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 that 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 “elements 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 considered 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, also considered components. The wind pressure then varies on these components and cladding based on 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.
Our Commentary:
Think of MWFRS as ‘what happens to the balloon’ – it could slide, fly up, or flip over in wind. We look at the unit and consider effects as a whole.
Components & cladding consider the effects of smaller ‘bursts’ of wind that occur in any given part of the building, not paying attention to the structure as a whole.
Both must be reviewed for a structure’s integrity.
Components & cladding are a function of the area of consideration – the probability that Mother Nature will blow her full fury at all places …at the same time… So think of a door opening. There’s more of a chance that the area near the lock gets a maximum 3-second burst in that one area than the whole door, so the tributary area is smaller, but the pressure is higher. The door is more likely to receive its tributary area of a 3-second burst than the door + sidelites + transom, so the frame as a unit has a slightly lower pressure than its parts.
Taking this up to the entire structure (beyond 100sqft per ASCE 7) approaches MWFRS pressures which also have to be looked at from windward directions, leeward directions, and combinations thereof with multipliers of pressure at given areas. It’s not easy, and the designer must purchase & study ASCE 7 and review the range of software applications to assist with design. Have a thorough understanding before construction, or contact us to help.
Last Update: February 15, 2024
