Wind Load Analysis: MWFRS vs. C&C
ASCE/SEI 7-10, Minimum Design Loads of Buildings and Other Structures, lists two methods for calculating wind pressures: Main Wind Force Resisting System (MWFRS) and Components & Cladding (C&C). Choosing which method to use when designing uplift connections for trusses can raise a number of questions for building designers, code officials and truss designers.
What is the best method of wind load analysis to use in truss uplift connection design and who is responsible for this work?
Selecting a method of wind load analysis depends on whether you are designing uplift connections for an individual member or a system. Let’s start with some definitions for ASCE 7-10.
Either of these methods can apply to trusses, depending on the situation. By definition, a truss is an assemblage of structural elements, which would put it into the MWFRS category. The minimum uplift connection forces are provided in Table R802.11 of the International Residential Code (IRC) and Table 2308.10.1 of the International Building Code (IBC) for rafters and trusses used in conventional light-frame construction. Both of these tables are developed using the MWFRS method as indicated by the reference in Footnote e to Figure 6-2 of ASCE 7-05 and Chapter 28 of ASCE 7-10.
Table R802.11 of the 2006 and 2009 IRC states:
Table 2308.10.1 of the 2012 IBC states:
However, a truss also receives wind load directly from the roof sheathing (i.e., cladding) and therefore acts as a component, which puts it into the C&C category. This crossover is illustrated in C26 of the Commentary for ASCE 7, which lists roof trusses as examples of both MWFRS and C&C (bold added to definitions to emphasize key concepts):
The truss industry uses a combined analysis, incorporating both the MWFRS and C&C method, to generate wind uplift and downward pressure loading conditions. MWFRS applies to the assembly of multiple parts, while C&C covers an individual part. SBCA recommends this hybrid approach. Most two-dimensional software analysis programs offer a choice of wind analysis methods when applying wind loads.
Using this combined analysis, truss or rafter uplift connections, at the plate line or as attached to a header, beam or girder, should be designed for wind load using the MWFRS analysis method, and individual truss or rafter members should be designed using the C&C analysis method. Similarly, gable frame uplift connections should be designed for wind uplift loads using the MWFRS analysis method, while individual members of the gable frame should be designed using the wind applied downward pressure loads developed through the C&C analysis method.
|Truss Uplift Connection||Individual Truss Member|
|Gable Frame Uplift Connection||Individual Member of a Gable Frame for
Downward Pressure Loading Conditions
|Rafter Uplift Connection||Roof Covering, Wall Covering|
Issues to Watch
Regardless of the design method used, the truss designer needs as much loading information as possible from the building designer in order to design the trusses. The building designer is responsible for providing the structural design documents and all of the load and dimension information necessary to design the trusses. If a project does not require a licensed professional building designer, the owner or the owner’s agent is responsible for providing this information. Although the IBC and IRC require all applicable design loads to be listed by the building designer in the structural design documents, this information is often lacking or not available to the truss designer at the time of design. If the truss designer does not have this information, assumptions may have to be made that can easily hamper an accurate and affordable design being performed.
Problems can arise if the end reactions on the truss designer’s or truss design engineer’s truss design drawings are different than the building designer’s calculation of roof-to-wall anchorage forces. If this occurs, the issue falls under the building designer’s scope of responsibility, per TPI 1-2007 and AISI S214-07, to resolve any differences in the reaction forces.
For more information on determining loads, along with a cover sheet that can be used by the building designer to define the loads or by the truss designer to get the loads approved, see the SBCA Load Guide available as a free Excel download at sbcindustry.com/loads.php. The wind tab in particular discusses the design methodology for MWFRS and C&C. For more information on the SBCA Load Guide and its use, see these SBC Magazine articles available: Introduction to TLG – Part 1 and Introduction to TLG – Part 2.