re items
1. your concrete supply house should have plastic inverted basket
supports in different heights for rebar . These are used instead of the
concrete blocks with the wire tie imbedded in them, but either will
work. No rebar should be exposed to earth.
2 & 3 - K7LXC did much of my 2x HDX589 install. To support the 6 anchor
bolts, we bolted them to the base and then welded 1/2" rebar near the
top and the bottom, across all sides. This "subcage" was removed from
the base and then wired to the perimeter cage, actually in the 589 base
design the anchor bolts just fit inside the main cage. Since there is
no wood framing supporting the anchor bolts it is easier to pour and
finish the concrete BUT, make sure the tie wiring is really tight so the
bolt cage can't move. All the anchor bolts and the main cage were
wired/welded together, so we created a pretty good Ufer ground system.
You can add more rebar as vertical ties between the top and bottom
cages, several (8x #6 IIRC for the 589) of those are standard on the
larger UST tower bases, it is surprising they aren't called out for the
BX. Diagonal braces are also good, wire tied to keep the cage from
distorting when placed. You don't want rebar moving around when pouring
the concrete or be wondering later during a big storm if it stayed in
the right place. If the anchor bolts move, you could be in big
trouble. As long as any rebar is about 3" from the concrete edge, you'd
have to go really crazy to add too much rebar.
4 - I think the objective is to have good drainage/drying and no
vegetation/debris around the bolts/legs.
5 - Never-Seez really lubricates and without double nuts or wired nuts,
I think the risks of loosening outweigh the benefit. My 589s had no
torque specs for the nuts, so I called the factory and talked to
engineering. Their spec (literally) "apply the maximum torque that a
grown man can apply". hmmm... Check the specs for the grade steel
bolts you have (should be on the prints) on the Portland Bolt web site
and you will be amazed at the torque levels they specify, about 800
ft-lbs for my bolts. I could have done that, but decided that 175
ft-lbs was the high side of the UST "spec" and used a torque wrench for
the anchor bolts and pivot bolts to that torque. Does Rohn have a
torque spec?
Rent a concrete vibrator to compact the pour and remove air pockets. If
you have any forms you need to not overdo the vibrating near them or you
will blow out the form.
Have fun!
Grant KZ1W
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
> _______________________________________________
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