[TowerTalk] FW: engineering help? Antenna weight for US Tower TX472

[Matt]

Just a word of caution regarding lowering crank-up towers (increasing
overlap) to reduce bending stress...   It is true that lowering the tower
decreases the overall moment reactions on the tower from the wind load,
however that strategy does not consider the stresses on individual tower
elements.   There are some tower designs that have extra horizontal
reinforcing bars placed in the vicinity of the pressure points of the
overlapping tower sections when fully extended.   When the tower is
partially lowered, these horizontal bars no longer line up with the pressure
points and the moment forces are transferred into a region of the truss
without the extra strengthening that the horizontal bars provide.   That is
OK if the tower is lowered enough so that the developed moment reactions do
not exceed the strength of the tower section "at that point".   The
calculation of those localized stresses is not trivial.

A case in point would be to only lower the tower a little, where in it is
probable that the reduction in stress fails to offset the increase in
localized stress.   Conversely, if the tower design does not incorporate
these horizontal strengthen members, then the point is moot.

Matt
KM5VI

-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
Patrick Greenlee
Sent: Thursday, June 11, 2015 1:20 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] engineering help? Antenna weight for US Tower TX472

Rick, I think you hit the nail squarely on the head. As the overlap goes to
its minimum the leverage of the extended tower portion vs the overlapped
region's leverage begins to bind the overlapped portion with side load.
Some users of the DB42 add on aerodynamic winglets to balance the center of
effort (or center of lateral resistance or drag, if you prefer) so that
winds do not create a torque around the vertical axis.

I wonder how much weight, strategically located and fastened so as to not
change electrical characteristics it would take to make the antenna
"neutral?"

I have a DB42 on my shop floor in 4 boxes awaiting it's turn on my priority
list.  Its going on a Tashjian tower with a dead weight carrying capacity of
550 lbs.  I too estimate the weight of the load at
300 lbs or so without cables and coax.  I haven't gotten past the musing
stage but it looks like there is plenty of margin to do some
counterbalancing if I have stiction issues with the raising or lowering.  It
is power down with a 1/2 horse power motor via a 40:1 so it will come down,
smoothly or otherwise.

Patrick   NJ5G


On 6/11/2015 12:36 PM, Richard (Rick) Karlquist wrote
>
>
> On 6/11/2015 10:07 AM, kr2q at optimum.net wrote:
>> Hi all,
>>
>
>> 2.  What point is the weak link for the 200 pounds?
>>
>
> Here is the surprising thing I ran into with my HDX-5106, a MUCH 
> bigger tower.  It has a MonstIR on top, figure
> 250 lbs.  Also a short mast and a prop pitch, figure another 50 lbs.  
> Figure another 100 lbs for cables, so lets call that 300 lbs.  Should 
> be fine.
>
> What happens is that when I get within about 5 feet of full extension, 
> I start to get noise and vibrations.
> The working theory is that this is due to IMBALANCE of weight, rather 
> than just the weight itself.  The sections start to bind a little when 
> the overlap gets small.
>
> I removed the antenna to change the cables in the tower, and ran it up 
> and down a few times, and the problem went away under those 
> conditions. The MonstIR has a very flexible boom that exascerbates the 
> imbalance problem.  Once it starts to lean a little, the boom flexes 
> in the direction that makes it worse.
>
> What I am doing as a workaround is manually stopping the raising winch 
> just before the problem begins.
> I know from experience where this point is.
>
> Rick N6RK
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