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Author Topic: Rohn 25G tower and base  (Read 8288 times)
WX4SNO
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« on: August 26, 2013, 05:40:51 PM »

Hello everyone,

I recently bought a gently-used 40' Rohn 25G tower that was originally installed back in the 1990's by a telephone/internet company.  According to the guy that I bought it off of, the tower had about 10-12 different antennas on it when the company went under and left it to him; he also mentioned that it had been un-guyed and stood up to some strong wind events and ice/snow storms in the WV mountains.  I'm planning on installing only 30' of it without guy wires (don't have the space) and it will only have a small 144/440 vertical, a scanner antenna, and a weather station anemometer attached.  I'm also going to buy either the 3' 4" short base section (R-SB25G) or the 5' base section (R-SB25G5).

My question is, what would you all recommend for the size of the concrete base?  I found a reference online from Rohn that suggest I use a 6x6x4 concrete slab with the base section submerged inside that.  Is this overkill?  We have hard soil, made up of bits and pieces of sandstone and quartz.  I'm just curious if a 3x3x3 or a 3x3x4 concrete base would suffice?  To that end, which base section would be most appropriate for my situation...the 3' section or the 5' section? 

Thanks in advance!
WX4SNO
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K3GM
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« Reply #1 on: August 26, 2013, 06:28:00 PM »

Anthony, I implore you to look at the Rohn website for specs on unguyed 25G.  You'll find that standard 25G was not made to freestand with any more than a VHF vertical mounted on it.  If you proceed as planned you are facing potential disaster and at the very least, injury if you climb even 30' of unguyed 25G.  Your base plan is totally inadequate. 3'x'3 is the recommended base for a guyed tower. If you work atop three sections of tower, the overturning forces at the base are tremendous.  I seriously rethink you plans, perhaps bracketing it to the house.  But please, don't attempt to freestand it.
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WX4SNO
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« Reply #2 on: August 26, 2013, 06:51:42 PM »

I'm actually not planning on climbing it (I hate heights), rather a group of friends and I will walk the tower up into position...would this not work (small 144/440 and a scanner antenna) for an un-guyed tower that I DON'T plan on climbing?  I won't be bracketing it to the house because I'm interested in keeping the WMO standards for the anemometer (1m tower away from buildings) and I know the Weather Service uses unguyed towers for their meteorological instruments...would I just need a larger concrete base?
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NJ1K
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« Reply #3 on: August 26, 2013, 07:19:27 PM »

If you want a free standing tower, get one that is meant for a free standing installation.  It will be much safer.

Even though Rohn 25 can be installed as free standing for small heights, I wouldn't recommend it even if it's not populated at all. 

One trade-off between guyed vs free standing is teh free standing requires a whole lot more concrete, but no guys.  Guyed towers require much less concrete but you also need guys, guy anchors and a larger footprint and more periodic maintenance. 

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W9GB
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« Reply #4 on: August 26, 2013, 07:53:14 PM »

Install the Rohn 25 G foundation PER SPEC !
Rohn has ALL of that material FREE. Page 41-44 foundations.
http://www.rohnnet.com/rohn-25g-tower

2-page (IF you don't use pin pier, Rohn has a short section ~4 feet for foundation).
http://resources.tessco.com/attachments/ROHN_25G_Standard_Foundation_Designs_245159.pdf

Need a Rohn Document??
Rohn Documentation Control
Chalmer Post
Phone: 309-566-3044
Chalmer.Post@rohnnet.com

Rohn 25G brochure
http://www.rohnnet.com/resourcesmodule/download_resource/id/593/src/@random48eced0c124b9/

Tower Accessories
http://www.rohnnet.com/resourcesmodule/download_resource/id/630/src/@random4a7c4930d495c/
« Last Edit: August 26, 2013, 07:57:33 PM by W9GB » Logged
N4KD
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« Reply #5 on: August 27, 2013, 06:06:13 AM »

Anthony, I implore you to look at the Rohn website for specs on unguyed 25G.  You'll find that standard 25G was not made to freestand with any more than a VHF vertical mounted on it.  If you proceed as planned you are facing potential disaster and at the very least, injury if you climb even 30' of unguyed 25G.  Your base plan is totally inadequate. 3'x'3 is the recommended base for a guyed tower. If you work atop three sections of tower, the overturning forces at the base are tremendous.  I seriously rethink you plans, perhaps bracketing it to the house.  But please, don't attempt to freestand it.

