Leah I need to talk to you before you go any further. Please call me at 972
1060. Thanks Van
----- Original Message -----
From: <towertalk-request@contesting.com>
To: <towertalk@contesting.com>
Sent: Monday, April 11, 2005 5:43 PM
Subject: TowerTalk Digest, Vol 28, Issue 54
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> Today's Topics:
>
> 1. Re: Guying a self-supporting tower - Yes (Doug Renwick)
> 2. looking for a very lightweight telescoping mast -- preferably
> motorized (wayne burdick)
> 3. Re: Guying a self-supporting tower - Yes (Jim Lux)
> 4. Re: Real numbers for Rohn BX-64 Re: guying (Steve Maki)
> 5. Re: TowerTalk Digest, Vol 28, Issue 51 (George Lee)
> 6. Re: Real numbers for Rohn BX-64 (Chris Pedder)
> 7. Re: Real numbers for Rohn BX-64 (Jim Lux)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Mon, 11 Apr 2005 11:55:04 -0600
> From: Doug Renwick <ve5ra@sasktel.net>
> Subject: Re: [TowerTalk] Guying a self-supporting tower - Yes
> To: Steve Maki <steve@oakcom.com>, towertalk@contesting.com
> Message-ID: <425AB9F8.1060905@sasktel.net>
> Content-Type: text/plain; charset=us-ascii; format=flowed
>
>
> Well said Steve. It has been reassuring to see a number of individuals,
> including yourself, privately and on this reflector, challenge this
> myth that sensible guying of a three legged self supporting tower is
wrong.
> For me this thread has become more of a study of human behaviour than
> about guying towers. You see, I have first hand experience with the
> successful guying of such towers and I am not prepared to remove those
> guys...than would just be stupid.
> Unfortunately there are still those who stick with the myth for their
> own convoluted reasons, and will continue to try and sway others.
>
> Let me end by quoting from Leo Tolstory:
>
> "I know that most men, including those who are at ease with problems
> of the greatest complexity, can seldom accept even the simplest and
> most obvious truth if it be such as would oblige them to admit the
> falsity of conclusions which they had delighted in explaining to
> colleagues, which they had proudly taught to others, and which they
> had woven, thread by thread, into the fabric of their lives."
>
>
> Steve Maki wrote:
>
> >
> > I agree that it's a shame common sense is so distrusted nowadays...
> >
> > Since I was the one who led it astray (don't they all go astray?) - my
> > point is that unless I'm mistaken, adding a sensible guy system to a
> > small self support tower will generally increase load capability by a
> > large amount - keeping in mind long boom twisting issues of course.
> >
> > So the oft repeated flat out statements that self supporters are
> > compromised by guys should not be left unchallenged because it's
> > a wrong headed thing to say.
> >
> > You may argue on the insurance and legal issues all you want,
> > I'm only talking about the strength of the tower.
> >
> > Steve K8LX
>
> --
> Doug Renwick VA5DX
> PO Box 50, Clavet, Saskatchewan, Canada S0K 0Y0
> VE5RA@sasktel.net
> First VE5 5BDXCC, 160M DXCC & 9BDXCC
> What profit it a ham if he gains all the awards yet forfeits his soul?
> I'll run the race and I will never be the same again.
>
>
>
> ------------------------------
>
> Message: 2
> Date: Mon, 11 Apr 2005 09:22:05 -0700
> From: wayne burdick <n6kr@elecraft.com>
> Subject: [TowerTalk] looking for a very lightweight telescoping mast
> -- preferably motorized
> To: towertalk@contesting.com
> Message-ID: <22eaa7c86be942a41f4b4bed130ae640@elecraft.com>
> Content-Type: text/plain; charset=US-ASCII; format=flowed
>
> Hi all,
>
> I'm hoping to put up a compact, lightweight rotatable dipole or
> 2-element yagi for 20-10 meters. This may not be a very efficient
> antenna, at least on the lower bands, but I have limited space
> available. The antenna may be home-brew and will weigh just a couple of
> pounds.
