TJI joist are available with a range of web height. Check with you
local distributor or TJI for load properties of the different sizes.
On 2/28/2014 9:51 AM, Patrick Greenlee wrote:
When mounting a tower on top of your roof you need to consider the
down thrust on the tower base and what part of your structure holds
up against that thrust. Guy tension increases these down forces. Wind
can greatly increase these down forces, even to the point of
structural failure of the roof structure because sideways wind forces
translate into down forces via the guys.
Of course the tower base should be placed right above good strong roof
structure but note that the roof was not designed to hold up against
the force of the mast. There are simple modifications/additions that
will greatly strengthen the roof in the area of the mast's down force.
A thrust member, a good solid timber or metal piece (tube or
rectangular metal) can be mounted vertically from the point under the
roof where the mast is positioned on downward to something capable of
taking the load.
OK, what can "take the load?" a timber or stiff metal reinforcement
positioned on and at right angles to the ceiling joists can distribute
the down force of the mast over several joists. Ideally you will
mount the mast over or very near to a load bearing wall in the
building and a timber, tube, or other brace from the top of the wall
to the point below the mast will pass the down force to the wall.
Personally, for vertical timbers I prefer at least a good straight
grained 4x4 or a metal tube that is stronger than the mast. For a
horizontal item to place at right angles across ceiling jopists I
prefer what are called "TJI" (trus joist, maker's spelling, not mine)
These are very strong and stiff for their weight. Essentially a TJI is
a wooden "I" beam. Think of two 2x3 boards with grooves down their
lengths attached to say a 12 inch wide length of OSB (Oriented Strand
Board, cheap substitute for plywood, NOT PARTICAL BOARD!) These TJI
are used as "I" beams and when placed on top of ceiling joists at
right angles to the joists the load placed on the TJI is distributed
to the underlying joists. The TJI being quite stiff can span several
joists. They can be shimmed to account for their flex under load
placing thicker and thicker shims as you progress out from the center
where the mast down load is.
I am not a structural engineer nor an architect by degree or
certification so consider the above as food for thought not detailed
instructions for any specific undertaking. Offer void where
prohibited by law, your mileage may very, no refund without a valid
receipt, no tickee no laundry, I didn't stay in a motel, I didn't play
anyone on TV. In fact I was never there and I have witnesses to that...
Patrick NJ5G
.
On 2/28/2014 12:31 AM, Roger (K8RI) on TT wrote:
On 2/27/2014 12:50 PM, Wilson wrote:
I hope this is easily translate and hope it is useful.
Wilson has asked the important, basic questions.
These are some additional thoughts and a question:
I have to ask if the 20 meter height above ground will be mounted on
the roof, or ground. On a 5 to 7 meter high flat roof we'd be
looking at a 13 to 15 meter tower (roughly 40 feet). "Depending" on
the roof structure and antenna size, it might be a lot for the roof
to support when the wind is strong. Even a 10 to 12sq ft (About 3.6
sq meters) is a lot of leverage when applied to a 15 meter lever.
Concrete and heavy timber roofs should be able to handle it.
A 20 meter ground mounted tower might be simpler from a structural
approach, but getting it up could be difficult. At present there are
still too many unknowns to be able to recommend one approach over
another
The tower in the following link is well beyond the capabilities most
of us have.
http://www.rogerhalstead.com/ham_files/skyhook.htm It is 60 meters,
self supporting and is entirely home brew, but it was engineered by
professional and licensed engineers. It was completely assembled by
two hams. Note the one ham working at a 100 or 120 feet (30 to 37
meters) and the gin pole they used. This story was in QST
20 meters (65 feet for those in the US) is not out of the question,
but I need to emphasize it can be very dangerous work. I don't know
your background or ability for climbing and working at heights,
access to safety equipment, or help.
With the steel available on the market and a safety factor for home
built it will take the equivalent of 4 sections and would need to be
built in the shape of a windmill tower. Triangular, not square.
It would need to be built in place unless you have access to heavy
equipment to lift it up and stand it in place.
Using steel angle, which could possibly be purchased with a
galvanized coating could be done with little or no welding. Bolting
the sections together would require care to make sure the joints are
strong enough. Because of the likely wind load it would require much
stronger bracing than a typical wind mill tower and a reasonable
overlap of vertical sections.
Wood is normally easier to work with and easier to get, but does
deteriorate much faster than steel with age. The wood available here
is no longer of high quality and is not well cured/dried. 2 X 4s tend
to twist, or warp. I lay them out on supports so air can get to all
sides. I rotate them (top to bottom)twice a day for the first week
and once a day after that in a heated (10 to 15C) shop with a
dehumidifier. Higher temperatures may cause them to dry too fast. Out
doors in summer They need to be protected from rain. The sections and
pieces should be bolted together, rather than nailed although I'm
sure opinions will vary on that. Use large washers on the bolts.
I tried clamping new 2 X 4s to a steel frame (without rotating) while
they dried, but in less than 4 days they twisted enough to break with
enough force to throw pieces a meter long as much as 3 to 5 meters.
When it broke, it was loud enough to scare me
73
Roger (K8RI
Hi Vikas,
Others will chime in, but I think 20m freestanding is more than you
could likely get built/installed safely.
So please tell us the following:
Do you have a flat roof, or peaked?
Roof material?
What services are available locally, welding, woodwork?
What materials are available locally, steel, aluminum, wood?
Many towers have been built of wood, which is available nearly
everywhere.
Do you plan to do the building, or hire it done?
A modest installation will be many times easier than an ambitious one.
With 3-5m, on a rooftop, a three element Yagi or two element quad
will help you greatly, with reasonable effort and danger.
It's reasonable to use a short tower on the roof and let the mast go
through the roof and be rotated by hand, if you have trouble finding
a rotor.
Wilson
W4BOH
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