Wind Loads on Buildings

Buildings and structures are designed and constructed to resist wind loads, or wind pressure, as opposed to wind speeds. Wind speed, although a significant contributor, is only one of several factors that impact wind loads acting on a building. Other factors that affect the calculated design wind loads (pounds per square foot, or psf) include:

• Combined gust factor and external pressure coefficients (GCpf)
• Combined gust factor and internal pressure coefficients (GCpi)
• Design velocity pressure

Design velocity pressure varies depending on several factors, including mapped wind speed, exposure category, topographic effects, wind directionality, elevation factor, and other specific building characteristics. All of these are covered in the following sections.

Mapped Wind Speeds and Risk Category

Mapped wind speeds are based on the probability of winds attaining certain speeds per geographic area. They are developed from an analysis of wind speed data collected during severe wind events and assuming a set of reference conditions. Current wind speed maps include a more comprehensive analysis of wind speeds for both coastal and non-coastal areas than was previously available. The wind maps in ASCE 7-16 vary substantially from previous versions. In most parts of the country, particularly in the West, winds speeds have decreased.

The risk categories introduced earlier also apply when designing for wind loads. Mapped wind speeds for Risk Category III and IV buildings, for which the potential consequences to human life and economic impact are greatest, are higher than mapped wind speeds for Risk Category II buildings. The ultimate design wind speed (Vult, in miles per hour) for the establishment of wind loads is determined using wind speed maps contained in three IBC Figures: Figure 1609.3(1) for Risk Category II buildings, Figure 1609.3(2) for Risk Category III buildings, figure 1609.3(3) for Risk Category IV buildings, and Figure 1609.3(4) for Risk Category I buildings.

Surface Roughness and Exposure Categories

One of the basic decisions in the design of wind-resistant buildings is determination of exposure category based on the “surface roughness” of nearby terrain. Surface roughness depends on natural topography, vegetation, and existing built structures. Obstructions upwind of the building site can exert a drag force on wind, slowing the flow of air close to the surface of nearby terrain (see Table 4).

For each wind direction considered, an exposure category that reflects the surface irregularities of the terrain must be determined for the site at which the building or structure is to be constructed (see Table 5).

Exposure B is the most common exposure category in the country and is often the default exposure category. However, it is important to recognize that exposure category varies based on conditions at the site and that buildings are often sited on properties which fall under Exposure C and D, with attendant higher wind loading.

Topographic Effects

Abrupt changes in the landscape can affect wind speeds. For this reason, topographic effects are accounted for when calculating wind loads using the topographic factor, or Kzt. Kzt considers the wind “speed-up” effects that occur when wind encounters an isolated hill, ridge, or escarpment.

Wind Directionality

For most wood-frame buildings with wind loads determined in accordance with ASCE 7 load combinations, the effect of wind directionality is accounted for by use of a wind directionality factor. This load reduction factor, called Kd, accounts for two effects: “(1) the reduced probability of maximum winds coming from any given direction and (2) the reduced probability of the maximum pressure coefficient occurring for any given wind direction.” ASCE 7-16 includes a table (Table 26.6-1) which displays the wind directionality factors to be used for each structure type.

Elevation Factor

A new elevation factor has been added in ASCE 7-16 that accounts for changes in air density at elevations above mean sea level. The revised standard includes a table of factors for elevations up to 6000 feet and a formula that can be used to calculate the factor for any elevation.

Figure 3: Effect of openings in a partially enclosed building, courtesy of Florida Building Code Commentary

Effect of Openings on Internal Pressure Coefficients

When wind encounters a building, the airflow changes direction and produces several varying effects. Exterior walls and other vertical surfaces facing the wind (windward side) and perpendicular to its path are subjected to positive pressures (loads). Wind continues to flow over and around the building, resulting in suction or negative pressures (loads) on sidewalls, the leeward wall and, depending on geometry, the roof. Wind pressures on a roof vary based on roof slope and location on the roof, with greatest wind pressures occurring at roof edges.

Openings in the building envelope can significantly affect the wind pressures imposed on building elements. Depending on the location and size of openings with respect to wind direction and building porosity, external and internal pressures can act in the same direction to produce higher forces on the walls and roof.

An example of this effect of openings is illustrated in Figure 3. Here, pressures occur on both exterior and interior building surfaces because of a large opening in the windward wall. As this scenario demonstrates, wind enters the building, exerting positive pressures against all interior surfaces. This opening has the net effect of producing high internal pressures that act in the same direction as the external pressures on the roof and leeward walls, adding to the overall pressures on these building elements.

Windows and doors are not considered openings if they are likely to be closed during a design storm event. Therefore, at a minimum, windows and doors need to be rated to resist the positive and negative design wind pressures.

Figure 4: Roof and wall-sheathing zones for wind design, courtesy of Florida Building Code Commentary

External Pressure Coefficients

ASCE 7-16 categorizes wind loads according to the building systems they impact. Main wind force-resisting system (MWFRS) wind loads act on the frame and foundation of a building.

Components and cladding (C&C) wind loads act on cladding elements such as shingles, siding, and windows. Loads are calculated differently for each.

For the design of wood-frame buildings 60 feet in height or less, the “envelope procedure” is the most used method for determining MWFRS loads. External pressure coefficients from the envelope procedure have been developed to represent critical loads on the main structural elements, which are “enveloped” to reflect induced actions on a building from various wind directions for diverse building geometries, roof heights, and roof slopes.

As per ASCE 7, the MWFRS is “an assemblage of structural elements assigned to provide support and stability for the overall structure … [that] generally receives wind load from more than one surface.” Examples of MWFRS elements include roof trusses, roof and floor diaphragms, and shear walls. Individual elements of the MWFRS may also be subject to C&C load requirements. These include sheathing, cladding, wall studs, individual roof rafters and elements of a roof truss, and short-span trusses resisting wind loads from cladding.

External pressure coefficients for C&C represent peak pressures which occur over specific areas on the building’s exterior surface. Localized negative pressures associated with C&C loads almost always control design of exterior sheathing and cladding elements, including wall sheathing, wall cladding, roof sheathing, roof cladding, and the attachment of the sheathing and cladding to the building's structural frame.