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

[Grant Saviers]

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
>   
>
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