From: "T. Eric Gillham PE" <teric(--nospam--at)gk2guam.com>
Date: Wed, 7 Mar 2001 16:20:29 +1000
I would like to hear some opinions on this
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
The design wind speed I am using (owner
specified) is 180mph (our design wind speed by law is only 155mph), and using
UBC94, I get 82psf as the wind stagnation pressure.
For the 180mph wind, the dish will be
"pointed" straight up and secured with locking pins. I am considering the
antenna to be a minor structure, and due to its curved nature, I am applying a
Cq factor of 1.4*(2/3)=.933.
So here is the question: Imagining the
antenna ready for a typhoon to be akin to a mushroom with an inverted cap,
should I be considering uplift on the bowl face of the dish?
Given that wind can pass both below and
above the dish, part of me says no, since this really isn't like a building that
completely blocks the wind. Furthermore, the bottom side of the dish is
curved, so perhaps this would be something along the lines of an inverted wing,
which would lead me to conclude that there may be some pull DOWNWARD.
However, the open top complicates matters.
Anyway, it would be great to hear from
someone who does this sort of analysis regularly, and anyone else's comments
would be more than welcome. 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. OTMs for the structure