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@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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