Main Wind Force Resisting Systems Vs Components Cladding
Question:
As an engineer, I have noticed truss designers in some high wind states routinely using “Main Wind-Force Resisting Systems” wind pressure coefficients as opposed to “Components and Cladding” coefficients to design for wind uplift. A roof truss is not a main wind-force resisting system and would have to have a tributary area of more than 1000 sq. ft. before qualifying for the lower Primary Frame coefficients. In my experience this practice is routine. I have to wonder why the design software would even allow the selection of “Primary Frames.” I resolve the problem in my truss specs but apparently few other designers are even aware of the problem.
Answer:
The primary guide to all types of loads on structures is ASCE/SEI 7-10 which is referenced by both IBC and IRC. In ASCE 7 commentary there are examples of roof trusses in both MWFRS and C & C. The industry usually designs for both MWFRS loads and C&C loads.
In considering wind loads on trusses according to IBC 2012 2308, IBC 2012 1601, (IBC 2015 2304, 1601) or ASCE/SEI 7-10, the following assumptions are made unless specified otherwise by the governing code:
Main wind-force resisting system design coefficients are considered to determine:
• Uplift reaction values (unless component and cladding design coefficients for uplift are required by the building code official).
• Trusses or girders that function as drag struts, since they function as part of the structure.
• Trusses that use purlins instead of sheathing on the top chord, since the sheathing is not directly applied to the truss.
Components and cladding design coefficients are considered to determine the resistance required to be provided by the truss, with the exceptions as listed above (unless component and cladding design coefficients for uplift are required by the building code official). Wind load is based on an effective wind area (ASCE/SEI 7-05 6.2, ASCE/SEI 7-10 26.2) determined as the span length multiplied by an effective width that need not be less than one-third the span length.
If you’re looking for a load calculation tool, try downloading SBCA’s Load Guide spreadsheet. The C6-Wind tab includes information about wind and trusses. See also SBCA Research Report 1507-11: Truss Uplift Design.