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RE: IBC 2007 Wind calcs.

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The issue is that what was in the UBC for wind was rather straight forward.
It was simple calculation of P = Ce*Cq*qs*Iw, where:

P is the design pressure
Ce is a factor to take into account exposure type (i.e. rural with no
obstructions, urban with similar obstructions, etc), 	gust effects and
Cq is a pressure cofficient for the structure or the portion of the
structure under cosideration qs is a basic wind pressure that is a function
of the wind speed (basically 0.00256 * (wind velocity)^2) Iw is an
importance factor

It was a very simple, straight forward process.

ASCE 7 has always been a little more complicated historically.  It has been
rather similar for MWFRS pressures, but had a separate method for Components
& Cladding (C&C) that took into accound the tributary area of the element.
Over all, that has been a major difference between the UBC implementation
and ASCE 7...effects of the "wind sail size" for what ever element that is
under consideration.  Personally, this is where I think the UBC has always
been weak.  Generally, speaking, however, the older ASCE 7 versions were
only a little more complex than what was in the UBC (it tended to look a lot
more complex due to the nomenclature...the "variables" appear more "complex"
or "bigger", but in reality it is not that much differnet).

The more current versions of ASCE 7 have mucked things up quite a bit.
There is now a Kzt factor which takes into account topographic effects (i.e.
hills, enscarpments, etc and how they speed up wind, etc).  They have added
in a method for flexible buildings which literally feels like rocket science
to me.  They have move to more variances in pressures for MWFRS over
different part of the structure surface.  In general, the methods have
gotten more complex and require more calculations and thought...more
variables in the formula...more coefficients to look up in a wider array of
tables and figures.  Not to mention the organization of ASCE 7 leaves a lot
to be desired at times.


Adrian, MI

-----Original Message-----
From: Gary L. Hodgson and Assoc. [mailto:ghodgson(--nospam--at)] 
Sent: Friday, February 15, 2008 8:03 AM
To: seaint(--nospam--at)
Subject: Re: IBC 2007 Wind calcs.

I have been sort of following this wind discussion and wonder if you 
could enlighten me. Since we don't use your codes, I am wondering what 
it is about these new ASCE provisions that has got all your knickers in 
a knot.  Can you send an example or copy of the particular section.  If 
it is too big, don't bother.  I am just curious.

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