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RE: What Exposure to use?

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Thanks Roger,
I don't feel so "ignorant" asking for the intent of the code as I feared
others might have considered what seems so obvious to them. However, in
almost twenty years of engineering, I have lived most of those years in a
city where the issue never came up. Now, like you, I have seen the effect of
a sand storm and open exposure.
I think Bill Polhemus answered it very well and Scott Maxwell insinuated the
same argument that water reduces the friction on the wind as protrusions
from even scrub oak and sand can do on land. In private conversations, I
asked if there was any difference between the wind off the Great Salt Lake
or that in the Bonneville Salt Flats. This would, at least, tell us how the
building departments in the Salt Lake Utah area consider the effects of wind
in these regions.

Thanks again,
Dennis

> -----Original Message-----
> From: Roger Turk [mailto:73527.1356(--nospam--at)compuserve.com]
> Sent: Tuesday, June 12, 2001 4:43 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: What Exposure to use?
>
>
> Dennis,
>
> I have the same questions.  I was in southeastern Arizona and
> could look out
> across the valley for 40 miles or more with nothing obstructing my vision
> or the wind.  It was the same as standing on the sand dunes of
> the barrier
> islands on the south shore of Long Island, N. Y. and looking out over the
> Atlantic Ocean.  I have stood on a bluff on the north side of
> Tucson that was
> 100 feet or more above the valley floor and pondered this wasn't any
> different than the top of a 10-story building.
>
> Basically, I consider the site and use what I feel represents the
> conditions.  If I thought that Exposure D was most appropriate, I
> would use
> it.
>
> I would not try to cut it too tight with pole signs.  Traffic signs
> (admittedly not the most wind resistant in the world) have a tremendous
> amount
> of rotational flutter in even the lightest of breezes.  That the
> flutter is
> not ordinarily visible in pole signs does not mean that the same
> forces are
> not there.
>
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona
>
> Dennis Wish wrote:
>
> . > This is where I started to read more into the code than was
> written. For
> . > my own edification, why is there a likelihood of greater wind
> exposure
> . > over a body of water for say a mile than in the middle of a
> desert for
> . > the same distance. I suppose this is what I did not
> understand. In most
> . > case we use Exposure C in areas where there are homes  - even
> farm homes
> . > in surrounding areas where I can see that we take into consideration
> . > trees, houses and other structures that can block a gust of wind.
>
> . > However, here in the low desert it is very likely to have one
> building and
> . > nothing taller than a rock for a mile or more. Although I
> read the code
> . > for what it said, I wondered if the interpretation was such
> that only a
> . > body of water is considered unobstructed by anything that
> might act to
> . > reduce or deflect the wind from a structure. If this is true,
> why would
> . > it not be true in an open area such as might be found in the
> Dakota's, or
> . > the Panhandle and deserts known as "Dust Bowls".
>
> . > I hope you see my point - it's simply one of learning.
>
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