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RE: What Exposure to use?

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Dennis, it has little to do with the "exposure" (the name is a misnomer,
IMO), but simply stated, with the "boundary layer" conditions. We deal with
wind as a fluid that is moving close to a "boundary"--that is, we deal with
the boundary layer.

The various "exposure levels" are actually estimates of "roughness" ("rough
estimates"?) of the surface over which the fluid passes. In a major urban
area, the surface is VERY "rough" (i.e. it has an average amplitude that is
significant); hence, "Exposure A" which is never used except for high-rise
construction in an urban center (for obvious reasons). "Exposure B"
represents urban/suburban "roughness" where you have a scattering of low
structure. "Exposure C" is actually a rural/plains area.

"Exposure D" essentially means NO obstruction to the wind (hence the
requirement for at least one mile "fetch").

In all these cases, it isn't the basic SPEED of the wind, but the shape of
the velocity curve within the boundary layer as it approaches the surface
itself. That's what the Kz value is for.

BTW, "Exposure A" is very rarely used, because nowadays a major high-rise is
going to be designed using the results of Boundary Layer Wind Tunnel
testing. And hardly anyone feels comfortable with Exposure B.

So, as a rule, Exposure C is pretty much it, unless you can justify using
Exposure D (only for coastal construction).

William L. Polhemus, Jr., P.E.
Polhemus Engineering Company
Katy, Texas
Phone 281-492-2251
Fax 281-492-8203

> -----Original Message-----
> From: Structuralist [mailto:dennis.wish(--nospam--at)]
> Sent: Tuesday, June 12, 2001 5:46 PM
> To: seaint(--nospam--at)
> Subject: RE: What Exposure to use?
> Thank all,
> 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.

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