[TowerTalk] Windload calculation query

Rick Karlquist richard at karlquist.com
Wed Nov 25 13:27:45 PST 2009

Dick Dievendorff wrote:
> I have tower specifications that describe a tower's windload capacity,
> which
> I take to mean the capacity if all the antenna were installed at exactly
> the
> top of the tower.
> However, I plan to install a couple of antennas on a mast that extends
> from
> the top of the tower.
> Is there a basic calculation that I could perform (say, with some Excel
> spread sheet formulae) that would help me plan the tower capacity?
> I have the windload numbers for each of two antennas.

There is no general formula for this.  Different towers will be
affected differently.  You would have to get the UBC calcs for
your tower, and then modify them for the extra loading.  Having
analyzed UBC calcs for the various towers I own, I can tell you
that each would be affected differently, depending on where the
weak point is.  You might think it would be at the bottom, but
on my HDX-5106 it is in the middle.  It also depends on the
relative contributions of the wind load on the tower itself
compared to the antenna load.  Some towers can barely hold
themselves up, like my MA-550.

I found that by reading the calcs and understanding them, I was
able to modify the calcs for special cases.  It's not rocket
science.  (I am an electrical, not structural, engineer).
I did my own calcs for a building permit that was issued,
even though I am not a registered engineer.  I guess the bldg
dept thought I sounded like I knew what I was doing.

Rick N6RK

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