JIMMIE wrote:
On Mar 9, 12:53 pm, Jim Lux wrote:
MTV wrote:
Allodoxaphobia wrote:
On Sat, 07 Mar 2009 15:03:33 -0600, MTV wrote:
Phase 2 was first digging the 36' x 36" x 45" hole for the base, then
securing the tower rod base and pouring the concrete.
A 36 foot deep hole ought to hold up a freestanding 100' tower --
or more! :-)
It's actually 52" deep; the tower mfgr said min. of 45"; ARRL handbook
says 6' but only 30" sq, depending on the type of soil.
I don't know that I'd use the ARRL handbook as a source for
structural/civil engineering data. The figures in the "assembling a
station" chapter in my 1990 handbook are just an example for a TriEx
LM-470 or for a Wilson ST-77B, not a general recommendation for all
towers. In fact, the last sentence in the section is:
"Once you have that information, contact the engineering department of
your tower manufacturer or a civil engineer"

Following the mfr recommendations is probably the best bet. (and that's
if they're the *current* recommendations.. engineering and construction
standards are always evolving, what was acceptable in 1960 might not be
acceptable today)

That said, there's a lot of possible tradeoffs in tower bases, even for
the same kind of tower in the same location. skinny and deep vs wide and
shallow is one. If you're not taking the manufacturer's recommendation,
then you probably need to get someone with some engineering expertise to
tell you whether what you want to do is reasonable.

The design of the base depends a lot on the local soil. An antenna
installed in
the red clay of North Carolina may be a lot smaller than one on a
Florida sand hill.
Local guidance should always be obtained. If you wing it on your on
you may
be held responsible for damage to your neighbor's home, insurance may
not
pay for damages. I was advised to get a permit when I contacted the
city engineers
office about installing an antenna. If you get a permit and install
the antenna properly
and it falls on your neighbor's house their insurance pays, you are
not responsible.
On the otherhand if you didnt do it by the numbers you can be held
responsible
and your insurance may not help you.


Jimmie

Regardless of the soil that your tower is sited upon, the total mass of
the concrete must exceed the weight of the tower, the antenna's fixed
upon and the estimated wind force that the tower will see. This is so
that the center of gravity of the tower/antenna system is below ground
level.
Thirty yeas ago my local radio club installed a new tower. The tower was
180 feet tall with a 4 bay antenna. We had a hole 10feet on a side dug
and filled with concrete. When the tower did fall it bent in two about
80 feet up and folded over. The base formed the bottom of the
replacement tower and is still in service.

Dave WD9BDZ