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Re: Antenna Dish

[Subject Prev][Subject Next][Thread Prev][Thread Next] Is that a 3sec wind speed,fastest mile,fastest 1/4 mile (tornado), or 1 minute average (Hurricane)? Ask owner what and why 180mph? Is it for Hurricane or tornado forces? This thing also if used for communications needs a high importance factor. Use the Durst curve in ASCE7-98 to convert tornado, hurricane, or 3sec gust speeds into fastest mile so you can design with UBC 94 or appropriate code.
My thoughts on the uplift VS downward force.
Have you ever tried to throw a Frisbee upside down?
Correct me if I am wrong but I believe it falls to the ground with minimal if any lift, so that would tell me that with the antenna facing up you would be inclined to get downward forces from the wind. The wind gets caught on the inside of the bowl and has a hard time escaping while the wind on the bottom of the bowl easily flows around.
Hope these comments help.


From: "T. Eric Gillham PE" <teric(--nospam--at)gk2guam.com>
Reply-To: seaint(--nospam--at)seaint.org
To: "seaoc list" <seaint(--nospam--at)seaint.org>
Subject: Antenna Dish
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

Thanks in advance for any comments.

T. Eric R. Gillham PE
teric(--nospam--at)gk2guam.com


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