Minimum Design Loads for Buildings and Other Structures (ASCE/SEI 7-10), published by the American Society of Civil Engineers (ASCE) Structural Engineering Institute (SEI) categorizes wind loads according to the parts of a building they affect:
But why the difference? After all, it’s all the same wind, right?
Well, yes and no. It’s true that there’s nothing intrinsic about wind that actively seeks out different parts of a building. But different parts of a building are more vulnerable to certain wind effects than others.
Atmospheric wind is not a uniform phenomenon: With few exceptions, it continuously changes speed and direction. When meteorologists remark that winds are approaching from the north, for example, what they mean is that air is generally moving from north to south on average. But a pedestrian walking outside will experience rapid changes in that wind, which we call gusts. The bulk wind may approach at 10 mph (16 kph) from the north, but individual gusts approach with different speeds from all directions. It’s simply the nature of the turbulent atmosphere in which we live.
It’s these variations in gust speed, direction, duration, and size that are responsible for the different wind loads that affect different parts of a structure. Imagine a hypothetical cubic building that measures 33 feet (10 meters) on each side, about three stories tall. This is the characteristic dimension. Gusts that are larger than this dimension will effectively appear like a sustained wind from the standpoint of the building. These are the gusts that act on the building as a whole and tend to push, pull, and twist its frame and foundation.
On the other hand, gusts that are smaller than the characteristic dimension tend to create more localized effects. A window here, a shingle there, and so on. Generally speaking—and this is a very rough simplification—a gust most acutely affects structures that are smaller than it is. A window-sized gust just doesn’t seem that big to a building.
In reality, gusts come in all sizes, forming a continuum from those as large as mountains and valleys, all the way down to the very smallest gusts that friction between air’s constituent molecules can dissipate. The building codes simply divide gusts into building-sized and component-sized features because that’s what architects and builders are accustomed to working with. And it’s this division that’s responsible for separate loads for MWFRS and components and cladding.