T. Eric Gillham wrote:
"I am designing a foundation for an antenna dish. The dish itself is
parabolically curved, and has a dia of 36 ft, and it is 8 ft deep. It sits
atop a 5 ft dia cylinder that is 25 ft tall......The design wind speed I am
using (owner specified) is 180mph our design wind speed by law is only
155mph).......One final note: the antenna manufacturer submitted calcs for
the design of the bolts securing the base of the riser to the mat
foundation, and they list a service level overturning moment. The equations
used were supposedly developed at MIT, and I understand that they are
industry standard for this type of design."
If I was in your shoes I would be inclined to use the manufacturer's loading
provided it is well supported technically. Did they give you a copy of the
MIT report which they based their design on? You should probably ask for one
if they didn't.
Code type wind coefficients are crude. I would only use them as a rough
check if you have good information based on wind tunnel testing.
I do have a copy of an old 14 page "bulletin" titled "Wind Forces and Loads
Produced by Parabolic Antennas". It was published in 1987 by an antenna
manufacturer named "ANDREW" (whoever they are) which provides design
information for all angles of wind. It includes some general formulas of the
F=CAV^2 type with the coefficient C plotted graphically for wind angles from
0 to 180 degrees. 0 being full frontal on the antenna. It has further tables
for the company's particular antennae. They tend to be much smaller than
your 36 footer (generally 15 foot max). If you'd like a copy of this send me
your fax number and I'll fire one over.
As a final thought from the cold North, is the antenna subject to icing in
combination with high wind?