>
> I'd like to keep the antenna at roof level except when I'm actually
> using it, thereby minimizing scrutiny by Neighborhood Aesthetics
> Monitors. This calls for a 20 to 30 foot telescoping mast made of
> small-diameter tubing. If it's motorized, and goes up quietly, so much
> the better! I can guy it at the top if necessary, and it will be
> clamped to the side of the house.
>
> Anyone have suggestions on where to find such a mast?
>
> Thanks,
> Wayne
> N6KR
>
>
> ---
>
> http://www.elecraft.com
>
>
>
> ------------------------------
>
> Message: 3
> Date: Mon, 11 Apr 2005 11:38:59 -0700
> From: Jim Lux <jimlux@earthlink.net>
> Subject: Re: [TowerTalk] Guying a self-supporting tower - Yes
> To: ve5ra@sasktel.net, Steve Maki <steve@oakcom.com>,
> towertalk@contesting.com
> Message-ID: <6.1.1.1.2.20050411112845.02709588@mail.earthlink.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> At 10:55 AM 4/11/2005, Doug Renwick wrote:
>
> >Well said Steve. It has been reassuring to see a number of individuals,
> >including yourself, privately and on this reflector, challenge this
> >myth that sensible guying of a three legged self supporting tower is
wrong.
> >For me this thread has become more of a study of human behaviour than
> >about guying towers. You see, I have first hand experience with the
> >successful guying of such towers and I am not prepared to remove those
> >guys...than would just be stupid.
> >Unfortunately there are still those who stick with the myth for their
> >own convoluted reasons, and will continue to try and sway others.
>
>
>
>
> So, with respect to your sensible guying scheme..
>
> Are you loading the tower with more load than the mfr recommended for the
> freestanding case?
>
> What kind of tower, what guying strategy?
>
>
>
>
> ------------------------------
>
> Message: 4
> Date: Mon, 11 Apr 2005 15:45:24 -0400
> From: Steve Maki <steve@oakcom.com>
> Subject: Re: [TowerTalk] Real numbers for Rohn BX-64 Re: guying
> To: towertalk <towertalk@contesting.com>
> Message-ID: <425AD3D4.9080205@oakcom.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Jim,
>
> Thanks for going through those calcs, it's much appreciated.
>
> Just a couple of questions:
>
> Why did you choose 60% guy anchors instead of the commonly
> recommended 80%? I agree though that 60% is probably in that
> "sensible" range, but just barely.
>
> I'm not quite sure how you so quickly came up with a number
> for base bending moment in the guyed configuration. It seems
> unlikely to me that it can be even 1/4 of the unguyed number,
> unless the guys were left slack.
>
> But even so, 5700 lbs compression vs. 19,000 compression
> is quite an improvement, don't you think?
>
> I do note that you're still worried about increased stress on
> on the middle of the tower (column buckling). Are you speaking
> of straight downward force collapsing the middle of the tower?
>
> Or a bend, where the downward force helps it along once it gets
> started?
>
> Steve K8LX
>
> Jim Lux wrote:
> > At 09:16 AM 4/11/2005, Steve Maki wrote:
> >
> >>Jim Jarvis wrote:
> >>
> >>
> >>>THAT SAID, a few short comments:
> >>
> >>
> >>Since I was the one who led it astray (don't they all go astray?) - my
> >>point is that unless I'm mistaken, adding a sensible guy system to a
> >>small self support tower will generally increase load capability by a
> >>large amount - keeping in mind long boom twisting issues of course.
> >>
> >>So the oft repeated flat out statements that self supporters are
> >>compromised by guys should not be left unchallenged because it's
> >>a wrong headed thing to say.
> >>
> >>You may argue on the insurance and legal issues all you want,
> >>I'm only talking about the strength of the tower.
