1. Are you sure that the water table will be so shallow at your location?
2. Consider someone with a larger backhoe. A mid-sized hoe can easily get
to the depth you need.
3. Once you dig down past the top soil, if the ground conditions are so
loose that material is sloughing off so much as to make the excavation
unsafe, then the "manufacturer foundation" calculations probably aren't
valid for your soil conditions anyway.
4. Consider shoring up the sides of the excavation - it's the safest way.
5. I have successfully used foundations with "reasonably" different
dimensions based on static overturning (tip-over) of the foundation without
any credit for the benefits of compressive soil strength. This will give a
design that uses more concrete but is conservative. Assuming the tower is
centered on the foundation the formula is:
S = 2 x M/W
Where:
W = total weight in lbs of concrete foundation ( approx. 140 x depth x S x
S)
M = design overturning moment in ft-lbs of tower at max wind load (from
tower mfg)
S = width in ft of the base (shortest side if rectangular)
If you don't know M, a 4 x 4 x 7.5 foundation would normally be good for
about 35,000 ft-lbs., but recommend that you confirm this with tower mfg.
Disclaimer: Follow the requirements of your local jurisdiction, it is best
to consult with a local structural engineer even if not required by the
jurisdiction.
I hope some of these ideas may help and good luck with your project.
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Mike
Reublin NF4L
Sent: Friday, January 10, 2014 3:28 PM
To: towertalk reflector
Subject: [TowerTalk] Base equivalent
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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