# Antenna Dish

• To: "seaoc list" <seaint(--nospam--at)seaint.org>
• Subject: Antenna Dish
• 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 one:

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), 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 are:

MIT calcs -                             1200k-ft
UBC94 no uplift                      1000k-ft
UBC94 with uplift (Cq=.7)      1900k-ft