> >>
> >>Steve K8LX
> >
> >
> >
> > OK... some real numbers.. off the Rohn BX data sheets which I found on
the
> > web. Whether or not this is a tower anyone would actually use, or
> > contemplate guying is sort of immaterial. It's representative...
> >
> > First off.. the limiting load on the actual lattice work sections is
> > determined by.... drum roll.... buckling failure of the vertical members
> > within the lattice. These are about 12.5" long, and have a radius of
> > gyration of 0.505 inches (for the bottom section), for a slenderness
ratio
> > of 24.8.
> >
> > The allowable leg load for this tower section (BX-8) is 20,250 pounds.
To
> > compare to straight compression failure, the cross sectional area is
.6043
> > square inches, the material is spec'd as 45,000 psi yield cold rolled
steel
> > for a compressive failure limit of 27,194 pounds. That's almost 50%
more
> > than the failure load due to buckling.
> >
> > So, raw compression strength of the steel is NOT the issue here.
> >
> >
> > Now... let's look at some guying strategies...
> >
> > Free standing, with the nominal 6 square ft antenna and 20psf wind on a
BX-64..
> >
> > The total load in the leg is 19,260 pounds (fairly close to the max
> > allowable, eh?), of which 19,100 pounds is the wind load and 160 pounds
is
> > the weight of the structure.
> > (37,770 ft pound moment, 1.978 ft moment arm at base).
> >
> >
> > Now, let's add three guys...
> > Let's say we use 3/16" EHS as the guy material, and put the guy anchors
60%
> > the tower height out. (so the angle between guy and tower is 30
> > degrees...). The guys will be about 74 ft long (64/cos(30))
> >
> > The breaking strength is about 4000 pounds, it weighs 73 pounds/1000 ft.
> > Each guy will weigh about 5-6 pounds, so we can probably neglect the guy
> > weight. (it's small compared to 19,000 pounds)
> >
> > Now, it's been oft asserted that the guys should be tensioned to 10% of
> > breaking strength. Sure, in this case, where you are using the guys as
> > "safety cables" you might tension them less, but let's start there...
The
> > tension is, therefore, 400 pounds, per cable.
> >
> > The additional downforce is now 400*3*.866 pounds or about 1040 pounds.
In
> > the no wind situation, that's no big deal. To the tower, it's just like
it
> > weighs 1500 pounds instead of 476, and divided over the three legs,
> > compared to the leg max allowable of 20,250, it's pretty small.
> >
> > Now, let's look at putting some wind load on the system. It's a tapered
> > tower, and the calculations would be somewhat involved (because it's
really
> > an elastic system, both the tower and the guys would stretch, etc.)
> >
> > However, we'll approximate, using Rohn's moment calculation of 37,770 ft
> > pounds. At the top of the 64 ft tower, this is about 590 pounds. So,
> > assuming the wind comes from the direction of the guy, the guy is going
to
> > have to resist the 590 pounds.. assuming no deflection (so the angles
> > remain the same) requiring a tension of 590/sin(30) = 1180 pounds.
That's
> > an increased downforce of about 1400 pounds, for a total of 2440
> > pounds. Yes, it won't be quite that much, because the downwind guys
will
> > relax a bit, etc.
> >
> > -------------------
> >
> > Some load will still be transmitted as a bending moment to the base,
> > loading the downwind leg. Whipping out the handbook (always dangerous,
but
> > it will give us a rough idea...)
> >
> > For a uniformly loaded canteliever beam, the maximum moment is at the
base,
> > and is 1/2 w*l^2 (w is the load per unit length)... conveniently, Rohn
has
> > calculated this for us, and it's the 37,770 ft lbs.
