend on the
strength of the sheeting or whatever is under the shingles and its ability to
keep the eyebolt or other fastener from moving sideways, expanding the hole,
and working the sealing material. A flashing above the fastener to divert
water from above may be a good idea.
People successfully mount TV and other VHF or UHF antennas and even small HF
antennas to tripods or other arrangements on the roof, but be cautious when you
start putting EME arrays and other large square footage on it. You are off to
the right start in calculating the loads. Get some professional help, or at
least read all the right handbooks, to determine what loads the house, mast,
and other parts can safely withstand.
73 de WOØW
jljarvis wrote:
> 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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