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

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My company does third-party review of truss designs for truss manufacturer's using the various standard wood truss design programs.  To the best of my knowledge all programs utilize what is called a "hybrid analysis."  In this analysis the truss is designed for MWFRS pressures applied to the ENTIRE truss, considering various load cases.  The resulting uplift reactions shown on the truss print are taken as the maximum reaction from the MWFRS load cases.  This analysis considers all members and connections under combined loads (compression/tension and bending).
 
A secondary analysis is performed to assess C&C loads on the top chord where the top chord is checked for bending only (not combined loads).  This "hybrid analysis" is described in a article by Charles C. Hoover, Jr., titled "ASCE 7-93 Wind Load and Its Application in the Design of Metal Plate Connected Wood Trusses," Wood Design Focus.
 
The analysis procedure for truss is analogous to the design of a wall stud--combined loads are checked using MWFRS and pure bending is checked for C&C.
 

 

Regards,

 

Eric Tompos, PE

Vice President

NTA, Inc.

Ph: 574 773-7975

Fx: 574 773-5739