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Re: What Exposure to use?

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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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