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Re: Roof Design Components and Cladding or MWFRS - ASCE7-02

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Refugio,

I think an example might help.

Take a moment frame that is part of the lateral system of the building.
Let's assume that there are both exterior columns as part of the moment
frame (i.e. columns that are at the exterior of the building and that are
directly "exposed" to wind pressures from the walls) and that there are
some beams at the top level of the moment frame that also act are roof
framing members (i.e. they are "directly" exposed to wind uplift at the
roof).

In such a case, one would apply the MWFRS loads to the frame to get it to
"act" as the lateral frame.  In other words, it received the MWFRS loads
and resists those loads so that the building will not "fall over".  This
results in moments in the beams and columns as we would typically expect
when analyzing a moment frame.

In addition, the INDIVIDUAL members must still be INDIVIDUALLY designed to
resist C&C loads.  This means that exterior columns will have to be
checked for out of plane wind pressures and the roof beams will have to be
checked for out of plane wind pressures (i.e. uplift) just as if there
were NOT part of a moment frame (i.e. just like some beams or columns that
might be next to them that are NOT part of the lateral system).

The biggest reason for this "other" check is that C&C wind pressures will
be larger than MWFRS pressures (if all other things are equal).  Wind
pressures per the ASCE method are a "function" of the tributary area of
the "item" (frame, beam, column, etc...in other words element or
group of elements working together).  The idea is that in reality wind
pressures vary over the surface of the structural, but it would be "too
complicated" for us to accurately model that variation.  Thus, we use an
average pressure.  As such, when the area that effects the element or
group of elements is small, then there is a higher probability that the
area will be in a localize zone of higher pressure...thus we use a higher
average pressure.  When the area is much larger, then there much more room
for variation and thus you have more zones of lower pressure...this
results in the use of a lower average pressure.  The function difference
between MWFRS and C&C is that MWRFS will have a VERY large tributary wind
area.  As such, it will have a low average wind pressue.  An element to be
designed for C&C loading will have much smaller tributary wind area and as
such will generally have a higher average wind pressure.

Another way to think of it is that if you remove the MFWRS from the
building, it will in theory not stand up (i.e. the whole building).  If
you remove an individual item resisting C&C loading, then just a local
small portion of the building will fail.

HTH,

Scott
Adrian, MI

On Tue, 31 Jan 2006, refugio rochin wrote:

> Thanks all.  Got it covered now.  Hopefully for future engineers and there
> understanding, there is some further information in the code pertaining to
> this problem.  One reason it would really be nice to have this list service
> directly collaborate with code commentary.  To have the code online, and be
> able to see such commentary next to it.  Of course there is a lot of
> commentary.
>
> Thanks again.
> Refugio
>
> On 1/31/06, bcainse(--nospam--at)aol.com <bcainse(--nospam--at)aol.com> wrote:
> >
> > Refugio-
> > When you design a truss, you usually treat it as a pin connected framework
> > with panel point loads. However, in reality, if sheathing is applied
> > directly to the truss, the portion of the top chord supporting the sheathing
> > functions as a flexural member supporting either vertical or suction loads
> > and the bottom chord may do likewise for an open building.  Also, the
> > connectors connecting that member to the remainder of the truss must support
> > these loads to be sure the loads are delivered as panel point loads.  Beyond
> > this point, I would design the basic truss as a panel point loaded truss
> > with MWFRS loads.  Another way of looking at it is that you are preventing
> > localized failure so the basic structure can function.
> >
> > It is really no different than what we do with equipment loads.  Make sure
> > the loads are carried into the structure so the equipment does not move.
> > Then the basic structure is designed for normal loadings.
> >
> > Bill Cain, S.E.
> > Berkeley CA
> >
> >
> > -----Original Message-----
> > From: refugio rochin <fugeeo(--nospam--at)gmail.com>
> > To: seaint(--nospam--at)seaint.org
> > Sent: Tue, 31 Jan 2006 13:59:13 -0400
> > Subject: Re: Roof Design Components and Cladding or MWFRS - ASCE7-02
> >
> > The specifics of how to design for these two cases is not clear.
> >
> > Seems, if the truss is designed to meet C&C loads, then there is no need
> > to design it for MWFRS loading.  The only thing the roof would be doing in
> > the MWFRS case would be transferring load to the shear walls, and such, to
> > the ground.
> >
> > It appears this is over design?  Or is this what is specifically meant?
> > This would be somewhat in conflict with the main definitions in the main
> > code section, and the last quote in the ASCE7-02 commentary that I mentioned
> > where it discusses long roof truss.
> > Refugio
> >
> > On 1/31/06, David Topete <davetopete(--nospam--at)yahoo.com> wrote:
> > >
> > > Refugio,
> > >
> > > To quote from Alan Williams "Seismic and Wind Forces" design exmaples
> > > book, it says the following:
> > > "... Components include purlins, studs, fasteners, and roof trusses.
> > > Some elements, such as roof trusses and sheathing, may also form part of the
> > > main-force wind resisting system and must be designed for both conditions."
> > > This book is based in IBC 2000, which references ASCE 7-98 by default.
> > > HTH.
> > >
> > > DT
> > >  ------------------------------
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> >
> >
>

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