> >
> > Now lets assume that it's uniformly loaded, and the base is fixed, but
the
> > (top free end) is supported by a reaction force (from the guys). The
peak
> > moment is still at the base, and is 1/8 w*l^2, or one quarter of the
> > unsupported case. That's about 9440 ft lbs. Using the same 1.987 ft
> > baseline, that's a vertical load (on one leg) at the bottom of the
section
> > of 4752 lbs. Plus 158 pounds for the weight of the tower, plus about 800
> > pounds for the resultant of the guys. We're up to around 5700 lbs.
> >
> > One would need to go through this for all the sections (because the max
> > allowable for the sections varies from 5300 at the top to 20,250 at the
> > bottom).
> >
> > So, if it were a uniform load (which it's not) and if the tower were of
> > uniform stiffness (which it is not), it looks superficially safe. At
least
> > at the rated capacity...
> >
> >
> > Of course, if you're just going to load it with the rated capacity, why
> > bother putting guys on it. That tower is only rated with 6 square feet
at
> > 20 psf (70 mi/hr). If you think that by putting guys on it, you can
safely
> > handle 10 square feet at 90 mi/hr, you're dreaming... That's 2.5 times
the
> > wind load at the top... what was before a moment from the antenna of
8000
> > ft pounds is now 20,000 ftlb. And, your guys are going to be exerting a
> > downforce of about 4500 pounds.
> >
> > All bets are off for a casual analysis.
> >
> > -----
> >
> > All this foregoing analysis makes the fatal assumption that the
structure
> > is a rigid body (which is what all those guy and tower compression
> > calculations assume), but it is NOT. It will flex.. Different parts
are
> > going to bend differently, and the stresses will divide in a way not
> > intended by the mfr. If you look at the chart of expected loads vs the
> > strength, there's not a huge margin anywhere in the system (it's a well
> > balanced design from that standpoint.) The allowable loads at the
splices
> > neatly match the allowable loads on the vertical members at that
> > joint. The load on the leg is typically about 90-95% of the allowable
> > load on the leg.
> >
> > This isn't some sort of design where you're working at 20-30% of the
> > ultimate capacity. Even with a simple rigid body analysis, you're
pretty
> > close to the edge. I haven't even looked at the loads on the diagonal
> > braces (those look like they are loaded about about 30% of allowable),
or
> > the effect of torsion (clearly this is important, given the big warning
> > about avoiding antennas with large twisting moments at the top of the
sheet).
> >
> > Torsion raises the specter of eccentric loads on the vertical
> > legs. Eccentric loads on columns dramatically reduce the maximum
allowable
> > load. The existing design assumes that the legs are loaded symmetrically
> > and axially... Let this be a warning to those who would hang something
off
> > a leg (or bend the rolled profile or drill holes )that violates that
> > assumption... there's not a lot of design margin.
> >
> > I suspect that if you start digging into the analysis, and actually
looking
> > at the distribution of loads as it flexes and is restrained by the guys,
> > you'll find a serious column buckling problem in the middle.
> >
> >
> >
> >
> > ----
> >
> > This analysis is truly a back of the envelope calculation... anyone is
> > welcome to refine it, correct it, or provide counter examples...
> >
> >
> > Jim, W6RMK
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > _______________________________________________
> >
> > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> ------------------------------
>
> Message: 5
> Date: Mon, 11 Apr 2005 10:13:21 -0500
> From: "George Lee" <geokr5c@cablelynx.com>
> Subject: Re: [TowerTalk] TowerTalk Digest, Vol 28, Issue 51
> To: <towertalk@contesting.com>
> Message-ID: <001201c53ea9$00520ea0$d30dffce@toshibauser>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
> Thank you Tom.
>
> Hope this completes guying a freestanding tower.
>
> George KR5C
>
>
> "The taking of privileges away from those hams who qualified for them is
> just about as outrageous as is the giving away of privileges to those hams
> that have not qualified for them"
> John B. Johnston former Chief, Amateur Division of the Federal
> Communications Commission. From
> WorldRadio March 2005 issue.
