If the roof can support the load, a stand-alone base with a mast might be
all you need. These bases used to be made of a wooden structure with sand
bags on it to weight it down and resist the overturning moments. But now
hollow plastic bases are available that can be put in place empty and then
filled with water. These require no penetrations of the roof, which is very
important if you do not own it!
73,
Steve
N6SJ
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Saturday, September 27, 2014 11:11 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Re: Alternative Method of Tower
Guying(Correction)
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@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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