ASCE 7 separates wind loading into three types: Main Wind Force Resisting System (MWFRS), Components and Cladding (C&C), and Other Structures and Building Appurtenances.

MWFRS is defined as “(a)n assemblage of structural elements to provide support and stability for the overall structure.” Typically, members which receive loading from two surfaces are designed to resist MWFRS loading. Let’s say you had a steel moment resisting frame building with metal roof and wall panels as pictured above, you would design the moment frames and the spread footings at the base of the frames to resist MWFRS loading from the lateral wind pressure on the wall panels and positive/negative wind pressure on the roof panels.

C&C is defined as “(e)lements of the building envelope that do not qualify as part of the MWFRS”. Let’s take the metal building again shown above as an example. The metal roof and wall panels would be considering cladding. The overhead door, walk door, and window would be considered components. Also, the roof purlins and wall girts are receiving loading from the cladding and are therefore considered components as well. The wind pressure then varies on these components and cladding based upon their respective effective wind area.

Other Structures and Building Appurtenances are defined as “rooftop structures, rooftop equipment, solid freestanding walls, freestanding solid signs, chimneys, tanks, open signs, lattice framework, and trussed towers”. Let’s say we were to add a rooftop mounted mechanical air conditioning unit to the roof purlins of our metal building, we would use this section of the code to determine the appropriate wind loading on the mechanical unit itself.

Article provided and updated by Zach Rubin, EI

Mar 8, 2018   9183    Codes & Standards, Engineering Principles    
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  1. Main wind force applies to buildings. Rooftop equipment is classified as ‘other structures’ and behaves differently than a larger enclosed structure. MWFRS does not apply and the forces imposed by the ‘other structures’ calculations should be used to analyze that component.

  2. Hi, for the added equipment surface area contributing to the building MWFRS, would you use the MWFRS pressure or the rooftop structures pressure to check the MWFRS? Any reference on this case?

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