Just a bit of pencil scratching on the back of an envelope shows a base
like this to be far from overkill, especially considering it is based on
"normal" soil. When I drive a medium sized tractor around my place this time of
year and see 12 inch deep ruts happening I know my soil can't be "normal".
If one is fortunate to purchase really good concrete, it and associated
rebar will weigh in at about 4,000 pounds per yard or about 120,000 pounds in
this case.
On flat ground, no soil resistance, it will take 60,000 pounds of force to
just begin lifting one edge of the foundation. A tower planted in the
middle creates a two to one lever requiring 120,000 pounds of force to begin
lifting one edge of the foundation.
Assuming 90 mph winds, the tower alone, with no antennas could exert
upwards of 10 pounds per lineal foot and SSV tower may well be more than this.
But, for the back of envelope calcs we can use 10 pounds. At an average
height of 140/2 or 70 ft, this totals 1400 X 70 = 98,000 ft lbs of force.
That leaves 22,000 ft lbs for the antennas, rotator, mast and coax. Using
the same 90 mph wind speed which will impart a load of about 30 pounds per
square foot load, this allows 22,000 / 140 / 30 = 5.2 sq ft for the antennas
system.
This is all simple enough but only a trained engineer can do the exact and
proper calculations to take into account your soil conditions, wind speeds
in your area and then apply appropriate safety margins to keep you and
himself out of legal troubles in the event of a tower failure.
Reinforcement of the concrete is a critical issue. While all this 120,000
pounds of force is happening, the foundation has to remain one solid chunk
of material with no cracks. This is serious stuff with really bad
consequences if done improperly.
Overkill? I don't think so!
73,
Gerald K5GW
In a message dated 1/18/2011 7:18:36 A.M. Central Standard Time,
K7LXC@aol.com writes:
In a message dated 1/15/2011 7:22:06 P.M. Pacific Standard Time,
towertalk-request@contesting.com writes:
> Looking at the Rohn base specs, I am quite surprised at the
requirements,
assuming I read it right. For a base, without piers, the specs call for a
14'-3" X 14'-3" X 4' concrete base ( 30.1 cubic yards).
> To me that seems like considerable over kill.
Umm, so you're an engineer who has calculated the relative forces to
arrive at that conclusion? I suggest you follow the LXC Prime Directive
to
"DO what the manufacturer says" in order to construct a reliable and safe
installation.
Cheers,
Steve K7LXC
TOWER TECH -
Professional tower services for hams
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