Richard Elizondo
Senior Consultant
Ionos Communications
361-887-6700
<mailto:relizondo@ionoscom.com> relizondo@ionoscom.com
_____
From: Richard Elizondo [mailto:relizondo@ionoscom.com]
Sent: Friday, June 05, 2009 9:59 PM
To: 'kr7x1@verizon.net'
Subject: RE: Re: [TowerTalk] Engineering advice on above ground foundation
"In Rudy's case the section modulus (b x d x d / 6) of the foundation area
is 11x11x11 / 6 = 221 ft cu."
I think I am missing something here 11x11x11 ?
Secondly, I am not try to argue the fact that both of us actually came to
the same conclusion, maybe with different safety factors, but the block will
work,,,,, just needs some engineering advice due to weather concerns.
My calculation were based on worst case scenarios, assuming very little
interaction from his soil, similar to the self supported Valmont we built at
little elephant mountain for the US military.
I think we can both agree, we will see structural failure long before over
turning with this tower.
Just curious, Is it typically common to use a safety factor of 3 on this
type of structure?
Maybe I have just been working with Mark Malouf too long, but this seems a
little low for something with dynamic loading.
Richard
_____
From: kr7x1@verizon.net [mailto:kr7x1@verizon.net]
Sent: Friday, June 05, 2009 6:22 PM
To: relizondo@ionoscom.com
Cc: TOWERTALK@contesting.com
Subject: Re: Re: [TowerTalk] Engineering advice on above ground foundation
Richard:
You are wrong about this.
The resistance of a 11x11x5' block of concrete to over turning moment is the
area section modulus of the geometric area (in plan) of the foundation block
multiplied by the allowable bearing pressure of the soil underneath minus
the verical bearing pressure due to the weight of the tower, antennas and
foundation concrete.
In Rudy's case the section modulus (b x d x d / 6) of the foundation area is
11x11x11 / 6 = 221 ft cu. You can assume that the allowable bearing pressure
would be 2000 psf ( IBC 2006 Table 1804.2 class 4 soil ) this can be used
without soils testing. The bearing pressure under the foundation due to
vertical is (11x11x5x150 + 3000) / 11 x 11 =
750 psf so the net moment allowable pressure would be 2000 - 750 or 1250
psf. The moment that would develop this pressure is Fb=M/S or M = Fb x S.
M= 1250 x 221 = 276,000 ft- lbs with a safety factor of 3. This ,assuming a
90 foot lever arm without considering the wind force on the tower section,
would be a lateral force of 276,000 / 90 = 3067 lbs lateral at the 90'
level.
As you can see the foundation with respect to wind load on the tower is
adequate.It is the frost heave potential that will be the kicker.
Regards
Lonberg Design Group, Ltd.
H.S. Lonberg, P.E.,S.E. / KR7X
President
Jun 5, 2009 10:55:38 PM, relizondo@ionoscom.com wrote:
The following are some things that could help in the review process.
Type of concrete and rebar used.
Soil samples.
Pictures of the installation.
Was the excavation belled into the soil at the lowest part of the form?
>From what you have explained, her are my opinions, and only opinions, do
not
take my word as proof, have it reviewed by a PE.
Your foundation design is what is known as a "Gravity Block". It relies
primary on shear weight and very little on vertical earth frictions.
With that information in mind, we use the following gross mathematics:
11x11x5 = 22.41 Cu Yards of concrete
Weight assuming 3500psi or better concrete = 73953 lbs (not including
reinforcing steel - Rebar)
Using standard leverage formulas it would take 8.0kips of force at the tower
top to topple this foundation, however that has no assumed safety factor. If
we use the industry standard safety factor of 10-1 this leaves only an
acceptable overturning momentum force of .827 kips (approx 827 lbs). Now
these factors only hold in perfect world scenario, this foundation was
designed with very little room for safety/fudge factor. If your contractor
was a grade A guy and did everything by the book, in my opinion, and only my
opinion, this would be an acceptable amateur radio tower installation, since
it does not have to meet current Rev. G. industry standards. :)
However, please do not take my word for it. Have the construction and
engineering reviewed by a PE from your area, who is familiar with your local
weather concerns.
Richard Elizondo
Senior Consultant
Ionos Communications
361-887-6700
relizondo@ionoscom.com
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of scottw3tx@verizon.net
Sent: Friday, June 05, 2009 5:17 PM
To: john@kk9a.com; TOWERTALK@contesting.com
Cc: r_bakalov@yahoo.com
Subject: Re: [TowerTalk] Engineering advice on above ground foundation
I agree with KK9A. Your project needs to be reviewed/studied/stamped by a
PE licensed in your state before you do anything else! Safety is first,
second, and third! Don't risk anyones life or property with the existing
base!
73, Scott W3TX
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of john@kk9a.com
Sent: Friday, June 05, 2009 6:13 PM
To: TOWERTALK@contesting.com
Cc: r_bakalov@yahoo.com
Subject: Re: [TowerTalk] Engineering advice on above ground foundation
Instead of relying on the undisturbed earth to hold your tower in position,
you are basically relying on a 60,000 pound weight with a 10 foot square
footing to hold up a 90 foot tower. Since this is a drastic change from
the tower manufacture's design, you should not erect your tower until your
have an engineer evaluate your tower base. It's unfortunately that you have
gone this far with the project and probably spent thousands of dollars on
this base, but you can loose much more if it should fail. There may have
been other solutions or ways to install a base below the water table.
Perhaps AN Wireless can assist you with the base design?
John KK9A
To: towertalk@contesting.com
Subject: [TowerTalk] Engineering advice on above ground foundation
From: Rudy Bakalov
Date: Fri, 5 Jun 2009 11:41:58 -0700 (PDT)
I have a rather complex situation that certainly can benefit from
the collective wisdom and experience of this reflector.
My property has a very high water table and as a result my builder and I
decided to build the tower foundation mostly above ground. Specifically, we
dug
a hole, filled it with compacted gravel that is above the water table,
covered
the gravel with very thick sheets of plastic, and inserted 2' styrofoam
boards
around the base. Then we poured the concrete. As a result, only about 1.5'
of
the foundation is below ground, 3.5' are above. The overall dimensions are
10'
x 10' x 5'.
Now I wonder what are the additional engineering issues we need to address
to
make sure the foundation will last and the 90' AN Wireless tower will stay
up.
Currently, we have identified the following next steps:
1) Waterproof the concrete using silica based solutions, such as Penetron,
Xypex, etc. as permanent solution against water getting into the
foundation2)
Add extra waterproofing using tar/asphalt-like solutions to make sure we
have
extra protection should cracks emerge3) Add styrofoam boards to prevent
against
freezing (it gets as cold as -25F in Ontario)4) Backfill around the
structure
to create a mound that takes the rain and melting snow water away from the
concrete. I have also proposed we layer more plastic sheets about a foot
below
the mound surface to make sure no water gets near the foundation
Any further comments and suggestions will be greatly appreciated.
Rudy N2WQ
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