Well in my case, I contacted the engineer, and he suggested adding 9
vertical rods tied to the mats to suspend the upper mat over the lower.
Rohn catalog descriptions aren't sufficient for my county's building permit
process.
The HyTower I'm installing has an instruction manual that specifies no
rebar, and it's also rated for 75 mph. My county won't accept less than
"wet stamped" plans from a PE licensed by Washington State, and I'm in an 80
mph wind zone area, near a bluff / ridge line with some wind enhancement as
a result. The engineer I engaged provided plans and strength calculations
for 85 mph, and his certification is a necessary part of the permit process.
I just need to add a little rebar and my base is a foot wider than the
HyGain manual describes. And I've had someone with the right credentials
examine my specific situation and develop suitable plans. I have confidence
in those results. More importantly, the county's building department will
have sufficient confidence to grant me the building permit I require.
Fortunately my county has a very good understanding of the difference
between amateur radio towers and commercial towers and the planning officer
(zoning) was very encouraging. I need to jump through the building permit
hoops. There are no zoning restrictions. I'll do it their way and thank them
for their help.
I think it would be OK to suspend the upper mat by tie wires from the form
or boards placed across the form, and cut the wires flush after the concrete
sets, although I initially worried about the tie wire rusting and somehow
working its way to the rebar. I think it would probably be OK to cut the
wires right after the concrete is poured and push the wires into the plastic
concrete, as the top mat won't sink very far in thick concrete.
But what I plan to do is add 9 vertical bars to hold the top mat above the
bottom mat, perhaps add some diagonal cross bracing to reduce "racking", and
set the 9 vertical bars on 3" concrete blocks to hold the whole cage up so
that all the rebar is no closer than 3" to the edge of the concrete after
the pour. Another equally attractive idea is to build a "cage" out of
smaller rebar (say #3) with suitable cross members to make it rigid, and
then tie the specified 20 #5 elements to that cage.
I'm really grateful for all the ideas I've received. I feel that I am now
much better informed, and I feel much more confident about doing this
myself.
73 de Dick, K6KR
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Gary "Joe" Mayfield
Sent: Thursday, September 16, 2010 10:17 PM
To: 'Tower and HF antenna construction topics.'
Subject: Re: [TowerTalk] Newbee concrete rebar questions
Would you want to do this (vertical rebar) if it is not in the prime
directive? My simple mind would say if some rebar is good, more rebar is
better...... Is that really true, or should I do what the manufacturer
says?
Rohn, HDBX tower drawings do not show any vertical rebar, nor do the specs
call for it. In fact they claim you can use wire mesh instead of rebar if
the surface area is the same. I just figured the tower bolts were the
vertical component.
Correct me if I am wrong, but I thought when you put the concrete in, the
top 'mat' of rebar could be suspended with wires, and then the wires could
be removed as the concrete level reaches the appropriate height. I do not
think the rebar will work its way down through the concrete.
73,
Joe kk0sd
P.S. We'll see what everyone else says as I am wrong a lot!
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of Scott Reed
Sent: Thursday, September 16, 2010 3:30 PM
To: Tower and HF antenna construction topics.
Subject: Re: [TowerTalk] Newbee concrete rebar questions
I don't do towers on a regular basis, nor concrete, but from what I
have done, I have never seen a foundation specified with no vertical
rebar members. Stand the vertical members on the ground and as Perry
says tie them to the horizontal memebers. The place you get the rebar
should have "tie wire" as well. For the most part you wrap it
diagonally around the joint and twist it so it holds the horizontal
members where you want them.
On 9/16/2010 3:53 PM, Perry - K4PWO wrote:
> Even though not "specified", you can add vertical rebar to form a true
> "cage". Then you just support the rebar off the bottom of the hole. You
> would probably want at least one diagonal on each side to prevent
"racking"
> of the cage if you "tie" it together. A "X" would work even better.
> I had a basic welded "skeleton" of a "cube" built out of rebar and then
tied
> in the additional horizontal rebars at the bottom and top.
>
> 73 de Perry - K4PWO
>
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Dick Dievendorff
> Sent: Thursday, September 16, 2010 2:28 PM
> To: towertalk@contesting.com
> Subject: [TowerTalk] Newbee concrete rebar questions
>
> I have never personally installed rebar.
>
>
>
> My tower and guy anchor foundation drawings specify a "mat" of #5 (5/8")
> rebar , 5 rods each way, top& bottom, just below the top surface and just
> above the bottom surface of the concrete block.
>
>
>
> I know to suspend the bottom mat at least 3" above the bottom of the hole
> with little concrete blocks so that the bottom rebar mat ends up 3" above
> the bottom of the concrete so that it is adequately covered.
>
>
>
> I also need to suspend a top 5 x 5 mat about 1-1/2" to 2" below the top
> surface of the concrete block. If it were going to be later protected
from
> the elements, I'd be able to suspend it with wires from the wooden form
and
> cut the wires flush with the foundation after the concrete sets. But since
> this tower foundation is going to be exposed to the elements, I'm worried
> that even if I cut the wire off flush, the wires themselves will become a
> conduit for corrosion to get to the embedded rebar.
>
>
>
> What techniques are used to support the top "mat" so that it ends up about
> 1-1/2" below the top finish surface?
>
>
>
> I found "high support chairs" in some literature, but the top mat is about
> 28 inches from the bottom of the hole, and the "chairs" don't seem to be
> that high.
>
>
>
> Maybe I can find some vertical plastic rods to tie the top mat to the
bottom
> mat and the plastic rods could stick out the bottom.
>
>
>
> I'm sure this is a well-understood fairly basic technique by those who do
> this. But I need to be educated.
>
>
>
> 73 de Dick, K6KR
>
>
>
> _______________________________________________
>
>
>
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>
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>
>
>
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>
--
Scott Reed
Owner
NewWays Networking, LLC
Wireless Networking
Network Design, Installation and Administration
Mikrotik Advanced Certified
www.nwwnet.net
(765) 855-1060
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