I know you say you don't want to deviate from Rohn's design but if I was
putting in this tower I'd be slightly apprehensive about this particular
footing design. There's no vertical steel, just the two horizontal mats. This
means that if any tension forces develop in the concrete, the tower is
depending on the tensile strength of concrete. Concrete by itself is strong in
compression but weak in tension. A lot of engineers believe the tensile
strength of concrete should be assumed to be zero because if you get a crack,
the tensile strength then becomes zero.
It could be this is an old design and Rohn has a newer design on file. It might
be worthwhile to call them and ask if they have an updated footing design with
vertical steel that forms a cage in all 3 dimensions.
A side benefit of having vertical bars is that it addresses your 2nd question
because you can wire tie the upper mat to them.
73 Jim K6OK
> Date: Mon, 11 Jun 2012 16:31:32 +0000
> From: Missouri Guy <n0tt1@juno.com>
> Subject: [TowerTalk] Concrete tower fountains plans...some
> questions
> To: towertalk@contesting.com
> Message-ID: <20120611.163132.2808.0.N0TT1@juno.com>
> Content-Type: text/plain; charset=us-ascii
>
> Hi Gents,
>
> I'm planning to erect a new 68' self-supporting BX tower and
> I'm
> going over Rhon's specifications for the concrete
> foundation.
> (I have NO desire to deviate from those specs.)
>
> A few questions:
>
> 1. There's a grid of 3/8" rebar about 3" up from the
> bottom
> and is about 4ft square and flat. What's the best
> way to support that off the bottom of the hole while
> the concrete is being poured around it? And will
> the supports allow water to enter and corrode the
> rebar?
>
> 2. There's also a grid of 3/8" rebar about 3" below
> the
> TOP surface of the foundation. The rebar is placed at
> 12" centers.
> I'm *thinking* that the grid can be suspended from the
> wooden
> jig I'll build to hold the anchor bolts firmly in place
> while the
> concrete is being poured. From your experience, would
> that be the best/easiest way?
>
> 3. I've read/heard somewhere that rebar in concrete
> tower foundations should be connected/bonded to the tower
> ground system. Seems logical, but is that true?
>
> 4. The Rohn spec sheet shows the TOP of the
> fountation
> 6" above grade. Why the 6"?...to allow for "growth" of
> the
> soil around it? Why not, say 3" and dig the foundation
>
> hole a little deeper?
>
> 5. The 1-inch anchor bolts have a nut and washer
> above
> and below the U-brackets that support the tower. Is
> it
> customary to place some anti-sieze compound on the
> threads to easily allow minor adjustments due to any
> settling
> of the foundation...say in 5-10 years or so?
>
> 73 and thanks for your input,
>
> Charlie, N0TT
>
>
> ------------------------------
>
> Message: 3
> Date: Mon, 11 Jun 2012 11:24:57 -0700
> From: David Gilbert <xdavid@cis-broadband.com>
> Subject: Re: [TowerTalk] Concrete tower fountains
> plans...some
> questions
> To: towertalk@contesting.com
> Message-ID: <4FD637F9.4040005@cis-broadband.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
>
> 1. Concrete is porous as all get out and water
> moisture will get to
> your rebar no matter how integral it is. It's the
> alkalinity in
> concrete that keeps the rebar from rusting. Typically
> a piece of
> concrete like a portion of a CMU block is used to support
> the rebar off
> the bottom of the hole so that the coefficients of expansion
> are similar.
>
> 2. Yes, suspending the rebar with steel wire
> works. Just make sure
> it's strong enough to withstand the force of the pour, and
> string the
> wires in a trapezoidal fashion so that the rebar can't swing
> side to side.
>
> 3. Yes .. bond the rebar to the tower base and its
> ground system. In
> most jurisdictions that is required by code.
>
> 4. Best to stay with the Rohn recommendations, but I'd
> go with six
> inches clearance from ground just to make sure there was
> never any
> standing water around my tower base.
>
> 5. Makes sense, but I don't see any downside to it so
> why not do it on
> general principle?
>
> 73,
> Dave AB7E
>
>
>
> On 6/11/2012 9:31 AM, Missouri Guy wrote:
> > Hi Gents,
> >
> > I'm planning to erect a new 68' self-supporting BX
> tower and I'm
> > going over Rhon's specifications for the concrete
> foundation.
> > (I have NO desire to deviate from those specs.)
> >
> > A few questions:
> >
> > 1. There's a grid of 3/8" rebar about 3" up from
> the bottom
> > and is about 4ft square and flat. What's the
> best
> > way to support that off the bottom of the hole while
> > the concrete is being poured around it? And will
> > the supports allow water to enter and corrode the
> rebar?
> >
> > 2. There's also a grid of 3/8" rebar about 3"
> below the
> > TOP surface of the foundation. The rebar is
> placed at 12" centers.
> > I'm *thinking* that the grid can be suspended from the
> wooden
> > jig I'll build to hold the anchor bolts firmly in place
> while the
> > concrete is being poured. From your experience,
> would
> > that be the best/easiest way?
> >
> > 3. I've read/heard somewhere that rebar in
> concrete
> > tower foundations should be connected/bonded to the
> tower
> > ground system. Seems logical, but is that true?
> >
> > 4. The Rohn spec sheet shows the TOP of the
> fountation
> > 6" above grade. Why the 6"?...to allow for
> "growth" of the
> > soil around it? Why not, say 3" and dig the
> foundation
> > hole a little deeper?
> >
> > 5. The 1-inch anchor bolts have a nut and washer
> above
> > and below the U-brackets that support the tower.
> Is it
> > customary to place some anti-sieze compound on the
> > threads to easily allow minor adjustments due to any
> settling
> > of the foundation...say in 5-10 years or so?
> >
> > 73 and thanks for your input,
> >
> > Charlie, N0TT
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
> >
>
>
> ------------------------------
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
> End of TowerTalk Digest, Vol 114, Issue 18
> ******************************************
>
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