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Author Topic: Concrete base for tower  (Read 3453 times)
KK7UV
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Posts: 23




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« on: May 22, 2011, 04:35:29 AM »

I recently acquired a small crank-up tower with no manufacturers marks and unable to find a look-alike.  It is 3 nested sections of triangular construction.

Bottom section:  14.5 ft.   8" triangular
Middle section:  14.5 ft.  6" triangular
Top section:  20.5 ft.  4" triangular

It extends to 44 feet full length.  I estimate the weight at around 100 lbs.  I plan to put a small 9-pound TV rotor and a 20 pound HexBeam on top.

Without manufacturer's recommendations, are there online calculators or a text reference for determining what I need for a concrete base?




« Last Edit: May 22, 2011, 08:42:47 AM by KK7UV » Logged
KG4RUL
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« Reply #1 on: May 22, 2011, 07:43:41 AM »

There are way too many variables for such a general calculator to be useful.  Find out who made it and get their installation instructions.  Your life and limb may depend on it.
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KK7UV
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Posts: 23




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« Reply #2 on: May 22, 2011, 08:22:51 AM »

Agreed.  It's not going up until I get a confident design for the base.  Have been looking for the manufacturer since last fall with no luck.
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KA5N
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« Reply #3 on: May 22, 2011, 10:41:12 AM »

If you haven't already done so, the first thing to find out is what must you provide for the permitting process (if you have one and today you probably do) as the permit board often requires engineering specs and blue prints covering the concrete base and rebar cage. 
Of course you could just dig a hole and pour concrete and hope all works out OK.  Or find specs from a similar tower and copy that installation.  Of course if anything goes wrong, guess who'll pay out the A**. 
Your tower may not turn out to be inexpensive at all.
Good Luck
Allen
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WB2WIK
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« Reply #4 on: May 22, 2011, 01:42:42 PM »

It's not how much the antenna weighs, it's the wind loading (surface area) that counts, and really the only thing that matters.

Based on the dimensions given, it seems that tower is designed to be guyed.  I really doubt an extendable tower with an upper section that's only 4" across on a face could possibly be self supporting with any kind of beam antenna on it.  As such, the foundation isn't so important and mostly prevents the tower from sinking into the ground: The guys do the actual supporting.
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KK7UV
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« Reply #5 on: May 22, 2011, 02:13:36 PM »

Yes it will be guyed.  The top of the base section at 14 feet will be attached to my garage.  The top of the middle section will have 3 guys.  Top wind speed here is around 45 mph and greatly buffered by surrounding forest canopy so actually this thing is not likely to see anything over 25 or 30 mph wind gust.

The wind load on the HexBeam is 5 sq.ft.   with probably another .5 sq.ft. on the rotator.
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WB2WIK
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« Reply #6 on: May 23, 2011, 09:28:40 AM »

Yes it will be guyed.  The top of the base section at 14 feet will be attached to my garage.  The top of the middle section will have 3 guys.  Top wind speed here is around 45 mph and greatly buffered by surrounding forest canopy so actually this thing is not likely to see anything over 25 or 30 mph wind gust.

The wind load on the HexBeam is 5 sq.ft.   with probably another .5 sq.ft. on the rotator.

You have to add in the wind loading of the coax used to attach the beam, also.  If everything combined is about 6 square feet, that's quite a lot.  It's like holding a 2-1/2 foot square solid plate against the wind above the top of the tower.  In a 30 mph wind, you'd never be able to hold it, it would twist you right off the tower.

I suspect the "top" of the tower, where the rotator is, will need to be guyed also.  You can't use a lot of guying tension on this kind of tower or you can buckle the top section; but light guying with probably 30 lbs tension or so will help enormously.
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KF7CG
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« Reply #7 on: May 23, 2011, 10:55:20 AM »

From what I hear, that is a manufacturer unknown TV antenna tower of yesteryear. Many manufactureres, most small, cranked these thing out for home TV use. They were designed to support a very small rotator and a "deep fringe" TV antenna fed by flat ribbon TV lead in.

It was most likely freestanding. Where I live in the country side there are a few examples of the cranks still around but many more of the 30 to 40 freestanding Rohn 20 design look alikes and many of th Spalding design look alikes with derelict TV antennas on top. Very light duty, but Radio Shack put out a tubing telescopic mast that extended guyed to 50 feet and was supposed to hold a RS rotor and "Deep Fringe" antenna.

Good luck with the tower! Be Careful!

David
KF7CG
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KI4Z
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Posts: 37




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« Reply #8 on: May 27, 2011, 03:53:19 PM »

5 cubic yards of reinforced concrete in a hole 6' deep will work.  Anything less requires some engineering talent.

mark
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K1CJS
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Posts: 5997




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« Reply #9 on: May 27, 2011, 08:28:29 PM »

Try taking pictures of the tower, especially the end sections where the winch and the pulleys are attached, and take them to a local club.  One of the hams there may be able to help you.  If that is a old TV antenna tower, and you can locate a TV shop that has been around for a while, they may be able to identify it for you by the pictures. 

In any case, you would first want to have it thoroughly inspected and tested to insure it is still useable, and won't collapse the minute you hoist it up.  Be sure it can withstand the weight of the rotor/antenna you're going to put up and for for the purposes you want.

Good luck!
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N4JTE
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Posts: 1155




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« Reply #10 on: May 28, 2011, 07:12:26 PM »

Sounds like a Aluma Tower form early eighties, had one motorized, and was lucky with FL one story home and attached to house at 10 ft. lasted for many years.
Bob
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MM0KTC
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Posts: 5




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« Reply #11 on: July 04, 2011, 04:49:16 AM »

as previpous said, the base is there to prevent the tower from sinking, if it is the type that requires to be supoorted by wires. if not, then the base plays a big part in that it resists the overturning efect of the wind loading.
as you have said that the fist section will be fixed to a building, then the base needs to hav a large surface area as opposed to mass. you could use steel plate, if you could something about 2m x 2m, this would prevent it sinking, but has very little mass.
you would be better with a base of around 1.5x1.5 by 0.5 deep. this would provide mass and a good surface area to stop sinking.
plenty of guy wires as well, to resist the wind loading and you will be fine.

Keith
MM0KTC
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W8JX
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« Reply #12 on: July 04, 2011, 08:13:06 AM »

you would be better with a base of around 1.5x1.5 by 0.5 deep. this would provide mass and a good surface area to stop sinking.
plenty of guy wires as well, to resist the wind loading and you will be fine.

While I feel 5 cubic yards is way over kill for a non self supporting tower of this type, what is suggested above (and I assume it is metric too) is not enough in that any tower concrete base must be well below any possible frost line and on firm soil too.  4ft x 4ft x 4ft which is a little over 2 cubic yards is more than enough here. Also it is the depth of the base more than its width for a given volume/mass that gives more stability. If this was a self supporting tower I would still do 4 x 4 but I would go down 6 feet or so.  As you go deeper, the side trust surface area and stiffness of soil increases and hold firmer and with better stability than same mass of concrete in a shallower hole.
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