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Re: [Fwd: Wind Load on Parapets]

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Reza,

	I would deal with your design as follows:

For Chilliwack, q(1/10) = 0.48 kPa = 9.94 p.s.f.
		q(1/30) = 1.63 kPa = 13 p.s.f.

	The wall is "on the ground" unless there is sufficient space between
the roof and the parapet that significant wind can pass under in order
to increase suction on back side (I'll leave it to you to decide what
"sufficient" and "significant" mean and how to interpolate between "on
the ground" and "above the ground" if there is a small gap).   Cf = 1.2

	Cg = 2.0 for structural framework (you use q(1/30) for structural
framework design); and Cg = 2.5 for cladding (you use q(1/10) for
cladding design) using the simplified procedure.

	Note: if you were designing a 20 + story building (which I doubt for
Chilliwack) and if you were using a detailed approach for the gust
factor for the building you could still make a good case for using Cg =
2.0 for design of the parapet framework.  The detailed approach is used
to determine peak dynamic response of the "building" to variations in
the wind speed; since Hook's Law applies to the building framework the
stresses in the framework will be maximum when the building reaches its
maximum lateral deflection (or drift).  Deflections of the parapet are
unlikely to correspond to deflections of the main building, therefore,
even if you determined Cg for the building to be 3.0 + you would
probably still use 2.0 for the parapet.

	The force on your wall would now be

	f = 13*1.2*13 = 31.2 p.s.f (total for both sides)

for design of the structural framework.

	For cladding you would use 2.5*9.94*Cp, where Cp is the appropriate
shape factor for the specific cladding location.  Cp would probably be
about 0.9 on the up wind side and 0.5 on the down wind side or perhaps a
bit more at the edges.

	As an aside to our American friends, this procedure is probably (in
theory, at least) superior to either the "three second gust" or the
"fastest mile" approach to wind loading design because it tailors the
wind loading to the specific building or structure being designed.  In
practice, however, there's probably not that much difference since the
shape factors are often one digit values, (+-) 10% or 20%.

	I hope this is helpful.

				Regards,

				H. Daryl Richardson

Reza Dashti Asl wrote:
>
> Daryl,
> Thanks for your input. I guess my problem is that this gives a very high
> load on parapet. Using free standing wall table:
>
> Fn=Cf.q.Cg.Ce.h.L  (h and L=height and length of parapet)
>
> Cf=2.0 for L/h>10  for walls above ground
> Cf=1.2 for L/h>10  for walls on the ground
>
> Ce=1, Cg=2.5, q=13 psf (for Chilliwack, BC)
>
> Fn=5q.h.L  ==> Fn=65 psf   for walls above ground
> Fn=3q.h.L  ==> Fn=39 psf   for walls on the ground
>
> Considering the factor given in UBC-97 for parapet walls=1.3 (inward or
> outward), Thess values seem too high. If you actually use this table for
> parapet design, which value will you consider: above ground or on the
> ground?
>
> Since Canadian Code is in some ways similar to ASCE 7-95,I'd appreciate
> comments from our colleagues who use that code.
>
> Thanks again,
> Reza Dashti P.Eng.
> Vancouver, BC
>
> >From: Daryl Richardson <h.d.richardson(--nospam--at)shaw.ca>
> >Reply-To: <seaint(--nospam--at)seaint.org>
> >To: "seaint(--nospam--at)seaint.org" <seaint(--nospam--at)seaint.org>
> >Subject: [Fwd: Wind Load on Parapets]
> >Date: Wed, 30 Jan 2002 13:26:20 -0700
> >
> >Fellow engineers.
> >
> >       I sent the attached to the list but the scanned attachment made it too
> >long so I sent it direct to Reza.  I have now detached the scanned
> >attachment so the rest of you can see the text of what I sent.
> >
> >                               Regards,
> >
> >                               H. Daryl Richardson
> ><< message3.txt >>
>
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