ROHN 25G foundation

<< < (3/7) > >>

KB5MD:
I was told years ago to place about 6" of crushed rock (gravel) in the bottom of  the hole and set the  tower section upon this. It allows the water to drain out the bottom of the tower, rather than drilling holes in the tower legs, which will weaken them.  I have a 50' rohn 25g with  a Mosely TA-33 on top and it has been thru several wind storms during the past 20 years. There are no guys. The tower is made to  stress from the ground. The base is a 3'x3'x3' chunk of reinforced concrete,(about 2.5 tons of weight)
One other thing, tell the readymix guys to add 2% calcium chloride to the mix and it will set up alot faster.  Hope this helps

k4kyv:
When I put up mine, I used #10 copperweld antenna wire and tent stakes for the temporary guys for the first section.  Then afterwards I used more of the copperweld but attached them to the permanent guy anchors.

I installed the first set of permanent guys at the 30 ft. level, but as the tower grew, I still used temporary copperweld guys every tower section till I installed the next set of permanent ones.  I never felt comfortable working at more than one tower section above a guy point.  I could stand on the ground and look at the tower and say, no way anything is going to  give way @ 2 or 3 sections above the guy point and I could safely swing on it like a trapeze artist, but when I climbed the tower it was an entirely different perspective. The top guy section appeared to be a long way down, and the tower would sway with my every move, and if I tried to lean back in the climbing belt it felt like the whole thing was coming down with me on it. No way;  I had to stabilise it with something.  About the only thing those temporary guys did was to stop the movement and make the tower seem more solid and stable, but that made me feel a lot more comfortable working.  Purely psycholoical, but that's crucial, since being less than fully at ease, in itself, can make a job like tower erection hazardous and vulnerable to human error.

As for drilling drain holes, if the tower is to be guyed, that shouldn't be a problem if the holes are kept small. The guys are what keeps the tower vertical, and although the tower will be stressed under wind conditions, it shouldn't be enough to cause a catastrophic failure.  Even if the tower broke completely in two (virtually zero likelihood), the bottom would still simply drop to rest on the concrete base and the guys would still hold it up.  It would take a tremendous lateral  force to displace the base of a tower, even if it were not anchored to anything, considering the downward force caused by the weight of the tower and antenna, plus the downward vertical component of the tension at each guy wire.

If the tower is to be self-supporting, I would be leery of drilling any kind of holes in the legs.  But in that case, it would be much better to purchase real self-supported tower components.

K1JJ:
Maybe it was already mentioned here somewhere -  (I didn't see it) for the main reason to use a base pin and floating base on a guyed tower - might as well emphasize so nobody makes the same mistake I did...

When the bottom section is placed solid in concrete, there is no way to get the guy wires perfectly balanced to produce no stress on the bottom section. Even if tensioned perfectly, guys stretch over time, wind storms occur,  and there will be uneven stress on the lower section in concrete. This is like a lever and can snap/fold the legs right at the base.  The biggest stress is right at the point of coming out of the concrete.

With a floating base pin, the tower base can rock slightly and compensate for guying and wind pressure changes. If you think about it, the top of the tower is also free to move about, which is a good thing... :-)

Back in 1985, my first tower was a 150'er Rohn 45 with its base placed in solid concrete. A fool at a popular ham store recommended it... :-)  I didn't know any better myself. Years later in 1994 I loosened the turnbuckles and used a hydralic jack to raise the tower a few inches - I cut the three legs and inserted in a base pin and plate.  Quite a job, but it worked.


Yes, John, go with turnbuckles. Tweaking the guys every year or so is part of tower maintainence. After a big wind or ice storm, you will find they probably need a some minor adjustments.

It's amazing how much vertical weight a guyed tower can take. For example, if each Rohn 45 guy is tensioned to 600 pounds, this is 1800 pounds per set. Multiply this time five sets and you're pushing upwards of 10,000 pounds including the tower weight.  (The guying weight load is somewhat less cuz of the angle not being straight down) The  tower needs to be very straight to handle a big load like this. Curve it or torque slightly and this rating drops markedly.   Torque on a tower is the big killer and why many come down.  I built up homebrew torque-arms for all of my guyed towers. (6 guys per level) This really helps when rotating a big load.

It pays to develop a good eye and sight straight up the tower to see which sections need guy adjustments. After some time, the tower can be tweaked very straight using your eye. Another way is to borrow/rent a transit.  Set it up two hundred feet away from the tower and sight right up the rail. Do this from each 120 degree leg angle view. (Three times)  Using this method I was able to find one of my guyed towers had straight section alignment, but leaned about 2 inches from center of gravity... :-)  No other way to do this... even the level didn't show it.

Good luck.

73,

Tom, K1JJ

WA1GFZ:
A friend not wanting to waste 5 feet of tower in the ground welded a frame with 3 vertical solid pins to hold the bottom section. This way water could drain around the pins. No, do not drill the bottom of a tower leg the hole will rot the leg in time.. Set it in stone so water can drain out.
Three pins seemed easier to deal with than a single center pin. I don't have my guys that tight Tom but I'm not kissing the clouds either. I just have my guys snug so it doesn't move. Reminds me I have not checked mine in a while.
I look up under each set of guys to make sure each level lines up. It is easy to make the structure straight.
knock on wood after 31 years. Guy anchor is where you want lots of concrete not so much the tower. As the tower moves it wants to pull the guy out of the ground or set the tower deeper in the ground. 

WQ9E:
OK,  John is putting up a fairly short tower consisting of Rohn 25, not the Eiffel tower.  Properly installed, a short base in concrete will work just fine for him.  Rohn designed short concrete bases for Rohn 25, 45, 55, and 65 series tower and if they are installed with proper gravel for drainage they will last as long as the rest of the tower which is a very long time indeed.  The biggest reason for using a pier pin installation is for very tall towers so that any deviation from plumb in the base is not transferred as a stress factor throughout the rest of the structure; that isn't going to be an issue with 50 feet of tower unless John chugs a fifth of bourbon before installing the base.  Just make sure your holes truly are properly spaced and the pipe tops are properly flattened and then use your level across the tops of the legs.

The funniest (since I didn't live anywhere near it) and worst Rohn 25G installation I am aware of was a CB type who obviously leveled across the rungs instead of the tops of the legs and the base was far from level.  He followed that up by using nylon ropes for guys and tried to pull it straight with those stretchy ropes.  A friend in the city engineering office later told me that they investigated after concerns from the neighbors and also found the guy anchors consisted of 3 small block chevy V-8's buried just below the surface with an eyebolt threaded into one of the head bolt openings-this was probably the strongest part of the entire installation.  By the way this was in Gulfport MS down in hurricane country.  

John,  If you need specs, diagrams, etc. for you 25 installation PM me.  I still have the Rohn commercial products catalog I got when I installed my 55G around 15 years ago.  It has all the basic information, specs, etc. necessary for installation and I can scan and email you what you need to set your mind at rest.  If you have an accommodating Rohn dealer nearby you can probably acquire your own copy and it is a handy reference guide.

Rodger WQ9E

Navigation

[0] Message Index

[#] Next page

[*] Previous page