I wonder if Rohn doesn't list their specs extremely conservatively. Just some back of the envelope calculations show that a 40 ft tower can withstand a 480 pound load at the top. That would be just on the edge, but it's a lot larger that any load that could be created by a 1 to 1.5 sf antenna. You could halve that and be safe with an antenna -- NOT A CLIMBER!

I'm sure there are a number of structural engineers out there that will tell me why these calculations are wrong and I'd appreciate the critique. So here it goes, a 4ft x 4f x 4ft foundation is about 2.4 cubic yards of concrete. Figure on 4000 #/yd and you end up with 9600 # of cured 'crete. That much would resist a moment of 19,200 ft-lbs, when you calculate from the center of the 4 ft cube.

Now look at the tower. Put a load at the very top of a 40 ft tower. The moment at the base is 40 ft times the load. We know the max moment is 19,200 ft-lbs, so divide to get the max load of 480#. Sure, there's a distributed load on the tower and it's not insignificant. I should calculate it, but maybe one of the experts can do so.

Then figure that a 100 mph wind has a dynamic pressure of 25 #/sf. A 1.5 sf antenna would produce a load of 38 # and a moment of 1500 ft-lb.

The conclusion is that there is more to research before making conclusion. The distributed wind load on the tower may be enough to severely limit the size of the antenna and it should be calculated.

Vy 73,
Dave N4KD
« Last Edit: August 27, 2013, 07:05:36 AM by N4KD » Logged
AD4U
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« Reply #6 on: August 27, 2013, 06:50:38 AM »

There are two 50 foot high un-guyed Rohn 25 towers about two blocks from me. One is at a wholesale seed business and the other is behind a furniture store.  On top of each tower is a Cel-Wave Station Master VHF antenna which is about 20 feet tall.  Both towers have been "up" at least 30 years.

These are two real-world examples that I see every day.  Your results may vary.  However I do not recommend going against Rohn's specs.

Dick  AD4U
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WX4SNO
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« Reply #7 on: August 27, 2013, 10:16:59 AM »

Thanks for the input folks!  I really appreciate the dialog. 

The guy I bought the tower off of just got back to me; he measured the concrete base where the tower was originally and said it was 30"x30"x30", so I'll just use a cubic yard of concrete and call it a day.  This is what Rohn calls for on their website.  I'll probably add a cage of re-bar to add some strength as well. With the original setup surviving 25 years of wind, snow, and ice in the WV mountains, I don't think there will be a problem.

I thought about using a scissor lift or bucket truck, so I called around to some local rental places and they're wanting around $200 to rent a scissor lift...that's for a day (no hour-by-hour rentals available).  I don't think I'll go that route.

I've decided this is what I'll do...for better or worse: The reason I don't want to climb the tower is that I'm a pretty big dude...6'4" and 295 Ibs, so I don't want to be putting all that force on the tower's legs and braces. So, I'm going to install the first 10' section and then attach my good 'ol extension ladder to it...secure it good to the first section of tower, then climb that up to 10' and install the second section. Then all I have to do is extend the ladder some more, strap it down further up, then haul the last 9' top section into place. I'd feel much better about doing this than climbing on the tower itself since most of my weight will be put on the rungs of the ladder and not the tower...

No easy way about doing it I don't think, not in my situation...I just don't have the yard to install guy wires since I live on the side of a mountain and my property isn't that big.

Thanks for all the comments and help!
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N6AJR
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« Reply #8 on: August 27, 2013, 10:30:07 AM »

I have 40 feet of rohn 25 up with a 3 element steppir on it.  I have a Glen Martin Hazer on it to make it easy to access the antenna. I used a 3 x 3 x 3  foot hole, with a small rebar type cage,  ( a bit wider at the bottom than the top ). I have it bracketed at the house at about 12 feet and then a second bracket at the roof line at about 15 feet. I also have a set of guys at the bottom of the Hazer. We get some intense winds here and it has been fine so far. Its your house and your responsibility, but I would recommend this as a minimum set up. and a little more  would not hurt.  ask around and see if there are any folks in the area that have done this and can give you a hand.  good luck.
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WI8P
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« Reply #9 on: August 27, 2013, 10:41:02 AM »

I am in the process of installing a used 25G now.  Mine will be 40 feet tall.  The Rohen base dimensions you quoted are for the 40g I believe, as the ones I down loaded from their site say 4' x 4' x 4.5' deep.  Those are the dimensions I used.  I also built a rebar cage on the 5 foot base section before putting it in the ground and pouring the cement.  I used 5, 10' sections of rebar, cutting each piece 3.2' (approx) long.  They are mounted on 12" centers.  To top it off, I used a fiberglass reinforced cement mixture.  Right now, I'm waiting for a crane to become available to mount the top two sections and the antenna.  At my age, I'm too old and fat to be climbing this thing, even though I know it would hold me.  Let me know if you have any questions I can help with.

