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RE: Wind Uplift on Awning

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Technically the only real difference between MWFRS and C&C loading is the
size of the "thing" that the wind is hitting (in reality, there are
practical differences in terms of the actual calculations).  The idea behind
a MWFRS pressure is that it is the pressure that is applied to the ENTIRE
structure...i.e. the entire structure is the "wind sail".  For C&C, the
pressure is only for that single particular element and C&C pressures are
thus a function of the size of the wind trib area of the element.  The
smaller the trib area of the element, the higher the pressure.  The intent
is that a MWFRS pressure is basically just nominally a C&C pressure for a
really large trib area (thus, the reason for section 6.5.12.1.3 which
basically allows one to use MWFRS pressures for C&C items with trib areas
greater than 700 sq ft).

Thus, the way I have always done it is that MWFRS is the pressure applied to
the building.  Then any element that gets resulting wind load due to the
load path of the lateral system gets design for that resulting wind
"reaction" as one case.  The second case then can be a wind load on the
element due to a C&C wind load being directly applied to the element
(nominally speaking).

Thus, a roof framing member might need to be designed to accomidate a
diaphragm force due to a MWFRS pressure acting on the whole building that
that "traces" the load through the member, but gets designed for a C&C load
for uplift bending.  Now, you can argue that the if the roof framing member
is part of a say a moment frame that it should be the MWFRS pressure that is
used for uplift.  This is due to the definition of C&C that ASCE 7 uses
(i.e. elements of the building envelope that do not qualify as part of the
MWFRS), but I think that is a crappy definition and to my knowledge is not
the intent behind the differences.  That moment frame beam should be
designed for MWFRS loads for when the moment frame is resisting the lateral
loads that make it do its "moment framededness" (nice madeup word), but the
roof beam should have an uplift load due to C&C loading as the actual wind
uplift trib area is MUCH smaller than the whole building.

Thus, I would still argue that it should be C&C loading as it should result
in a larger (thus, more conservative) wind pressure for your smaller element
(i.e. one canvas panel) than the pressure you would get from a MWFRS
pressure, which effect trib area is meant to be the whole structure.

Note, I did not go into why C&C pressures tend to be larger because I am
assuming that you know why already and did not want to appear to be
"insulting your intelligence" (which is never my intent, but I have been
told that I sometimes come across that way when I am merely just trying to
offer a detailed explanation...i.e. I get long winded...pun intended).  If
my assumption is wrong and you want that explanation (at least at I
understand it), then I would be more than glad to engage the long winded
mode and elaborate.

Regards,

Scott
Adrian, MI

-----Original Message-----
From: Bill Allen [mailto:t.w.allen(--nospam--at)cox.net] 
Sent: Friday, May 02, 2008 6:59 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Wind Uplift on Awning


But it is an element that is directly used to support the whole "structure".
If the cables are removed, the canvas is laying on the ground.

T. William (Bill) Allen, S.E.
ALLEN DESIGNS
Consulting Structural Engineers

> -----Original Message-----
> From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu]
> Sent: Friday, May 02, 2008 3:39 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Wind Uplift on Awning
> 
> Bill,
> 
> In my opinion it would be components & cladding unless it is an 
> element that is directly used to support the whole "structure"...and 
> even then you might still have to check both C&C and MWFRS.  Think of 
> a stud in an exterior shear wall that acts as the end post of the 
> sheallwall for overturning.  That stud would first have to be 
> checked/design for C&C wind loads for out of plane bending and shear, 
> etc.  Then it would also need to be checked for MWFRS loads for in 
> plane shear acting on the shearwall that results in tension 
> compression.
> 
> As to the specifics, I don't have my ASCE 7-05 in front of me at the 
> moment, so I cannot help with that at the moment.  I can take a look 
> later if someone else has not already offered one.
> 
> Regards,
> 
> Scott
> Adrian, MI
> 
>  ..... Original Message .......
> On Fri, 2 May 2008 15:24:16 -0700 "Bill Allen" <t.w.allen(--nospam--at)cox.net> 
> wrote:
> >I have a client who is creating a shade area by attaching a canvas 
> >awning
> between two exterior trellises. The canvas panels are 22'-0" x 16'-6" 
> and there is a gap of about 1'-8" between them. There area a total of 
> three canvas panels to form a shaded area of about 22'-0" x 38'-0".
> >
> >
> >
> >I'm a noobie when it comes to the ASCE 7 wind load provisions and I'm 
> >not
> really sure on what to use to calculate the wind uplift. My interest 
> is in the design of the cables and particularly the attachment of the 
> cables to the trellises. In my mind, I don't think these items are 
> C&C, but I could be wrong.
> >
> >
> >
> >What should I use, Method 2, MFRS or C&C?
> >
> >
> >
> >From what table should I obtain the gust coefficients?
> >
> >
> >
> >TIA,
> >
> >
> >
> >T. William (Bill) Allen, S.E.
> >
> >ALLEN DESIGNS
> >
> >Consulting Structural Engineers
> > V (949) 248-8588 .
> 
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