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

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It is not unusual for the top chord of a truss to receive loads between panel points, i.e. resist local loads in flexure. 
 
If you have purlins framing into a truss, I would design the purlins for C&C wind loads and the trusses for MWFRS wind loads.  If closely spaced trusses receive loads directly from the cladding, I would design the top chord for local C&C wind loads and would design the overall truss for MWFRS wind loads.
 

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

 


From: refugio rochin [mailto:fugeeo(--nospam--at)gmail.com]
Sent: Tuesday, January 31, 2006 9:08 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Roof Design Components and Cladding or MWFRS - ASCE7-02

A truss by definition will not be loaded at the panel points.  Otherwise there would be flexural stresses in the member, not just axial loads.

In reference to the chord, you must be referring to the diaphragm chord, and not the truss chord?  The truss chord would have already been evaluated in the MWFRS loading. 

Thanks for your input !  I think overall it is not a clear issue to most.  Perhaps it is something to be more clearly addressed at ASCE.?

On 1/31/06, Wesley Werner <wwerner(--nospam--at)conewago.com> wrote:
    Not quite. I take this section to mean that a long span roof joist has such a large tributary area that you can design the whole thing for MWFRS loading, but that elements such as chord members, must be designed for C&C loading. In other words, you can analyze the whole truss using MWFRS, but when you check a chord member size that is receiving wind loading from cladding, you would use C&C loading to check bending between panel points, for instance. C&C loads are for both the cladding and the component to which the cladding is attached. 
 

Wesley C. Werner


-----Original Message-----
From: refugio rochin [mailto:fugeeo(--nospam--at)gmail.com]
Sent: Tuesday, January 31, 2006 9:58 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Roof Design Components and Cladding or MWFRS - ASCE7-02

A quote from the commentary:
MWFRS - "Can consist of a structural fram or an assemblage of structural elements that work together to transfer wind loads acting on the entire structure to the ground.  Structural elements such as cross-bracing, shear walls, roof trusses, and roof diaphragms are part of the [MWFRS] when they assist in transferring overall loads."

C&C - Components receive wind loads directly or from cladding and transfer the load to the MWFRS.  Cladding receives wind loads directly.  Examples of components include fasteners, purlins,  ...  roof trusses.  Components can be part of the MWFRS when they act as shear walls and roof diaphragms, but they may also be loaded as individual components.  The engineer needs to use appropriate loadings for design of components, which may require certain componenets to be designed for more than on type of loading. (eg. long span roof trusses should be designed for loads associated with MWFRS, adn individual members of trusses should also be designed for C&C). ..."

To me, I interpret it that the roof truss, when it has cladding on it, will be designed for the MWFRS loading.  And when it is exposed, it will be designed for C&C.  But in the exposed case, there is only the tributary area of itself with the wind at C&C levels.

This is how I interpret.   Agree?
Refugio