Gene
I am not sure what you mean by a "moment force." There is both a moment and
a force involved. The force is the combination of all the arrays with the
wind load calculated. It is applied at the top of the tower and creates a
side loading on the top of the tower. There is also a moment. It is the
summation of all the moments applied to the mast and again it is applied at
the top of the tower. If the antennas hanging down are rigidly mounted to
a horizontal mast the moment will be the wind loading times the distance to
the top of the tower (read radial bearing). That moment will be in the
opposite direction of the moment from the antennas above the top of the
tower. Hence the total moment applied to the top of the tower will be
expected to be less than with the antennas above the top alone at least for
a static loading (Who knows what happens dynamically). It is hard to guess
how the combinaiton will affect the tower. Ordinarily I would prefer a
static horizontal force at the top of the tower rather than a static
moment, at least from the buckling point of view. The deflections can be
simulated broadly using superposition but it must be noted that the vertical
loads applied by the guys are not linear in the buckling mode so the
combination becomes more complicated. Still if the combination of wind
loads are less than the tower allowables, it is worth a try since the
applied moment may well be less than with the antennas above the tower
alone.
I certainly agree with you other comments though.
73
Bill W7VP
----- Original Message -----
From: <ersmar@comcast.net>
To: "Doug Rehman" <rehman@surveil.com>; "'Towertalk'"
<towertalk@contesting.com>
Sent: Sunday, June 04, 2006 3:52 PM
Subject: Re: [TowerTalk] Downward Mast Extensions
> Doug:
>
> Couple of comments:
>
> 1. Make sure your rotator will handle the added torque/twisting load.
> Assuming everything on this mast will be a rigid connection (no loose
> clamps to let the auxiliary masts pivot around the boom), then any wind
> load will be applied four feet from either end of the main mast (at the
> outer ends of your horizontal boom.) Could be a significant lever arm
> force.
>
> This force might not be balanced on either side of the tower if the
> wind comes in at the tower from a direction that shields one or the other
> down-hanging array. In other words, there could be a net torque applied
> to the main mast and rotator.
>
> 2. Wind force will be applied to the top of the tower just as if the
> down-hanging masts had been installed upward instead. That is, if the
> wind were to blow straight into the assemblage of antennas on this
> mast/boom combination, your tower top would be subject to the total moment
> force from all the antennas' areas, at whatever magnitude of moment they
> generate, given their mounting distances from the attachment points to the
> top of the tower.
>
> BOTTOM LINE: No such thing as a free lunch when it comes to wind loading.
>
> 73 de
> Gene Smar AD3F
>
>
> -------------- Original message ----------------------
> From: "Doug Rehman" <rehman@surveil.com>
>>
>> I'm in the conceptual stage of adding a few antennas to my tower. At
>> present, the 24' of mast above the tower is pretty well filled with
>> antennas
>> from 7 MHz to 1296 MHz. I plan on adding a few more UHF and microwave
>> antennas.
>>
>> What I am contemplating is attaching a cross arm on the mast just above
>> the
>> thrust bearing. The cross arm will probably be about 8' long. On each end
>> of
>> the cross arm, a 10' mast will be attached downward. This will allow me
>> to
>> rotate the antennas 360 degrees with the other antennas and to retract
>> the
>> tower. Here's a poor ASCII art idea of what I'm talking about ("U"
>> represents the tower itself):
>>
>> |
>> |
>> |
>> |
>> __|__
>> | | |
>> | U |
>> U
>> U
>> U
>> U
>>
>> Has anybody ever done this before or seen it done?
>>
>> Would the wind loading of the antennas on the "down hanging" masts
>> actually
>> offset against the wind loading of the antennas on the main mast? (This
>> presumes that the antennas are somewhat balanced on the two down hanging
>> masts to avoid adding an unbalanced lateral load on the main mast.)
>>
>> Thoughts, comments, criticisms?
>>
>> Tnx,
>> Doug
>> K4DDR
>>
>>
>> _______________________________________________
>>
>>
>>
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>
>
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