I have a TriEx Catalog from 1979 that shows the MW-65 as a tower that can
support a maximum of 5 square feet, two feet above the top of the tower,
at 50 MPH. I can scan the page and send it to you if you doubt it . . .
I would not trust this tower to hold ITSELF up with NO antennas on it at
70 MPH.
I also have, in the same catalog, an EIA wind chart that says the wind
pressure is 10 pounds per square foot at 50 MPH and nearly twice that much
(19.6 pounds per square foot) at 70 MPH.
Again, in the same catalog, is a UBC wind chart that says the wind
pressure is 8.26 pounds per square foot at 50 MPH and 16.20 pounds per
square foot at 70 MPH.
This is old data but how wrong can it be? The numbers are certainly
similar in size on both charts and the other important point is that the
wind pressure is close to twice as much at 70 MPH than it is at 50 MPH on
both charts.
It is tempting to believe that since the wind pressure is twice as much at
70 MPH as it is at 50 MPH, the tower will safely hold half as much antenna
at 70 as it will at 50. DON'T BELIEVE IT! The allowable antenna load
number at 70 MPH is much LESS than half of the 50 MPH number (it may even
be less than ZERO) and here is why:
The total amount of wind load a free-standing tower can take before
blowing over is equal to the wind load imparted by the antenna (rotator,
feedlines, etc) plus the wind load of the bare tower ITSELF. As the wind
speed increases, the load imparted by the antenna (rotator, feedlines,
etc) increases BUT SO DOES THE LOAD OF THE BARE TOWER. Certainly, you can
see that there is some wind speed where the load of the bare TOWER ALONE
is more than it can take and it will fall over. Up until that point, as
you increase the wind speed above 50 MPH, you have to decrease the wind
load allowed for antennas to make up for the increased wind load of the
bare tower itself. At some point, you run out of antenna load to take
away and this is where the bare tower is not safe with NO antennas on it .
. .
To really figure this out, you need to know what the wind load of the
tower itself is and what part of the total allowable wind load is
attributed to the tower and what part is attributed to the antennas. I
don't recall ever seeing the wind load of a bare free-standing crankup put
in print by any manufacturer . . . Frankly, I don't think they like to
talk about this, and I think it is because it would put free-standing
crankups in a poor light, showing they really can't safely support nearly
as much antenna as people desparately want to believe they can . . .
Fire away . . .
Stan
w7ni@teleport.com
Geo. Clute wrote:
> Messr. mj may writes:
> |Does anyone know the wind loading for a TriEx MW-65 at
> |70mph? The pages
> |that I have found don't list the MW-65.
> |
> |Thanks in Advance
> |Mark
>
> The tower is rated at 9 Sq foot of antenna, but I'm not sure at what
> wind speed. Around here in my den of opressive archives the
> construction manual shows that information. When located I'll provide
> you with more accurate info.
>
> vy 73,
> George Clute - W7LFD
> ACS, Inc. - Mercer Island, WA
> gclute@home.com
--
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|