[TowerTalk] [Bulk] Re: Alternative Method of Tower Guying(Correction)

[Roger (K8RI) on TT]

On 9/27/2014 2:10 PM, Grant Saviers wrote:

Stock lengths of Al through industrial suppliers is 24'. It comes in 
standard fractional wall thicknesses such as, but not limited to, 1/8th, 
3/16ths, 1/4, and in many alloys as well as tempers. As what they have. 
There are stronger and cheaper than 6061-T6.

73

Roger (K8RI)


> Assuming the building is of recent vintage and permitted, then there 
> is a code for the "live" roof load, and Rockville, MD has snow so it 
> might be around 20 lb/ft^2 or more. So that is a starting point.
>
> Given the roof is flat, there is a most likely a plate of plywood 
> under the weatherproofing. So the roof is pretty rigid in torsion. A 
> guyed tower will convert the guy loads into down force at the tower 
> base (plus the weight of the tower + antennas, etc) so that spot needs 
> to be strong enough (post or structural wall under) or you need 
> provide some means to spread the point load. The guys need to tie 
> their loads to the walls of the building or into the structural parts 
> of the roof.
>
> A self supporting tower is much more difficult, since it applies a 
> moment (torque) to the roof plate, upforce as well as downforce.
>
> You might consider how light you can make the structure, R45 seems 
> like overkill when a guyed mast will support most VHF verticals. Then 
> there is less windload to apply forces to the building structure. 21' 
> of 2" aluminum pipe or 20' of 2 1/2" sq steel tube x 1/8 wall as 
> examples, guyed at the 2/3 level.
>
> Just some ideas for consideration. As others suggest, getting 
> professional help is a very good idea, but you will need structural 
> drawings of the building.
>
> Grant KZ1W
>
>
> On 9/26/2014 6:47 PM, W2RU - Bud Hippisley wrote:
>> On Sep 26, 2014, at 8:36 02PM, Roger (K8RI) on TT 
>> <K8RI-on-TowerTalk at tm.net> wrote:
>>
>>> Don't forget that those guying forces plus are transferred to the 
>>> base of the tower (roof) when the tower is self supporting. A guyed 
>>> tower spreads the force over a wide area and the forces add 
>>> algebraically between the guys.
>>> All of that force is transferred to the base anchor with a self 
>>> supporting tower, so the roof will need to be much stronger than for 
>>> a guyed tower.
>> All the more reason they need to engage a Professional Engineer. It’s 
>> possible they shouldn’t even be _walking_ on the roof …
>>
>> Bud, W2RU
>>
> snip
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-- 

73

Roger (K8RI)


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