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RE: Truss-MWFRS or C & C

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I have some older responses to this issue in the archives.


A structural element should be designed for MWFRS pressures if it is part of the MWFR system and vice versa for Components and Cladding. This determination is made using the definitions of MWFRS and C&C given in the Standard (ASCE 7).  Note that these definitions do not mention anything about 700sf or effective wind area. The language about 700sf allows C&C to be designed using MWFRS pressures if the effective wind area of the C&C is 700sf or more.  This provision should not be interpreted to mean that no elements in the MWFRS will have wind areas less than 700sf. In fact, Effective Wind Area is not really relevant to MWFRS calculations.


Also note that, to be precise, the definitions of MWFRS and C&C are mutually exclusive. That is to say, the way the definitions are written, an element cannot be both MWFRS and C&C.   I am not aware of any provision in the standard that speaks to the design of structural elements to both MWFRS and C&C pressures.  You can find language that recommends this in Kishor’s Guide to ASCE7 and you might possibly infer it from language in the Standard Commentary.  It is left to your professional engineering judgment to determine if MWFRS pressures or C&C pressures or both must be used.  It is left to your professional engineering judgment to determine whether a structural element is C&C or part of the MWFRS system.  There are many other published documents that can help you make this determination.


For metal plate connected wood trusses, I suggest that you contact Kirk Grundahl at WTCA to get their latest opinion and advice for truss manufacturers on this issue.




Christopher Banbury, PE


Ark Engineering, Inc.

PO Box 10129, Brooksville, FL 34603

22 N. Broad ST, Brooksville, FL 34601

Phone: (352) 754-2424

FAX: (352) 754-2412


From: Ramesh Savaliya [mailto:savaliyaramesh(--nospam--at)]
Sent: Tuesday, February 20, 2007 10:31 AM
To: seaint(--nospam--at)
Subject: Truss-MWFRS or C & C


I always design trusses for C & C loading.  In which situation it should be designed for MWFRS load?

Aegis software have selection for both.




Ramesh Savaliya, P. Eng.


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