>
>
>
>
> ------------------------------
>
> Message: 6
> Date: Mon, 11 Apr 2005 21:21:30 +0100
> From: Chris Pedder <chris@g3vbl.co.uk>
> Subject: Re: [TowerTalk] Real numbers for Rohn BX-64
> To: towertalk@contesting.com
> Message-ID: <6.2.1.2.2.20050411210719.0200efe8@mail.plus.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> At 18:54 11/04/2005, Jim Lux wrote:
>
> >However, we'll approximate, using Rohn's moment calculation of 37,770 ft
> >pounds. At the top of the 64 ft tower, this is about 590 pounds. So,
> >assuming the wind comes from the direction of the guy, the guy is going
to
> >have to resist the 590 pounds.. assuming no deflection (so the angles
> >remain the same) requiring a tension of 590/sin(30) = 1180 pounds.
That's
> >an increased downforce of about 1400 pounds, for a total of 2440
> >pounds. Yes, it won't be quite that much, because the downwind guys will
> >relax a bit, etc.
>
> Excuse me, the tower is rated for 120lb (sic) headload and you want to
> apply 590 lb, albeit opposing the 120lb on the antenna? Do you think this
> is a sensible strategy?
>
> Chris
>
>
>
>
>
> ------------------------------
>
> Message: 7
> Date: Mon, 11 Apr 2005 14:43:14 -0700
> From: Jim Lux <jimlux@earthlink.net>
> Subject: Re: [TowerTalk] Real numbers for Rohn BX-64
> To: Chris Pedder <chris@g3vbl.co.uk>, towertalk@contesting.com
> Message-ID: <6.1.1.1.2.20050411142938.02708658@mail.earthlink.net>
> Content-Type: text/plain; charset="us-ascii"; format=flowed
>
> At 01:21 PM 4/11/2005, Chris Pedder wrote:
> >At 18:54 11/04/2005, Jim Lux wrote:
> >
> > >However, we'll approximate, using Rohn's moment calculation of 37,770
ft
> > >pounds. At the top of the 64 ft tower, this is about 590 pounds. So,
> > >assuming the wind comes from the direction of the guy, the guy is going
to
> > >have to resist the 590 pounds.. assuming no deflection (so the angles
> > >remain the same) requiring a tension of 590/sin(30) = 1180 pounds.
That's
> > >an increased downforce of about 1400 pounds, for a total of 2440
> > >pounds. Yes, it won't be quite that much, because the downwind guys
will
> > >relax a bit, etc.
> >
> >Excuse me, the tower is rated for 120lb (sic) headload and you want to
> >apply 590 lb, albeit opposing the 120lb on the antenna? Do you think this
> >is a sensible strategy?
>
>
> I'm not applying the 590 lb... that's the tower applying the load on the
> guy from the wind. On that tower, the wind load on the tower (excluding
> the antenna) is about 1050 pounds, spread out over the 60 odd feet.. a
LOT
> more than the little 120 pound load from the antenna. Rohn's calculations
> show anywhere from 9 to 5 square feet per segment of the tower as you go
up.
>
> One could make a simplifying assumption that the tower can be considered
as
> a rigid beam supported between two supports, so half the wind load appears
> at the bottom and half appears at the top guy (i.e. about 525 pounds on
> each).. Then you'd add the load from the antenna (120 lbs) at the top.. If
> you do this (which would be typical for a standard uniform section guyed
> tower like Rohn 25/45/55) then the load at the guy is actually higher than
> I used (525+120 = 645 lbs) with only 525 shear load at the base.
>
> This would be the load if the guy were perfectly slack before the wind
came
> up, by the way. You've also got to add in the preload on the guy in any
> real calculation.
>
> Well.. since you ask, no I don't think it's sensible, but then, the
purpose
> of the exercise is to show that there isn't necessarily ANY sensible
> configuration that's trivial to analyze.
>
>
>
> >Chris
>
>
>
>
> ------------------------------
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> End of TowerTalk Digest, Vol 28, Issue 54
> *****************************************
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
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