Wind Loading – MWFRS or C&C: Which is it?
There is an age-old debate in the building industry: Wind Loading. Each region seems to have its own set of rules. Why? Let’s have a look:
Designing trusses for wind load can be looked in a couple of different ways and will affect the member design and reactions that are produced. Which method to use can raise a number of questions for engineers, truss designers and code officials.
The two methods used are:
- MWFRS – Main Wind Force Resisting System
- This 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.” Structural elements such as cross bracing, shear walls, roof trusses and roof diaphragms are part of MWFRS.
- C & C – Components and Cladding
- o Elements of the building envelope that do not qualify as part of the MWFRS. Cladding receives wind loads directly. Roof covering and wall covering are examples of cladding. Components receive wind loads either directly or from the cladding and then transfer the loads to the main wind force resisting system. Examples of components include fasteners, purlins, girts, studs, roof decking, and roof trusses.
You can see that trusses can fall under either of these methods. Trusses are an assemblage of structural elements which would put it into the MWFRS category. Trusses also receive loads directly from the roof sheathing (cladding) and, therefore, acts as a component which puts it in the C&C category.
Which method should be used? Well, the truss industry uses a combined analysis of both methods to generate wind pressures for a truss. Truss design software does give you the option to choose the method to use for a truss design. The standard practice in the component industry is to design a truss for a combination of the two methods and to specify the uplift connections based on the MWFRS.
There may be instances that the specifications for a project may require different combined analysis as shown below:
- Loads and reactions based on MWFRS with additional C&C member design.
- Loads based on MWFRS with additional C&C member design and reactions
- Loads and reactions based on both MWFRS and C&C
- Loads based on MWFRS and C&C, reactions based on MWFRS.
In theory, the engineer of record should indicate the method to be used for the trusses, but, that is not always the case.
As a point of information, the C&C method will produce larger uplift reactions. On a recent project I had at a 60’-0” clear span truss. Wind load was 90 mph, Exp. C, Cat. III, 20-10-10 loading. Basing the uplift reaction on the C&C method produced an uplift reaction of 1400#. Using the MWFRS, the uplift was 776#. I verified which method to use with the engineer and the response was to base uplift reactions on MWFRS. This made a significant difference in hurricane anchors used for this project.
Refer to an article on partially enclosed buildings by Steve Kastner, P.E. here.
What has your experience been with this issue? How do you typically handle this? I look forward to hearing from you.
Bill Hoover – Design Professional
Gould Design, Inc.