[TowerTalk] TowerTalk antenna TOWER - Homebrew

Fri Feb 28 13:31:34 EST 2014

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
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk at contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>>
>>
>> _______________________________________________
>>
>>
>>
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