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Author Topic: Extremely confused....crank-up tower base  (Read 940 times)
KC0LTV
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Posts: 26




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« on: May 06, 2006, 01:53:06 AM »

A few years ago I acquired a used Tristao TMM-340SS crank-up tower.  The tower weights around 300 lbs, stands at 11'6" without telescoping, and raises to 34' (40' if you include the included 6' mast, which I intend on swapping with a 12' mast so I can install the antennas I wish to).  

Last fall, I dug the hole for the tower, and now I want to pour the cement for it.  However, I am confused...what exactly needs to go in the hole to make the form for the tower?  I have looked at other operators' pages, and they have used rebar cages, something I am utterly, completely unfamilliar with.  Will this be required for the small tower I am installing?  Can some other material be used?  The hole I dug is roughly 3' x 4' x 4'.  Does this sound sufficient?  The ham I bought it from recommended about 3 x 3 x 3.  

Thank you.
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K3GM
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« Reply #1 on: May 06, 2006, 05:38:23 AM »

I did some checking of other similar sized towers, and their recommended bases were 3'x3'x4'(deep), and 4'x4'x4.5'.  A rebar cage is necessary, but simple to build.  For your base, you need to make 4 rectangles 2.5'x2.5'. Make them from 3/8" rebar.  It's best to weld any joints, but you can also wire them, by twisting the wire tightly to bind the pieces.  Now stack the 4 rectangles together using 1/2" vertical sections of rebar.  You want spread the rectangles out evenly so no rebar comes closer than 3" to the concrete's surface. If the tower is bolted to the base, you will need to make a template from heavy plywood. Secure the J-bolts to the plywood with a washer and bolt on either side and embed them during the pour.  You also need to make a wood sill form to level the top surface, and to dress the sides.  That's the basics of it.  I guess it goes without saying that experienced help to assist you makes things much easier.
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KC0LTV
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Posts: 26




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« Reply #2 on: May 06, 2006, 11:34:37 AM »

Thank you.  I do have some close "acquaintances" who are skilled with welding, so they may be able to give me help.  Thank you.
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KA0GKT
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Posts: 555




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« Reply #3 on: May 06, 2006, 02:26:04 PM »

My only question would be, Is 4' deep sufficient for the bottom of the concrete to reach below the frost line in Minnesota?  4' was the minimum depth for frost-free ground in Eastern Nebraska, I would think you would need to go deeper in Minnesota...if only to protect from the additional load placed upon the tower when a Minnesota mosquito lands on one of the antennas :-D

73 DE KAØGKT/7

--Steve
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W5GA
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Posts: 430




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« Reply #4 on: May 08, 2006, 08:05:09 AM »

I always thought that something that was 2.5' square was, um.....square!  ;-)
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WB2WIK
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« Reply #5 on: May 08, 2006, 08:38:50 AM »

I hope you had that hole covered up all this time or it may not provide the support needed due to erosion.

The concrete base should be poured into a fresh excavation made in *undisturbed* soil.  There's a reason for this.  When you dig a hole and wait months or longer before you use it, if it rains during that time, the sides of the hole will erode, slide and become loosened.  (If you had it well covered up, then maybe not.)  Usually, the result is a hole that's wider at the "top" (surface) than it is at the "bottom," and that's just the opposite of what you want.

WB2WIK/6
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N3UMH
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Posts: 153




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« Reply #6 on: May 08, 2006, 01:46:22 PM »

I did this once... dug the hole months before I put the tower in.  Fortunately I had soil that was mostly clay and the sides stayed put.  (it was covered)

If the soil would have been more particulate and less clay, I'm sure it would have caved in all the rain.

As it is, the tower (28' tubular crankup) is still up.  Unfortunately I don't get to use it anymore, and haven't been able to convince my mom or sister to get licensed and on the air :-)

73,
Dan
N3OX
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KC0LTV
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Posts: 26




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« Reply #7 on: May 09, 2006, 05:52:27 PM »

Thank you for your comments so far.  They are very much appreciated.

About the erosion...I completely see what you mean.  However, do you think would it be OK if I dug the hole out perhaps six inches in each direction, and a bit deeper (it actually should be anyway)...that way none of the walls would be from the "eroded" soil?  

Thank you.
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KC0LTV
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Posts: 26




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« Reply #8 on: May 15, 2006, 11:36:37 AM »

Could somebody please help me with that last question?  I'm sure it went unnoticed, but I really need an answer.

Thank you for all your help.
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WB2WIK
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« Reply #9 on: May 15, 2006, 01:57:27 PM »

A "larger foundation" is always better than a smaller one, so I'd say "yes," for sure.

Concrete's heavy.  If you make the base large enough, you don't need a hole!

WB2WIK/6
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N2MG
Administrator

Posts: 127



« Reply #10 on: May 16, 2006, 07:38:22 AM »

From what I've read, welding rebar is not always a good idea. There are some types that are weldable, but the standard rebar stuff is not - the welding weakens it.

Look into it...

Mike N2MG
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W9RAY
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Posts: 48


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« Reply #11 on: May 16, 2006, 04:47:06 PM »

yes.. check the codes...

The ACI 318-99 Building Code Requirements for StructuralConcrete (which is referenced by all
 building codes in the US) states:
"7.5.4 -- Welding of crossing bars shall not be
permitted for assembly of reinforcement unless
authorized by the engineer."

"R 7.5.4 -- 'Tack' welding (welding crossing bars) can
seriously weaken a bar at the point welded by creating
metallurgical notch effect. This operation can be
performed safely only when the material welded and
welding operations are under continuous competent
control, as in the manufacture of welded wire fabric."

If you use ASTM A 706 reinforcing bar then no preheat is required for welding.
 most standard rod(ASTM A 615) requires 300 degrees F for a minimum of 20 minutes prior to welding.

However note that the intent of welding reinforcing
steel is focused on direct splices and connections to
other metal devices to be embedded in the concrete, not
for rebar cage assembly or tackng in lieu of wire tie..

but then again  I dont think you are getting a permit for this anyway .. :-)

http://www.k1ttt.net/technote/welding.html
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