To go that thin I would recommend using a number 6 or larger rebar on the
top and bottom. At that point you might be better off calling a local
concrete company to put the foundation in for you. They will be familiar
with local codes and how things are done in your area. I know that this
defies the do it yourself spirit but if you have a friend who is a
foundation engineer they may tell you how to do it safely. If you were
anywhere in AZ I would be more than happy to.
In reality most building codes will overdesign a tower foundation. There
are two failure criterion: falling over and settling. I guess you could
include lifting out of the ground as one as well. In places with little to
no seismic activity and deep groundwater falling over is unlikely as the
bearing capacity is very high as long as the concrete can absorb the
moments. A 4x4x7 footing would use its own gravity to do so in most
locales. If it settles...who cares. If it drops a couple inches the coax
and antennas may slack but the tower itself will be fine. If you have
earthquakes, or groundwater within the bearing zone, then falling over
becomes a concern.
Falling over is when people and things get hurt. If you live one a huge
vacant landscape where no one will get hurt if it falls then by all means
do whatever you like. If it falls and kills someone or damages a house then
you are liable. If you had a foundation engineer puts his stamp on a
document telling you what to do then he is liable provided it was build
just how he said, which is pretty easy to prove. If its not at least
permitted then your insurance may not cover any liability either.
I am a foundation engineer so I would feel comfortable designing the
foundation myself. I would way overdesign it because if it fell on one of
my kids I wouldn't be able to live with myself. I live on a large lot as
well so I have some buffer between my tower and my neighbors.
I do recommend calling a foundation or geotechnical engineer. A structural
engineer will design the steel in your foundation and that is all. The
geotechnical engineer knows the soil conditions. And is able to accept
liability for the design. A structural engineer worth his salt would call a
geotechnical engineer. Who is also capable of designing the steel in your
foundation.
On the topic of drilled piers, home depot rents a hydraulic anger with
enough auger to go 10 ft. Not a shovel and pick option, but cheaper than a
backhoe. And a much better foundation in most cases. I would batter the 4
piers at 15 degrees, put one in each direction, then use a pile cap to join
them together. A single #10 bar down the middle bonded to the steel in the
cap with the tower bottom bonded to the steel in the pile cap as well
would Ljubljana e the thing bullet proof in 90 percent of soil conditions.
After thinking about it I think the manufacturers recommend a raft
foundation which uses its own gravity to resist the loads and doesn't
transfer lateral loads onto the soil around it. Using the straw analogy
previously used, imagine a plastic straw embedded in a concrete block 4
inches cubed. Set it in a bucket of your favorite soil and try to blow it
over. Then shake the hell out of the bucket in a side to side manner. It
should stay put. The only problem would come when its on wet clay. That
would be more like putting the thing in a bucket of jello. It may fall
over at that point. But the clay will at least dampen the earthquake loads.
Earthquakes in saturated sands and up to fine gravel will cause something
called liquefaction. It basically turns the soil into quicksand. That's bad
and could also cause it to tip over.
An engineer familiar with your soil will know what to do.
Brian
KF7OVD
On Jan 11, 2014 6:40 AM, <john@kk9a.com> wrote:
> Would a hole 8 x 8 x 3 (5.3 yards) be a safe equivalent? I doubt that
> anyone on towertalk could answer your question without studying the soil
> conditions and the unmentioned tower that goes on it. It is probably less
> expensive to find a backhoe that can dig a little deeper or perhaps you
> can shore up the hole to safely dig by hand.
>
> John KK9A
>
>
> To: towertalk reflector <towertalk@contesting.com>
> Subject: [TowerTalk] Base equivalent
> From: Mike Reublin NF4L <nf4l@comcast.net>
> Date: Fri, 10 Jan 2014 16:28:11 -0500
> List-post: <towertalk@contesting.com">mailto:
> towertalk@contesting.com>
> The specs for my crank up call for a hole 4x4x7.5 feet. That works out to
> 4.4
> yds of concrete. The smaller backhoes that I can get to the site can go to
> a
> depth of 6 feet. Because of the water table, I consider it an unacceptable
> risk
> to put a man in the hole to dig out to 7.5 feet deep.
>
> A hole 5x5x6 is 5.5 yds.
>
> Would that be a safe equivalent to hold the tower up with the same wind
> load?
>
> _______________________________________________
>
>
>
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>
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