Hi Carl,
In a nutshell, we went with 55G for extra tower capacity and a safety
factor. With the icing and wind conditions we had to meet with our proposed
antennas at this height above grade, 55G had some extra load capacity, 45G
may have just been sufficient, and 25G was insufficient according to Rohn
specs and our structural engineer's analysis. For comparison, a MonstIR on
40 ft of 55G was just barely allowable according to the engineer's (FEM I
believe) model.
In the initial structural engineering work we had performed, the actual
tower weight was a far second-order contributor to the total axial load at
the base. It was interesting (and surprising to me) that most of peak axial
load comes from guy forces that are transferred to the tower base. Putting
1.5 inches of radial ice on each guy line really makes for some astonishing
axial load figures. In the end, the base axial load was indeed the most
important factor.
Anyhow, we decided to stay with 55G and add a very small axial load in
return for extra tower capacity (for future antennas, more severe weather,
climber's peace of mind, etc.).
73, Chris KA8WFC
-----Original Message-----
From: jeremy-ca [mailto:km1h@jeremy.mv.com]
Sent: Friday, August 17, 2007 12:51 PM
To: Christopher J Galbraith; 'towertalk'
Subject: Re: [TowerTalk] Our club's (W8UM) new tower and antennas
How come the extra weight of 55G when 45G or even 25G would suffice for that
small load. Ive been involved in several roof top installs at
commercial/industrial sites and roof loading weight was always the primary
concern.
Carl
KM1H
----- Original Message -----
From: "Christopher J Galbraith" <cgalbrai@umich.edu>
To: "'towertalk'" <towertalk@contesting.com>
Sent: Friday, August 17, 2007 3:14 PM
Subject: [TowerTalk] Our club's (W8UM) new tower and antennas
> Gang,
>
> For your enjoyment, here are some action shots of our new W8UM roof-top
> tower installation from 7/30/07. It is a 40 ft Rohn 55G tower on top of
> the
> 60 ft tall Electrical Engineering and Computer Science (EECS) building at
> The University of Michigan:
>
> http://www.umich.edu/~umarc/projects/towerConstruction.html
>
> And one view from the ground on Michigan's North Campus:
>
> http://www.umich.edu/~umarc/photos/tower/views/tower_Beal.jpg
>
> The new tower and antennas have evoked some positive responses from the
> engineering types in the vicinity ("it is huge!") and we hope it will
> serve
> as our most valuable visual recruiting tool (enticing students into the
> W8UM
> shack).
>
> The 4-el SteppIR at 105 ft is a DX killer on 20m (and should be on 14-30
> MHz
> when propagation improves) and the dipole seems to be doing well on 40m,
> too.
>
> We originally were going with a MonstIR for the HF Yagi but ran into a
> series of engineering problems. It is a long story, but our problems were
> mostly due to having structural engineers that did not have any experience
> with roof-top antenna installations. In the end, the 6-20m Yagi with
> 30/40m
> dipole was a good compromise between overall performance and
> size/weight/cost.
>
> I sincerely thank everyone on the list who gave us excellent advice
> throughout the design process. We learned a great deal in the year or so
> of
> putting the design together, and a lot of information came from list
> members' valuable experience. As you'll note on the web page, Hank
> Lonberg
> and Matt Strelow had a lot to do with our success, too!
>
> Support your college amateur radio clubs!
>
> 73, Chris KA8WFC
>
> The U-M Amateur Radio Club (W8UM)
> www.umich.edu/~umarc
>
> P.S. If you know any U-M alumni, please spread the word. We love to hear
> from former W8UM (and W8PGW, W8AXZ, even 8AX!) ops.
>
>
> _______________________________________________
>
>
>
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> TowerTalk@contesting.com
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>
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