KB1TOM
Tom
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N4KD
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« Reply #10 on: August 27, 2013, 12:52:54 PM »

I am in the process of installing a used 25G now.  Mine will be 40 feet tall.  The Rohen base dimensions you quoted are for the 40g I believe, as the ones I down loaded from their site say 4' x 4' x 4.5' deep.  Those are the dimensions I used.  I also built a rebar cage on the 5 foot base section before putting it in the ground and pouring the cement.  I used 5, 10' sections of rebar, cutting each piece 3.2' (approx) long.  They are mounted on 12" centers.  To top it off, I used a fiberglass reinforced cement mixture.  Right now, I'm waiting for a crane to become available to mount the top two sections and the antenna.  At my age, I'm too old and fat to be climbing this thing, even though I know it would hold me.  Let me know if you have any questions I can help with.

KB1TOM
Tom

This is really off-topic and I apologize. But, to continue, Tom, did you get a concrete truck to deliver the 2 1/2 yards you needed? I'm just curious, as I'm really having trouble finding companies that want to deliver short loads.

Vy 73,
Dave N4KD
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WX4SNO
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« Reply #11 on: August 27, 2013, 01:09:09 PM »

I am in the process of installing a used 25G now.  Mine will be 40 feet tall.  The Rohen base dimensions you quoted are for the 40g I believe, as the ones I down loaded from their site say 4' x 4' x 4.5' deep.  Those are the dimensions I used.  I also built a rebar cage on the 5 foot base section before putting it in the ground and pouring the cement.  I used 5, 10' sections of rebar, cutting each piece 3.2' (approx) long.  They are mounted on 12" centers.  To top it off, I used a fiberglass reinforced cement mixture.  Right now, I'm waiting for a crane to become available to mount the top two sections and the antenna.  At my age, I'm too old and fat to be climbing this thing, even though I know it would hold me.  Let me know if you have any questions I can help with.

KB1TOM
Tom

Okay...thanks for the info!  I'm going to go with a base of 3' x 3' x 4' since mine is only going to be 29' tall to be exact.  I'll also install a re-bar cage around the submerged section for added support.  I just don't see how over 3200 pounds of concrete will move with only a 30' tower above it...shouldn't be a problem unless we have a tornado come through.  What I am going to do after it's completely up is monitor it during the first few wind storms this winter.  If I see it swaying at all, I'm going to attach guy wires to it near the top.  I really don't have the room to put guy wires since they would run across my backyard and limit foot traffic, but if it comes to that, then that's what I would have had to do in the first place...

I'll have two options as to getting the tower up...I'll either walk the sections up with the help of a few friends, then lift it vertically and place it over the protruding section, or I'll attach a ladder and carry the sections into position (or borrow/make a gin pole)...I'll let you all know how this turns out...
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K3GM
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« Reply #12 on: August 27, 2013, 01:14:33 PM »

...I'll either walk the sections up with the help of a few friends, then lift it vertically and place it over the protruding section, or I'll attach a ladder and carry the sections into position (or borrow/make a gin pole)...I'll let you all know how this turns out...

Please do.  By the way, what are you using for a climbing harness, and positioning and fall arrest lanyards?
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W9GB
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« Reply #13 on: August 27, 2013, 01:15:44 PM »

Quote
This is really off-topic and I apologize. But, to continue, Tom, did you get a concrete truck to deliver the 2 1/2 yards you needed? I'm just curious, as I'm really having trouble finding companies that want to deliver short loads.
Last Rohn tower that I put up for my dad ~ 1989, we talked to local concrete company in early summer.  We told them our need, and they said as long as we were flexible they would "tack on" to a larger job in the area.

About 10 days later, we got a phone call, and were ready for them a couple of days later.
Took less than 15 minutes for the pour.  Driver was happy to get coffee and doughnuts.

Best to let a new foundation cure for 30 days, before installing tower.
« Last Edit: August 27, 2013, 01:17:48 PM by W9GB » Logged
WX4SNO
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« Reply #14 on: August 27, 2013, 01:16:54 PM »

This is really off-topic and I apologize. But, to continue, Tom, did you get a concrete truck to deliver the 2 1/2 yards you needed? I'm just curious, as I'm really having trouble finding companies that want to deliver short loads.

Vy 73,
Dave N4KD

I just called up a company near where I live and they wanted around $400 for 2.5 cubic yards of concrete delivered...too rich for my blood...good thing I've got a mechanical mixer! Grin
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