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Author Topic: Mechanical Thoughts About My Tower  (Read 3975 times)
KC4MOP
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« on: October 09, 2012, 02:35:02 PM »

I am very close to building my 40 foot tower. It kinda like a Rohn 25G. More likely US Tower Aluma whatever.
When mating the tower sections together, should I just push in until the screw holes match and use the appropriate hardware or continue to push the one section down until it rests on the flared part of the mating leg and drill new mounting holes??
My mechanical mind says that the upper section resting on that flared part of the mating section and drilling holes on the inner piece would be stronger. Finally get a chance to use my Photobucket.
http://s1263.beta.photobucket.com/user/mopman6/media/FLARE_zps701bc714.jpg.html
http://s1263.beta.photobucket.com/user/mopman6/media/BASE_zpsd6b9f058.jpg.html?sort=3&o=1
http://s1263.beta.photobucket.com/user/mopman6/media/BoltHole_zps05402fce.jpg.html?sort=3&o=0

THanks for any help.

Fred
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W9GB
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« Reply #1 on: October 09, 2012, 05:26:37 PM »

Fred -

This is the Rohn Guyed Tower Catalog (e.g. Rohn 25G).
http://www.rohnnet.com/resourcesmodule/download_resource/id/593/src/@random48eced0c124b9/

The pictures, diagrams, and drawings show proper assembly as well as specifications.
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KC4MOP
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« Reply #2 on: October 10, 2012, 03:26:47 AM »

Thanks for the info, but nothing really clear about the assembly of the sections and how they would mate together. The usual guidelines for the concrete pier and guying instructions for various products.
It seemed from a previous Rohn Tower  (A genuine Rohn product) I installed, that when the screw holes in the legs matched that the one end was sitting on top of the flared end, as seen in my picture. ( my picture is a Rohn look-a-like )
I'm gonna go with my gut and create a level work area and assure that the pieces are at maximum insertion and eyeball for any bowing of the two sections and drill new holes. The pictures shows that the inner leg bolt hole has slid past the outer leg hole for the mounting hardware.
The mechanical contact and the bolts making the final attachment seems to make more structural sense.
Fred
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K1CJS
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« Reply #3 on: October 10, 2012, 04:12:43 AM »

Just a couple of thoughts.  First, are all the holes like that when they're lined up?  That is, are they all not completely inserted to the flare?  If so, maybe that's the way the tower was meant to be assembled. 

Second, aluminum is really a soft metal.  Could be that the ends where the holes are were strengthened in some way--something that may be defeated by drilling new holes.

Third, anytime you remove metal from a joint (yes, even by drilling holes) you weaken that area. 

There are several things that you should check before you change the specs of that tower by assembling it in a way other than what is provided.  Finally, don't take these thoughts as gospel--as I said, they're just thoughts.
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K3GM
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« Reply #4 on: October 10, 2012, 04:39:03 AM »

Fred, it is normal when assembling sections together to have them mate past their hole alignment. DO NOT REDRILL!  It is also very important to use the prescribed hardware, and not just any bolt and nut. New sections are supplied with a bolt and nust set in one of the legs.  If these are old sections, the bolts may be missing.  Rohn sells a bolt pack to satisfy one section.  Four sections require 3 bolt packs, plus 1 for the bottom joint, and possibly 1 more depending what's going on top.

Occasionally, you may need a leg alignment tool when joining the sections, and then something to help break them apart years down the road. A TowerJack works great for this.  A small bottle jack works in a pinch for separation too.
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AA4PB
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« Reply #5 on: October 10, 2012, 05:14:31 AM »

DO NOT redrill the holes!!! The original tower was galvanized after the holes were drilled. If you drill new holes, the tower will rust around the edges of the new holes. It is not necessary for the sections to seat completely into the mating section. The Rohn instructions indicate that the bolts should be tightened enough to **slightly** squeeze the outer section to the inner section, making a firm connection. Some people don't do that because it makes the sections very difficult to get apart in the event you have to take it down.

MAKE SURE YOU GET THE ROHN BOLT KITS! These bolts are sized to fit the holes exactly. If these kits are the same as they were originally, the larger bolt is galvanized and the smaller is stainless.
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W8JI
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« Reply #6 on: October 10, 2012, 06:11:47 AM »

Fred,

I hope you installed the base correctly below the concrete, because above the concrete you did the worse thing posssible. You have the bolted joint just at the concrete.

Below the concrete, you have to have gravel and open leg bottoms to drain water. Otherwise the tower will fill with water and split or rust.

Above the concrete, you really should have been out of the concrete a few feet before the joint. This keeps water out, keeps areas that have galvanizing scraped away from dirt and snow and backsplash, and reduces mechanical stress on the joint.

The mechanical stress is not an issue with a bracketed or guyed tower, because if properly guyed or bracketed the majo load is vertical or torque and not bending moment. But it still is better, on your next tower, to get joints up off the pad. If the legs can't drain, you do have a majot issue that should be corrected.

The weakness in any tower is almost never at a joint, unless the joint is right at a rigid support. Redrilling the holes is a terrible idea, because it can only weaken things and encourage rust.

Follow the tower installation instructions. I'm not sure if you have 20G or 25G or some knock-off. How many horizontal braces does it have in each ten foot section?? That will help tell which tower type you have.

