[TowerTalk] Fwd: Rohn RSL tower - slightly off topic
hanslg at aol.com
Mon Jun 17 10:25:16 EDT 2019
The problem at hand is not the bending force but the twisting force on the tower (Many times overlooked). You can guy the tower to death, but if there is a twisting force large enough, the tower will break apart and collapse. The guy wires give very little, if any, support for a twisting motion. The only way to "guy" for that is to install a torque bar on the tower. You see them frequently on latter cell towers. It's a bar or triangle mounted near the top of the tower and connected to pair guy wires. A twisting motion will then stretch one guy wire while slacking the other. It is used when you want improved azimuthal alignment.
There are issues whether you can improve the strength of an unguyed tower by adding guy wires. Guy wires are adding a downforce which adds to the already existing force from the loads. I did the analyzes on my Universal Tower so I know. I found out, in my specific case, that by adding one set of guy wires about 2/3 up increased the ability to withstand wind from 85 mph to 135 mph provided I don't add a lot of weight to it. A guy wire attached to the top would not improve it rather make it worse.
And this was my $.03 worth. 73 de,
Hans - N2JFS
From: terry burge <ki7m at comcast.net>
To: towertalk <towertalk at contesting.com>; terry burge <ki7m at comcast.net>
Sent: Thu, Jun 6, 2019 5:09 pm
Subject: Re: [TowerTalk] Rohn RSL tower - slightly off topic
Guys, I really got to wonder about this. It seems to me IF your antenna load on the tower is a concern, why not just add some guy wires. A guyed tower can handle the wind forces so much better and you can sleep well at night knowing that. I personally have wondered about the integrity of free standing towers over time. Sure, they may look nicer (maybe) and have a smaller footprint outside the concrete base, but knowing how us hams always want bigger and better antennas, why not just guy a tower to make sure it will stand up to the wind forces. Maybe won't handle a tornado but then what tower would?
Just my $.02 worth.
> On June 6, 2019 at 1:30 PM Steve Maki <lists at oakcom.org> wrote:
> Seems like if the wind is in-line with the booms, there will be a small
> amount of torque even if the antennas are perfectly wind balanced (if
> they share the same side of the mast).
> And that small torque will be somewhat canceled if they are on opposite
> sides of the mast.
> -Steve K8LX
> On 6/6/2019 4:11 PM, George Dubovsky wrote:
> > I'm with you, Chuck. The torque is additive regardless of which side of the
> > mast the antenna(s) are on.
> > On Thu, Jun 6, 2019 at 1:32 PM Chuck Dietz <w5prchuck at gmail.com> wrote:
> >> Maybe I’m dense, but I don’t see how this does anything (much). If the
> >> back of both antennas has a larger wind area than the front, the torque on
> >> the mast is going to twist the mast in the same direction no matter to
> >> which side it is attached.
> >> From: k7lxc--- via TowerTalk
> >> Sent: Thursday, June 6, 2019 11:38 AM
> >> To: towertalk at contesting.com; bjtatum1 at att.net
> >> Subject: Re: [TowerTalk] Rohn RSL tower - slightly off topic
> >>> Hello-? ? I was at a friend's QTH recently viewing his new Rohn
> >> RSL100L10, a 100' self supporter put together with 10' sections R-10H
> >> through R-1. It is rated for 25 ft/2 wind load at 90 MPH, dropping back to
> >> 11 ft2 at 100 MPH. We were discussing his planned installation of a KT-36
> >> and M2 3 ele 40 meter yagi. He wishes to have the KT-36 about 15' above the
> >> tower top plate with the 40 meter yagi approx. 1' or 2' above tower top.
> >> Both antennas are fairly close in weight (approx. 95#), boom length (36')
> >> and wind load (10 ft2). ? I wanted to get opinions of folks experienced
> >> with large antennas and self supporting towers if this planned installation
> >> is OK for this tower.
> >> Hiya, Byron --
> >> The proposed configuration sounds good to me but I'm not offering an
> >> answer to your question. What I am offering is a tower technique that I
> >> highly recommend.
> >> Install the antennas on opposite sides of the mast. That way many of
> >> the wind vectors cancel each other out. This was determined by Dick Weber,
> >> K5IU, a PE and the article appeared in QEX years ago. Anything you can do
> >> to reduce the wind induced torque on a mast is a good thing. Did I mention
> >> this is highly recommended?
> >> Cheers.Steve K7LXCTOWER TECH
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