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

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See below.

Harold O. Sprague

> -----Original Message-----
> From:	Bill Polhemus [SMTP:bill(--nospam--at)]
> Sent:	Thursday, July 18, 2002 4:15 PM
> To:	seaint(--nospam--at)
> Subject:	More Wind Stuff - Canopy Wind Loading/Recurrence Interval
> I'm having a difficult time designing a canopy at the entrance of a mall
> building. In form it is simply four posts with a gable roof. I'm making
> the
> structural system out of CFS, and it has cladding such as stucco or EIFS
> and
> a steel panel roof.
> The problem is that the wind load calculated from ASCE 7-98 kicks this
> thing's butt.
> Since the canopy is right up against a building, I'm using the "partially
> enclosed" option. There is a TREMENDOUS amount of uplift, though, such
> that
> the windward legs will see almost 10,000 lbs of uplift EACH! This is just
> going to be ridiculous for the foundation design, which is going to have
> to
> be drilled shaft footings if these forces are rational. I'm sure these
> folks
> are NOT going to want to call out a foundation crew just for four footing!
> In addition using the CFS is giving me outrageous lateral wind
> deflections.
> So my questions are these:
> 1. What if any special allowances are available for a small stand-alone
> canopy structure like this? Use a lower exposure factor? What? The ASCE 7
> code is silent on this score.  
	You could call it an agricultural structure.   Just kidding.  There
is no good guidance here.  The pre-engineered metal bldg people have
contended with this for years.  Drop your spread footings down.  Through in
some top bar, and count on the soil and slab on top of the footing.

> 2. Instead of the 100 year recurrence interval for determining the
> deflection (which if I understand the ASCE 7 commentary, is what is used
> along the gulf coast in order to generate the wind speed contours), may I
> use a lower interval such as 25, 30, 15, even 10 years? If so, where would
> I
> go to figure out how to adjust the wind speed for a different recurrence
> interval?
	The AISC design guide on serviceability gives some guidance, but not
for canopy structures.  The pre-engineered metal buildings people are all
over the map on this one.  Some hold it to h/30 some h/100 and they use a 10
year wind.  There is no requirement for lateral drift for wind.  I wouldn't
have a problem with a simple canopy going to h/30 with a 10 year wind.  Go
to the commentary of ASCE 7 Sect. C6.5.5.  The ASCE 7-95 was a little easier
to follow.  Check it out in the commentary in Table C6-5.  

> As it is, I may have to rely on the existing structure for lateral
> support,
> and that will mean lots of field measurements, etc. This was supposed to
> be
> a quick 'n' dirty job, but I'm find it to be mostly just "dirty".
> Any comments would be appreciated.
> William L. Polhemus, Jr. P.E.
> Polhemus Engineering Company
> Katy, TX, USA
> Phone (281) 492-2251
> FAX (281) 492-8203
> email bill(--nospam--at)

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