Return to index: [Subject] [Thread] [Date] [Author]

Re: Antenna Dish

[Subject Prev][Subject Next][Thread Prev][Thread Next]
usually the manufacturer of the antenna will provide a tabulated data regarding the dish's various positions and the corresponding forces on its supports.

arman rivera

seaint(--nospam--at) wrote:

>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