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Re: What Exposure to use?

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Dennis,

Exposure D is clearly defined with regards to large open bodies of water.
Large open bodies of sand where not considered by the code writers I am
sure, and I can see your analogy.  If you do not feel that the exposure C
80mph forces are appropriate, increase the wind speed.  There are areas of
Ontario along the 10 Freeway corridor that require exposure C 90mph as a
minimum.  I would definitely consider the 10 corridor as a special wind
zone.

You have a lot of control over how you apply the designated wind
requirements.  If you use Exposure C 80mph, you don't have to take the
stress range up to the maximum, you can limit deflection or stresses to
within a range you are comfortable with.  It is a question of economy and
engineering judgment.

Exposure D 80mph is only 25% more than Exposure C 80mph.  From 80 to 90 mph
is 25%.  A self mandated 25% reduction in allowable stresses achieves the
same thing. Pick a level you are comfortable with.

Paul Feather
San Diego, CA
----- Original Message -----
From: "Structuralist" <dennis.wish(--nospam--at)gte.net>
To: "SEAINT Listservice" <seaint(--nospam--at)seaint.org>
Sent: Tuesday, June 12, 2001 2:54 PM
Subject: What Exposure to use?


> 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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