120 foot tower: questions.

[zguy1243]

I am in the process of installing my new tower at my home. The plan is a 100 to 120 foot Rohn 25G tower. I have installed a good number of other towers up to 60 feet or so, this will be the tallest that I have installed yet. Those of you with alot of tower experience please help me with the following questions (please no flamers or know it alls, helpful responses only)

1) According to Rohn technical documents it states that guys should be 80% of the total tower height from tower. My guy anchors are 75 feet from tower base. If I go 120 feet should I have any issues? The Rohn documents state you can do 65% of tower height in some cases of lower wind loading. My antennas will be small UHF 225-400Mhz discones and Create 100Mhz to 1300Mhz log periodic. The wind loading will be nill.

2)When installing all my other towers I concreted in a 10 foot tower section 4-5 feet in the ground in a 4x4x5 pad. I have worries about getting the tower perfectly plumb due to the extra height of this tower. I know a small error at the base is greatly amplified at that kind of height. I typically use a plumb bob to get the tower as plumb as possible. I am looking for any hints or advice on the subject of getting the base perfectly plumb.

I have been following the rohn technical documents as close as possible on all other guidelines of the install. The tower will be guyed 3 times before the 100 foot mark with 3/16 EHS.

 

[n4yek]

I am in the process of installing my new tower at my home. The plan is a 100 to 120 foot Rohn 25G tower. I have installed a good number of other towers up to 60 feet or so, this will be the tallest that I have installed yet. Those of you with alot of tower experience please help me with the following questions (please no flamers or know it alls, helpful responses only)

1) According to Rohn technical documents it states that guys should be 80% of the total tower height from tower. My guy anchors are 75 feet from tower base. If I go 120 feet should I have any issues? The Rohn documents state you can do 65% of tower height in some cases of lower wind loading. My antennas will be small UHF 225-400Mhz discones and Create 100Mhz to 1300Mhz log periodic. The wind loading will be nill.

The guys should be fine at that distance. As you already stated you will have very little wind load with those antennas. But you might want to think for the future and place them at 96 Feet just in case you ever do get larger antennas.

2)When installing all my other towers I concreted in a 10 foot tower section 4-5 feet in the ground in a 4x4x5 pad. I have worries about getting the tower perfectly plumb due to the extra height of this tower. I know a small error at the base is greatly amplified at that kind of height. I typically use a plumb bob to get the tower as plumb as possible. I am looking for any hints or advice on the subject of getting the base perfectly plumb.

The plumb bob is your best bet as you already use. I would install temporary guys to hold the base in place once you get it plumb. Pour about a foot or two of concrete and check it again to make sure it is still plumb and make any adjustments if needed. Once you pour the first foot or two of concrete and done your checks, the base should not move and you can go ahead and pour the remaining base. Make sure you use a vibrator to remove any air bubbles once your done.
I would wait a full month before installing the 120' tower on the base, give it plenty of time to setup so you don't have any sagging.
Periodically hose it down to keep it from cracking while curing at the surface.

I have been following the rohn technical documents as close as possible on all other guidelines of the install. The tower will be guyed 3 times before the 100 foot mark with 3/16 EHS.

Always best to follow the manufacturers guidelines. Have fun with your project.

 

[k9rzz]

Here's a link that will keep 'ya busy for a while (if you haven't been there yet):

http://lists.contesting.com/_towertalk/

 

[jim202]

I don't like to see a tower section placed directly into the concrete base. A number of issues
develop in doing this. The first is what you have already mentioned about trying to get it
plumb.

The more important issue is how to take into account the moisture that will condense
inside the tower legs. This will collect at the bottom of the tower legs and start to
cause two problems. First is that corrosion will start from the inside. Second and
more important is that this moisture will eventually collect enough to go above the
poured base. Come winter time, the moisture will freeze and split the tower legs
just above the poured base. Now you have tower legs that look like a pealed
banana. Not a good situation for a 100 foot tower to be in. The tower weight will
have the ability to mushroom out the split tower leg or legs and start a downward
movement with time. This slackens the guy wires and somewhere along the
process, the whole tower tends to come down in a rapid movement.

