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RE: What Exposure to use?

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Thank  you for the time you spent to organize this. It does help and
represents the comments I received from others. Although I don't want to
diminish the fine help you gave me, I do want to point out that I was
attempting to seek an "intent" which required "reading between the lines" so
to speak. Underlying my question (which Bill Polhemus has answered quite
well) is the key that water does not have the surface friction that will
slow down the wind as would sand or low protrusions (rocks, cactus etc.).
Roger Turk echoed my concerns - which is reasonable when you live in the
desert where you can have twenty miles of flat sandy area with no elevation
change or obstructions larger than a stone (such as on the Great Bonneville
Salt Flats).
I hope you see that while it appears to be a very straight forward
explanation, my childlike response has always been "Why?"
I think Scott Maxwell indicated the same explanation about the frictionless
quality of water as had Bill P.
Thank you again for taking considerable time to provide this chart to me.

Regards,
Dennis

> -----Original Message-----
> From: syed faiz ahmad [mailto:syedfaiz23(--nospam--at)hotmail.com]
> Sent: Wednesday, June 13, 2001 12:48 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: What Exposure to use?
>
>
> Gentlemen
> Iam impressed with this overwhelming response to this subject. Keep it up
> guys & congrats, of course for demonstration of this intellectual
> enthusiasm.
> Coming back to the subject perse, if i can describe the design problem it
> would be:
> - a sign
> - height                  = 25'-0"
> - signage surface area    = 60 sft
> -location:
>        -  on a road parallel to a free way
>        -  windy off the free way
>        -  free way lined with trees
>        -  area is open with industrial buildings close
>        - but the area is open for miles
>
> Well, Dennis, i have had experience of using UBC-94 & ANSI/ASCE  for
> lateral load design of structures. The description of Exposure conditions
> (for borh these Codes) are being reproduced below which make the issue
> crystal clear.
>
> EXPOSURE cATEGORY         UBC-94                   ANSI/ASCE
> ------------------------------------------!-----------------------
> --------EXPO-B
>     has terrain with bldgs, forests! Urban & suburban areas,
>           or surface irregularities,      ! wooded areas, or other terr-
>           covering at least 20% of the    ! ain with numerous closely
>           ground level area extending     ! spaced obstructions having
>           1 mile or more from the site.   ! the size of single family
>                                           ! Dwellings or larger.......
> ------------------------------------------!-----------------------
> ---------EXPO-C
>    has terrain which is flat &    ! open terrain with scattered
>           generally open, extending one-  ! obstructions having heights
>           mile  or more from the site in  ! generally less than 30 ft.
>           any full quadrant.              ! This category includes flat,
>                                           ! open country & grasslands.
> -------------------------------------------------------------------------
> -EXPO-D   represents the most sever expos-! flat, unobstructed coastal
>           ure in areas with basic wind    ! areas directly exposed to
>           speed of 80 mph or greater and  ! wind flowing over large
>           has terrain which is flat &     ! bodies of water. This exposu-
>           unobstructed facing large bodies! re shall be used for those
>           of water over one mile or more  ! areas representative of expo-
>           in width relative to any quad-  ! sure-D extending inland from
>           rant of the building site. expo-! the shoreline a distance of
>           sure-D extends inland from the  ! 1500 ft. or 10 times the
>           shoreline 1/4 mile or 10 times  ! height of the building or
>           the building height, whichever  ! structure, whichever is
>           is greater.                     ! greater.
> -------------------------------------------------------------------------
> So, you can now conclude as:
> - Expo-B is ruled out because:
>      - its not a terrain which covers 20% of the ground level area.
>        Nor, its urban or suburban area or wooded area. It also doesn't
>        have numerous closely spaced obstructions.
>
> - Expo-D is ruled out because:
>      - Its not the area which is facing large bodies of water.
> - Expo-C is more relevant & applicable because:
>      - The terrain in question is flat & generally open & as described
>        by you its extending miles together. All these description
>        qualify it fall in the Category of EXPOSURE-C & nothing else.
>
> Iam sure this fully explains & shall help in putting issue to bed!
> Best regards to all fellow Engineers.
>
> SYED FAIZ AHMAD, MENGG, Mem ASCE
> SENIOR STRUCTURAL ENGINEER
> SAUDI OGER LTD
> RIYADH, SAUDI ARABIA.
>
>
>
> >From: "Structuralist" <dennis.wish@gtenet>
> >Reply-To: seaint(--nospam--at)seaint.org
> >To: "SEAINT Listservice" <seaint(--nospam--at)seaint.org>
> >Subject: What Exposure to use?
> >Date: Tue, 12 Jun 2001 14:54:12 -0700
> >
> >This is the first time that I have had this one happen. I called
> the city
> >of
> >Indio California to obtain the local wind load design criteria.
> I was told
> >that to use an 80-mph wind, but when I asked what exposure to use, the
> >Building Official responded with - "Your the engineer, you
> determine that!
> >Use a minimum of at least Exposure C."
> >There is quite a difference between Exposure C and D which has
> me worried.
> >The sign is a 25'-0" tall pole with a 60-s.f. (6'-0" high x 10'-0" wide)
> >sign. The sign occurs in a windy off the freeway, however, the freeway is
> >lined with trees intending to create a wind break. This prevents
> sand from
> >blowing over and building up on the freeway.
> >How can an engineer determine the specific wind load conditions
> for a very
> >specific site if the building official will not provide the information?
> >
> >Any suggestions? The sign is for an Auto Body Repair store located on the
> >road running parallel to the freeway. The area is open with industrial
> >buildings close by, but the area is open for miles.
> >
> >Regards,
> >Dennis S. Wish, PE
> >Structural Engineering Consultant
> >mailto:structures(--nospam--at)engineer.com
> >(208) 361-5447 E-Fax
> >
> >
> >
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