Correction to my math: 700 lbs x 8 ft = 5600 ft-lbs; but you get the idea.
Thanks for the nice comments off-line.
Bob, W5LT
-----Original Message-----
From: W5LT [mailto:W5LT@verizon.net]
Sent: Thursday, August 14, 2008 6:33 PM
To: donovanf@starpower.net; towertalk@contesting.com
Subject: Re: [TowerTalk] Heights Towers Aluminum??
There must have been more at work here than the marginal addition of some
Heliax. I designed the fold-over mechanism for the Heights new tower series
a few years ago, in accordance with the Aluminum Association Specifications
and the Uniform Building Code, which include industry accepted design safety
margins. I am assuming the tower that failed was one of the newer design
(post 1995) units. The pre-1995 units were substantially over rated.
The tower sections have two parameters: a moment rating and a shear rating.
Both must be considered in the tower design for wind and also the tilt-over
case. For a typical Yagi antenna and associated rotor & mast, the forces
generated during fold-over are much less than (typically around 1/2) those
that would be experienced during a rated wind load event. For example, my
HTS tower is rated for 36 sq-ft projected area at 80 ft in my 70mph wind
zone, which yields a wind pressure at the top of the tower of about 700lbs.
That force generates a shear force at the apex the top section of that
magnitude, and a moment of 700 lbs x 8 ft = 2800 ft-lbs at the bottom of the
section. The top tower section must be chosen to carry those loads to the
next lower section, in addition to the loads from the wind area (or
self-weight) of the section itself. As the design progresses down the tower,
both the total moment and shear load grow and each tower section must be
selected to carry both. The fold-over case loads are derived similarly.
Analysis of the fold-over case for my tower shows that the shear and moment
loads are well balanced, that is, the actual load to the maximum design load
ratios are about equal for each section, and are less than half the
corresponding section rating.
It would be very unusual for a properly designed tower supporting its rated
wind load to fail in shear during the folding operation. The only way I can
think that could happen is that, as Frank suggests, the user added some low
wind resistance dead weight to the tower that exceeded the shear rating by a
significant margin. Perhaps a several hundred pound lead ball at the top?
The engineering ratings of the Heights tower sections are provided on their
website, along with the methods to engineer you own tower configuration.
73s
Bob, W5LT
-----Original Message-----
From: donovanf@starpower.net [mailto:donovanf@starpower.net]
Sent: Thursday, August 14, 2008 9:11 AM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Heights Towers Aluminum??
Don't forget that you must comply with the dead weight capacity of your
tilt-over tower. This is important, a nearby ham failed to comply with the
dead weight specification and his Heights fold-over tower collapsed as he
tilted it up. His mistake? He forgot to count the weight of his Heliax
feedlines!
Everything you add to your tower must be counted against the dead weight
specification, including all cables, masts, rotators and other accessories.
Give serious consideration to using an aluminum mast and light weight
antennas, feedlines and control cables.
Its especially important to minimize every pound of weight above the top of
the tower (i.e., mast and antennas and feedlines). Every pound above the
top of the tower adds very significantly to the moment on the lower tower
sections and the tilting base.
73
Frank
W3LPL
---- Original message ----
>Date: Thu, 14 Aug 2008 5:11:25 -0700
>From: Dennis <radioart@charter.net>
>Subject: Re: [TowerTalk] Heights Towers Aluminum??
>To: W3OA <w3oa@roadrunner.com>, towertalk@contesting.com
>
>Thanks for taking the time to respond Dick.
>
>Very good input on some of the issues I would be faced with. How do you
rotate the antenna at 40' or is it stationaly? Isn't there some kind of
ring assembly for rotating antennas on the sides of towers?
>
>Thanks Dick, Dennis, k0eoo
>
>
>---- W3OA <w3oa@roadrunner.com> wrote:
>> Hi Dennis -
>>
>> I have a 60' Heights tower with a fold-over kit and 10 feet of mast out
>> the top.
>>
>> It's been up for 5 years now and I'm very happy with it.
>>
>> You do need to watch how big an antenna you put on it. If the antenna
>> is too large you won't be able to lower the tower far enough to work on
>> most of the antenna from the ground. I suppose that's obvious but it's
>> a trade I thought about for a long time before I decide to get a
>> fold-over tower. I ended up with two Force 12 C-4s, one at 40 feet and
>> one at 70 feet. I can reach the feed points using an 8 foot step ladder.
>>
>> Another gotcha I found out through experience is that rotor brakes that
>> hold an antenna just fine when the tower is up may not hold the antenna
>> in position as the tower rotates down. The antenna, of course, wants to
>> rotate so its heavy end is pointed down and the break may not hold it in
>> any other position. This can be an even bigger problem with side
>> mounted antennas.
>>
>> I'd do it again 'cause I don't want to climb a tower.
>>
>> Good Luck - Dick, W3OA
>>
>> Message: 6
>> Date: Mon, 11 Aug 2008 20:46:23 -0500
>> From: "Dennis Petrich" <radioart@charter.net>
>> Subject: [TowerTalk] Heights Towers Aluminum??
>> To: <towertalk@contesting.com>
>> Message-ID: <A577548AEF344F668DEB831F901180B9@D5V7GZF1>
>> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>> reply-type=original
>>
>> Hello all who have aluminum towers.
>>
>> I am thinking of putting up the tilt-over self supporting aluminum tower
>> from Heights Towers. This one is 69' with the tilt-over kit and is rated
at
>> 21sq' at 80mph. and will take a 12' mast. I will be putting up 15sq' of
>> antenna.
>>
>> Anyone of you have experence with such a thing?? The whole tower tilts
over
>> so you can work on the antenna from the ground, an idea I like very much.
>> You can see it on their web page.
>> http://www.heightstowers.com/fold_over_kits.htm
>>
>> Thanks in advance for your comments
>>
>> Dennis, k0eoo
>>
>> >
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk@contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
>
>_______________________________________________
>
>
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|