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

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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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