From: "T. Eric Gillham PE" <teric(--nospam--at)gk2guam.com>
Date: Thu, 8 Mar 2001 10:38:03 +1000
I agree, but the parameters of the design make this point moot. Basically,
the dish will be point straight up (giving the least area presented to
horizontal wind) whenever wind speeds exceed 45mph (or in ancticipation of
an approaching typhoon). I have checked the structure for the non typhoon
case, dish pointed sideways to present the biggest area, and it doesn't
begin to control over the OTM resulting from typhoon wind when in the
T. Eric Gillham PE
From: Nacionales Alex [mailto:acnacionales(--nospam--at)yahoo.com]
Sent: Wednesday, March 07, 2001 10:20 PM
Subject: Re: Antenna Dish
Since the antenna dish can be rotated to face to
different angles. Why not compute the OTM for worst
case position, facing horizontally would give the
--- "T. Eric Gillham PE" <teric(--nospam--at)gk2guam.com> 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
> 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
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