73 Tom





     
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KC4MOP
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« Reply #7 on: October 10, 2012, 08:03:34 AM »

Thanks for the excellent replies and the thoughts on 're-drilling holes'. I visited a couple of school districts who have Rohn 45 towers 60 feet high. And the joints are not flush against the flare. There must have been a number of inches of space. So my theory is all wet.
I will pull my tower apart where the bolt holes match up and proceed with assembly.

And correct on not following construction notes 100%. The base should have been higher out of the ground to allow the bolts to be at least 1 foot above the concrete surface. I may have created a maintenance job for me to protect what I just screwed up. No turning back now.

Fred
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W8JI
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« Reply #8 on: October 10, 2012, 08:16:54 AM »

The most important thing is you properly guy the tower, that the tower is really 25G or some tower suitable for guying, and that the legs can drain water.

The first one of anything is never perfect, that's how we learn. I've probably installed a few hundred towers over the years, including some over 300 feet. I've also seen a few dozen that have failed, fortunately none of them have ever been mine. Seeing the towers that fail helps me understand how to not make mistakes more than any advice ever has.      :-)

The single most common failure has always been guying mistakes. Always by far.

So obviously that is the critical thing.
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W5DQ
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« Reply #9 on: October 10, 2012, 09:24:45 AM »

Thanks for the excellent replies and the thoughts on 're-drilling holes'. I visited a couple of school districts who have Rohn 45 towers 60 feet high. And the joints are not flush against the flare. There must have been a number of inches of space. So my theory is all wet.I will pull my tower apart where the bolt holes match up and proceed with assembly.

And correct on not following construction notes 100%. The base should have been higher out of the ground to allow the bolts to be at least 1 foot above the concrete surface. I may have created a maintenance job for me to protect what I just screwed up. No turning back now.

Fred

I recommend you put it together AS DESIGNED and don't try to redesign it on the fly. I would imagine that ROHN spent a pretty penny doing the structural analysis before going to production and allowing room for a shade tree mechanic to re-design according to hunches probably wasn't factored in.  Huh

I would call your 'theory' completely submerged and not coming up for air  Roll Eyes
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
K3GM
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« Reply #10 on: October 10, 2012, 11:17:35 AM »

Fred, I just got a look at your pictures. I have to tell you, in my 30 years of monkeying around towers, I don't think I've ever seen a hole misalignment of that magnitude.  
http://s1263.beta.photobucket.com/user/mopman6/media/BoltHole_zps05402fce.jpg.html?sort=3&o=0
Typically, it's perhaps at the most half a hole diameter.  These sections appear old, yes?  Is this degree of misalignment the same for all sections or just one?  I would carefully examine the female end of each of the leg tubes for possible distorsion or damage.

One other thought.  Rohn 20 & 25G look very similar, and will even bolt together.  Besides the visible bracing differences the 20G leg tubes have thinner walls.  Is it possible that you have a 20G plugging into a 25G or vice-versa which would allow for a greater insertion depth and the gross hole misalignment?
« Last Edit: October 10, 2012, 11:26:57 AM by K3GM » Logged
NR9R
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« Reply #11 on: October 10, 2012, 02:47:23 PM »

Fred,

This statement concerns me:
It kinda like a Rohn 25G. More likely US Tower Aluma whatever

It is essential to know exactly what model of tower you have before you can plan the construction properly.  There are many variations of TV antenna towers that look like Rohn 25G but have vastly different specifications.  Figuring out the type of tower you have should be the starting point.  If you are uncertain of the tower type then how do you know the structural specifications and mounting methods. 

Here are some critical questions: 

What is your resource for designing the foundation?

What tension will you use for the guy wires?

What is the maximum antenna wind load of the tower?

Will you be climbing the tower?

These questions cannot be answered by guesstimating.
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KC4MOP
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« Reply #12 on: October 10, 2012, 04:24:11 PM »

Thanks for the excellent replies and the thoughts on 're-drilling holes'. I visited a couple of school districts who have Rohn 45 towers 60 feet high. And the joints are not flush against the flare. There must have been a number of inches of space. So my theory is all wet.
I will pull my tower apart where the bolt holes match up and proceed with assembly.

And correct on not following construction notes 100%. The base should have been higher out of the ground to allow the bolts to be at least 1 foot above the concrete surface. I may have created a maintenance job for me to protect what I just screwed up. No turning back now.

Fred

I'll just replay what I entered earlier today.
I will pull the tower section apart enough so that the bolt holes will once again be in alignment. That will separate the joining pieces as I saw on another tower installation earlier today.
The tower itself is only 30 feet and with mast and antenna will officially be close to 38 feet and should not need guying. The antenna load is a three element 6M Yagi and a 36 inch magnetic loop antenna.
I'll be hiring a bucket truck to put the pieces together and insert the mast into the top section and rotor.
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W8JI
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« Reply #13 on: October 10, 2012, 05:39:46 PM »

We just don't want you to get hurt.

It would be a tragedy to have to have to call someone to (KC4)MOP you up.  Cry

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KC4MOP
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« Reply #14 on: October 11, 2012, 07:19:13 AM »

We just don't want you to get hurt.

It would be a tragedy to have to have to call someone to (KC4)MOP you up.  Cry


HA! Thanks It can get pretty messy.. I will be carefull
TNX
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