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Re: Antenna Dish

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     I would assume that it drains well in the vertical position.  The shallower the dish, the smaller the angle of attack is and the less predictable it is for uplift or down.  I was in my initiation hurricane in Houston years ago and watched the relative wind outside go from straight horizontal to about 15 deg upward from horizontal - this may have been due to the vertical walls of the building that I was in but then again it may not have been.  For a shallow dish I can see some possibility of attenuation which could make fatigue a factor.  The TIA is the code authority on tower structures.

Greg
    
-----Original Message-----
From: James Lane, P.E. <jamesalane(--nospam--at)hotmail.com>
To: seaint(--nospam--at)seaint.org <seaint(--nospam--at)seaint.org>
Date: Wednesday, March 07, 2001 7:04 PM
Subject: Re: Antenna Dish


>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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