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

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Refugio,
 
    A truss is loaded at the panel points, but unless your cladding is attached only at your panel points then your chord member is uniformly loaded and must act as a beam between panel points. This is why on bar joists you will see vertical members that by truss theory are zero force members. In reality they are supporting the top chord, so that it doesn't have to span as far. If you are designing a truss where the cladding is attached directly to the chord members like steel deck to bar joists or plywood to wood trusss than there is more design involved then just checking the axial forces.
 
 

Wesley C. Werner


-----Original Message-----
From: refugio rochin [mailto:fugeeo(--nospam--at)gmail.com]
Sent: Tuesday, January 31, 2006 11:08 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

On 1/31/06, refugio rochin <fugeeo(--nospam--at)gmail.com> wrote:
Oh, I've found it



On 1/31/06, refugio rochin <fugeeo(--nospam--at)gmail.com > wrote:
But also the MWFRS is defined as " An assemblage of structural elements assigned to provide support and stability for the overall structure.  The system generally receives wind loading from more than one surface. "  These two definitions together seem to say, if the truss itself is taking the force for MWFRS, then it is not C&C.  I do agree that roof decking and connections need to be designed for the C&C.  But it is not clear otherwise.

Wesley, can you point out the page number or reference to the dual design?


On 1/31/06, Keith Pabst < kpabst(--nospam--at)ha-pa.com > wrote:
I agree
 
 
Keith E. Pabst
-----Original Message-----
From: Wesley Werner [mailto:wwerner(--nospam--at)conewago.com]
Sent: Tuesday, January 31, 2006 9:22 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Roof Design Components and Cladding or MWFRS - ASCE7-02

Refugio,
 
    In the commentary where it gives definitions, roof trusses are listed under both C&C and MWFRS. My understanding of this dual listing is that trusses must be designed for C&C loading; and if they are acting as part of a frame or some other element in the MWFRS, they must be designed for those loads as well.
 
 

Wesley C. Werner


-----Original Message-----
From: refugio rochin [mailto:fugeeo(--nospam--at)gmail.com]
Sent: Tuesday, January 31, 2006 9:03 AM
To: seaint(--nospam--at)seaint.org
Subject: Roof Design Components and Cladding or MWFRS - ASCE7-02

I have a small fire to put out.

In design for wind on a roof, in the design of the trusses according to ASCE7-02, are these trusses to be based on Components and Cladding or on MWFRS?  We assume MWFRS because that is the main system for resisting wind.  However, Components and Cladding would indeed be used for connections and roof decks.  What are the engineers takes on these meanings?

Please reply ASAP.
Refugio Rochin, PE
Systems Engineering Ltd. 
www.systemscaribbean.com