[TowerTalk] Base equivalent

Sun Jan 12 01:30:26 EST 2014

Patrick

That sounds like quite the interesting setup. How well it works really
depends on the soil conditions and the design loads. Having a reinforced
foundation goes a long way. In this case the extra legs would probably add
stability, especially in soft ground conditions.

The well point is a fancy dewatering well, which has been in use for a very
long time. They just keep the water out of the hole.  Typically they are
used where you have a very large excavation for a basement.  I guess you
could use it for the tiny excavation needed for a tower but I think if you
are going to that expense a driven pile or drilled pier might be a cheaper
alternative. They use very similar equipment to install a well as to
install a drilled pier.

In all honesty many things work great in one situation where they don't
work at all in others (dewatering wells in clay are worthless, for example,
but in sandy soils they are a godsend). Its kind of like antennas: what
works great for some people doesn't work at all for others. It all depends
on each specific set of conditions. That is why I struggle with the idea of
a manufacturer providing specific foundation instructions when each site is
different and has different conditions. Considering the cost of a good
tower I would think they could at least provide the buyer with a
personalized set of recommendations for the foundation, or drop the price a
bit and recommend you seek recommendations from an engineer familiar with
your area.

Just my two cents.

Brian
KF7OVD
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 at 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 at 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 at earthlink.net>
> To: towertalk at 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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