You would be much better off following the directions that Rohn, now called
Radian Corp, have using a base plate. These plates have a drain hole in each leg
to let the moisture out and not cause the problems mentioned above.

Also make sure you use a tapered top section and not just try to tape up the top
tower section legs. You need to keep the rain water out of the tower legs.

Use a good grade of galvanized guy wire. Don't let any bare copper wire touch
the galvanized tower. It will leach out the zinc and cause rusting in a short time
frame. Been there and done that with towers in the past. Spent almost 20
years building towers for cellular companies. Seen a bunch of poor construction
in that time. Even told the company not to lease space at some sites based on
the poor condition of a poorly installed tower.

Don't forget to ground the tower with multiple ground rods and the guy anchor
points. The ground rods should be spaced at twice their length apart. Make
sure the ground connection to the tower is a bronze connector made for the
grounding application. Don't just take the copper ground wire and lay it against
the tower leg and use a stainless steel hose clamp to hold it there.

Jim

 

[gcgrotz]

jim202, sounds like you know what you're talking about. zguy1243 will do well to seek professional help. I've been in the wireless business for 25 years and have worked with many many tower crews. Some have been downright scary and some I would trust so much it wouldn't even be necessary to hang around and watch them.

120 feet of tower is nothing to mess around with. But I have to say zguy, have fun with it, I'm very jealous!

 

[n5ims]

I don't like to see a tower section placed directly into the concrete base. A number of issues
develop in doing this. The first is what you have already mentioned about trying to get it
plumb.

The more important issue is how to take into account the moisture that will condense
inside the tower legs. This will collect at the bottom of the tower legs and start to
cause two problems. First is that corrosion will start from the inside. Second and
more important is that this moisture will eventually collect enough to go above the
poured base. Come winter time, the moisture will freeze and split the tower legs
just above the poured base. Now you have tower legs that look like a pealed
banana. Not a good situation for a 100 foot tower to be in. The tower weight will
have the ability to mushroom out the split tower leg or legs and start a downward
movement with time. This slackens the guy wires and somewhere along the
process, the whole tower tends to come down in a rapid movement.

You would be much better off following the directions that Rohn, now called
Radian Corp, have using a base plate. These plates have a drain hole in each leg
to let the moisture out and not cause the problems mentioned above.

Isn't this why Rohn specifies to have your base section extend into a "compacted sand and gravel drainage bed" below the concrete pad?
http://www.radiancorp.com/ROHNNET/rohnnet2001/catalog/pdfs/25G/25G-2.pdf

 

[zguy1243]

Isn't this why Rohn specifies to have your base section extend into a "compacted sand and gravel drainage bed" below the concrete pad?
http://www.radiancorp.com/ROHNNET/rohnnet2001/catalog/pdfs/25G/25G-2.pdf

I have put the gravel drain bed in the hole before I poured concrete on that last towers I did. I see that there is a good number of conflicting opinions about sinking a section of tower in the base or using a base plate. I am very concerned with the tower being perfectly plumb. It seems that with a base plate you could have just as many plumb problems as you must have a perfectly level pad to attach the plate to.

 

[kd5dga]

It is not good to concrete in a section of tower. The base plate is the only way to go and that is for the ovious questions mentioned above. Make sure at the base of the tower that you run grounding for the base in a delta or triangle. If you ever take on a direct ligtning strike the chances of blowing out the concrete support is greater if the lower end of the tower is not protected. I have my 30 foot Rohn 25 tower resting on a base plate attached to a concrete pad and every leg is grounded. I had a lightning hit with 50 feet in the air and I did not have enough grounding and the concrete pad had blown apart at the top. Somewhere I had seen photos of direct lightning hit of commercial broadcast towers showing the base with cracking and burns where the operators were not properly maintaining their towers.