I really don't understand why it is a subject so hard to understand.
A self-supporting tower will have certain forces on its legs from a wind
pressure. Supplied with guy wires the forces from the wind will be reduced but
new forces from the guy wires will be added.
Now, the idea of adding guy wires to enforce the tower require the down forces
from the guy wires to be smaller than the forces from the wind. It is only the
compressing forces that are effected as the guy wires only add compression
forces to the legs (unless you have some very, every unusual conditions :-).
I think it is easy to realize that the guy wire anchor points have to be on a
reasonable distance from the tower (around 80% or more). Further, the bias
strength on the guy wires have to be even (equal) in order not to bend the
tower over (That's what a strain gauge is for). The attachment point on the
tower (for the guy wires) has to add equal force to all the legs and not add
strain to the welding of the cross arms. (It was suggested to me to wrap the
guy wires around the crossing arms, thereby adding compression force to the
welds. A guy wire attached directly to a leg may rip these weldings apart.)
How would guy wires reduce the strength of a tower with these things in mind?
Yeah, any idiot could topple a tower with attached guy wires but let's put some
sense into out lives.
Best 73 de,
Hans - N2JFS
-----Original Message-----
From: Roger (K8RI) on TT <K8RI-on-TowerTalk@tm.net>
To: towertalk <towertalk@contesting.com>
Sent: Fri, Oct 17, 2014 1:29 am
Subject: Re: [TowerTalk] Guyed + self supporting /2 ??
On 10/16/2014 9:29 PM, Steve Maki wrote:
> No you didn't read it correctly Roger. His opinion was exactly as
> stated, that a guy system could be designed for a (existing) self
> supporter that would favorably impact the load rating. Maybe only a
> little, maybe more depending on lot's of variables. And yes it was
> just "off the cuff".
And I'm going to stick with a specif statement as what you have is an
opinion. "Never guy a self supporting tower unless you have the
installation engineered" or the manufacturer's blessings. You only have
to pull one of those legs a few inches out of line to severely weaken it
for buckling and a strong wind can push the structure away from the guy
far enough to do that.
Houses, towers, and specifically self supporting towers move in the
wind. Look at the flashing around a chimney on a windy day. It's not
unusual to see an inch or two of house movement on a single story ranch.
I do have to admit, there are many "self supporting" Aluminum towers out
there that make me question their use of the definition.
You are welcome to do as you wish. I'd not want a guyed self supporter,
but my climbing days are over. Two strokes and a heart attack ended my
flying and climbing. Both strokes came with no warning as did the heart
attack.
I exercised, ate right, rode bike (10 miles at 20 MPH which is fast, but
it took years to get there) and one day I hung up the telephone, took a
step and my left leg went out like I'd stepped on ice. Never did get
the use of that ankle back.
73
Roger (K8RI)
>
> My point was, which I though was obvious:
>
> To say that you "must not guy a self supporter" implies that in every
> case, the tower will be weakened by the addition of a guy system. That
> is simply not true.
>
> If that is not what was meant, then it was misstated. Consider this an
> academic exercise if you can.
>
> That's all I'm going to say on this until I can call in a favor and
> get someone with a modeling program to run some reasonable *what if*
> case studies.
>
> -Steve K8LX
>
> On 10/16/2014 8:33 PM, Roger (K8RI) on TT wrote:
>
>> On 10/16/2014 7:02 AM, Steve Maki wrote:
>>
>> I see this as a safety issue and doing things differently than the
>> manufacturers design and limits is risky, even if they do design in a
>> substantial CYA safety factor. Changes can cost your insurance.
>>
>> Sorry, I can not accept what one engineer says "off the cuff". If I read
>> it correctly he did say the tower had to be designed that way and you
>> talked about it being silly to design one that way. So, which is it?
>> Just hooking guys to a tower (to add strength) could just as easily
>> reduce the strength unless the installation were properly engineered.
>> Even if as he says, you could add guys, would there be any net gain in
>> strength unless the tower were designed that way to begin with? If
>> there was a net gain, would it be enough to be useful. The steel angle
>> used in most self supporters is not designed to withstand a pull on it's
>> own. We'd need a band around the tower at that point.
>>
>> The purpose of guys is to add strength and stability. It's far better
>> and safer to start with the proper tower system to support what you plan
>> on, rather than beef any tower beyond its design limits. There would be
>> no other valid reason for adding guys. Reducing the base is just another
>> way of fiddling with the design limits. I much prefer to use towers
>> within their design limits.
>>
>> For any of us to just add guys to a self supporter is not smart and you
>> both said as much, but we shouldn't make statements that are likely to
>> encourage hams (without an engineering background) to go ahead and just
>> add guys. It puts them in the area of research and could likely void
>> their insurance.
>>
>> In the past, I've experimented with towers and antennas, knowing full
>> well if it faile my insurance would likely not cover it.
>>
>> Most ham, self supporters are crankups. There are a few exceptions.
>> http://www.rogerhalstead.com/ham_files/skyhook.htm but that is one that
>> took a lot of engineering and planning. The guys in the engineering
>> department donated a lot of time on that one. There are 3 semi loads of
>> steel in that tower (and a lot of money)
>>
>> 73
>>
>> Roger
>>
>>
>>> On 10/16/2014 2:49 AM, Roger (K8RI) on TT wrote:
>>>>
>>>>> After much repeated discussion on this forum over the years I asked a
>>>>> structural engineer about this, and he confirmed what you are
>>>>> saying -
>>>>> that basically a guy system could be designed for pretty much any
>>>>> *self supporter* to favorably affect it's load limits (within a
>>>>> reasonable footprint), but that it would be silly to start out
>>>>> designing one that way since it would be a waste of materials. If
>>>>> you're going to guy a tower, use a *guyed tower*.
>>>>>
>>>>> But when someone says *you MUST not guy a self supporter*, it
>>>>> makes me
>>>>> cringe.
>>>>
>>>> Why. A self supporter can be designed for guys, but most are not. If
>>>> they are not designed with the guy forces taken into account, you are
>>>> just gambling.
>>>
>>> Why? Because it's just not true to say that. Note that I'm not
>>> recommending anything, other than to speak accurately..
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
--
73
Roger (K8RI)
_______________________________________________
_______________________________________________
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
|