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RE: What Exposure to use?

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Thanks Bill,
This is the information I needed. Scott Maxwell also eluded to the
possibility that the water had a "lubricating" effect that helped to reduce
the friction of the wind against the surface.

I hope those who are reading this understand that it isn't simply my
ignorance of not understanding the word of the code. However, there is an
intent hidden in the rhetoric and some have interpreted the solution based
on conservative results - in other words, you can't be blamed for being
overly conservative. You can, though, lose a client who pre-bids the work
based on materials that they are used to providing for similar projects
designed by other engineers when you cut into their profit margin without
sufficient intelligent argument.

I would rather understand the intent of the rhetoric than to make a blind
assumption that overly conservative is better. There is an inherent fear of
retribution in the code that leads many to choose the more conservative
decision rather than discovering and arguing for a interpretation of the
code as a reasonable standard of practice. This is what your explanation has
provided - a reason make a decision without arbitrarily seeking conservative
at the expense of my clients.

This goes back to our expectation of the wood section of the code - why is
it not reasonable to expect clear and concise interpretation or explanation
of the original code without leaving us in limbo for two years to fend for
ourselves or simply choose the conservative solution and let our clients pay
the price?

Sorry - I'm on the soap box again.

Thanks Bill,
Dennis

> -----Original Message-----
> From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
> Sent: Tuesday, June 12, 2001 5:24 PM
> To: seaint(--nospam--at)seaint.org
> Subject: RE: What Exposure to use?
>
>
> 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)gte.net]
> > Sent: Tuesday, June 12, 2001 5:46 PM
> > To: seaint(--nospam--at)seaint.org
> > 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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