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

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HTH,
Greg Effland, P.E.
KC MO USA

-----Original Message-----
From: Bill Polhemus [mailto:bill(--nospam--at)polhemus.cc]
Sent: Thursday, July 18, 2002 4:15 PM
To: seaint(--nospam--at)seaint.org
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!

****************************************************************

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.

****************************************************************

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.

************************************

I am not aware of any special allowances for small stand-alone canopies

************************************

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?

**************************************

Typically PEMB companies use the 10 year wind for deflection calculations as
per the recommendations of the AISC design guide.  As always, engineering
judgement should be used and applied per the specific case to determine if a
more severe wind should be used.

If I recall correctly, ASCE7-9x has a table in the book (back I think) which
converts their wind interval into a 10 year wind and, IIRC, other wind
intervals also.  I can not recall off the top of my head what the conversion
is for ASCE7-98 (seems like +-.65) but with the older codes using fastest
mile winds vs 3 sec gusts, the conversion from a 50 year wind to a 10 year
wind was approximated by 10Yr = 0.75*50Yr (for pressure, not wind speed...
i.e. pressure is proportional to wind speed ^2).

**************************************

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".

**************************************

Don't know your actual specifics but a common bracing scheme that could be
used is partial height bracing from the eave to the bottom of a
facade/fascia (if it exists on this canopy).  Then use the bending
capacities of the columns to transfer the force the rest of the way to the
footings.  If you are doing this in both directions than you may want to
consider using square tubes (or rectangular) as the columns.

**************************************

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)polhemus.cc



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