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RE: More Wind Stuff - Canopy Wind Loading/Recurrence Interval

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I would agree with Greg.  The canopy would not fit the definition of
"partially enclosed" in ASCE 7-98.  As Greg pointed out, a "partially
enclosed" structure under ASCE 7-98 requires that "...the percentage of
openings in the balance of the building envelope does not exceed  20%."
("the balance of the building envelope" in this case means the sides of
the building that DOES NOT receive a positive external pressure; i.e. the
two sidewalls and leeward wall).  It would also not fit the definition of
an "open" structure since each wall must have "...at least 80% open.",
unless the canopy is completely free standing and far enough away from the
structure for the "backside" to be considered "open", which is unlikely.

Thus, the canopy should be designed as an "enclosed" structure.  I
would, however, likely use the provision for Roof Overhangs in section
6.5.11.4 since the "typical" enclosed structure would _NOT_ account for
any upward pressure created on the underside of the canopy.
Unfortunately, I did not notice anything in the commentary about the use
this section, so it is just my opinion.  As a result of this provision, it
is entirely possible that the uplift loads may be similar (or even higher)
to the "partially enclosed" structure (I can't really tell with out
running the numbers, but a Cp of .8 for the positive pressure on the
underside will likely make the uplift load rather hefty).

HTH,

Scott
Ypsilanti, MI


On Fri, 19 Jul 2002, Effland, Greg wrote:

> 
> First thing I would do is double check the definition in the specificed code
> (ASCE7-98, IBC 2000, or other specified code) of "partially enclosed."  In
> most of the codes I am familiar with, "Partially Enclosed" would not be
> justified for a canopy with 3 open sides.  Most of the time they would have
> 1 wall with a minimum of xx% open with the maximum % of wall openings in
> other walls limited.  Basically I see the "Partially Enclosed" category as a
> bag.  If the wind can blow in and have virtually no where to go, like
> blowing into a bag, then it seems reasonable.  On a canopy with 3 sides open
> the wind can simply go around the walls and will not be forced to fill the
> canopy area like a bag/baloon.  Most likely this would fall in either Open
> or Enclosed (refer to the specific code, as not every code has all 3
> categories either).  These other categories would decrease your design wind
> pressures.  In some cases Open might use less pressure than Enclosed...
> Personally for those cases I would go ahead and design for enclosed but
> there is nothing in the code requiring that.
> 



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