Brian, faced with a similar problem and using brute force and awkwardness as
a substitute for engineering training, I elected to excavate 4 trenches
radiating from the central excavation. I then put rebar into these 4
trenches, 4 horizontals ringed with stirrups and tied into the central cage.
These radial legs (props?) were about 16 inches wide and 36 inches deep
where they attached to the cage and tapered to 24 inches at the outer ends.
These "legs" were 3 ft long from outer end to central cage and the central
"rectangular lump" was 4x4 ft horizontally and 5 ft deep. The intent of the
radial legs was to provide leverage against tipping. Our code frost depth is
18 inches. We are in a rural area and have no permits or inspections. II
My motivation was to make better use of materials than just a big lump.
Would something like this be useful to the OP of this thread? For that
matter, is this approach useful at all in your professional opinion for
getting more performance for your buck in materials when depth of digging is
a cost factor?
Patrick NJ5G
-----Original Message-----
From: Brian Amos
Sent: Friday, January 10, 2014 5:05 PM
To: J Chaloupka
Cc: towertalk@contesting.com ; Jim Lux
Subject: Re: [TowerTalk] Base equivalent
As a foundation engineer my first thought is wow! I think that foundation
is way over designed. My second thought is if you have water at 5 feet you
may be better off having the backhoe guy bring a drill instead. A drilled
pier would possibly give you more bang for your buck.
Really calling a local geotechnical engineering firm is your best bet. I do
that sort analysis all the time. Someone familiar with the soil at your
location will know exactly what you need. And a couple hundred dollars is
about the right price point. You may even be able to save enough on
construction costs that you come out ahead.
In general a larger foundation area will give you more bang for your buck
than a deeper one. But if you get too shallow then you will need to put
some steel in the top and bottom of your foundation so the moment loads are
properly distributed in the slab.
A tower is all lateral and moment load on the foundation and very little
gravity loads. Your 4 yrds of concrete will weigh more than than your
tower. So a larger base will more than likely be better than the
recommended one regardless of depth as you will have more area to
distribute the loads. The depth is likely a frost protection method more
than anything.
Again I can't reiterate enough talking to a local engineer. Foundations are
very regional. Ones I design here in Arizona and Utah will likely be
different than ones my friends in Wisconsin design.
Brian
KF7OVD
On Jan 10, 2014 3:26 PM, "J Chaloupka" <boltsnutspins@yahoo.com> wrote:
If it were my dilema, I would go back to the tower manufacturer and tell
them my situation. Ask for a modified and certified specification for
the
specific instalation.
JIM
________________________________
From: Jim Lux <jimlux@earthlink.net>
To: towertalk@contesting.com
Sent: Friday, January 10, 2014 4:59 PM
Subject: Re: [TowerTalk] Base equivalent
On 1/10/14 1:28 PM, Mike Reublin NF4L wrote:
> 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?
>
I just thought of a better analogy than my straw in a milkshake one..
For a freestanding tower, the base is more about resisting movement, not
so much bulk and mass.
Compare a tootsie-pop and a long thin peppermint stick, each shoved
half-way into some stiff pudding or cake. The tootsie-pop is heavier,
but it's round and there's not much resistance to tipping the stick
over. The peppermint stick might be the same weight (in the "soil"),
but is harder to push over.
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