Code Provisions For Wind Loads

In recent years, wind loads specified in codes and standards have been refined significantly. This is because our knowledge of how wind affects buildings and structures has expanded due to new technology and advanced research that have ensued in greater accuracy in predicting wind loads. We now have an opportunity to design buildings that will satisfy anticipated loads without excessive conservatism. The resulting complexity in the determination of wind loads may be appreciated by comparing the 1973 Standard Building Code (SBC), which contained only a page and one-half of wind load requirements, to the 2002 edition of the ASCE 7, which contains 97 pages of text, commentary, figures, and tables to predict wind loads for a particular structure. As compared to a single method given in the 1973 SBC, ASCE 7 contains three methods for determining winds: the simplified procedure, the analytical procedure, and the wind-tunnel procedure. The controlling equations for determining wind loads require calculating velocity pressure as before, but are now modified to account for several variables such as gusts, internal pressure, and aerodynamic properties of the element under consideration, as well as topographic effects. Using the low-rise buildings' analytical procedure in ASCE-7 and applying it to the simplest building requires the use of up to 11 variables. An important criterion that influences the calculation of wind loads is the enclosure classification of the building. Three classifications are used: 1) enclosed; 2) partially enclosed; or 3) open. A building classified as partially enclosed assumes that a large opening is on one side of a building and no (or minimal) openings are on the other walls. As openings on one wall reach a certain size with respect to openings on the other walls, the building is classified as partially enclosed. Depending upon the wind's direction, this type of situation allows two conditions to develop: internal pressure or internal suction. Internal pressure occurs when air enters a building opening on the windward wall and becomes trapped, exerting an additional force on the interior elements of the building. Typically the internal pressures act in the same directions as the external pressures on all walls except the windward wall. Internal suction is a condition that exists when there is an opening on the leeward wall allowing air to be pulled out of the building. This results in the internal forces acting in the same direction as the external forces on the windward wall. The additional forces produced by this type of pressurization are characterized by requiring an internal pressure coefficient that is more than three times greater than that required for an enclosed building.

Another criterion that significantly affects the magnitude of the wind pressures is the site's exposure category, which provides a way to define the relative roughness of the boundary layers at the site.

The ASCE 7-02 and IBC-03 define three exposure categories: B, C, and D. Exposure B is the roughest and D is the smoothest. Consequently, when all other conditions are equal, calculated wind loads are reduced as the exposure category moves from D to B. Exposure B is the most common category, consisting primarily of terrain associated with a suburban or urban site. Accordingly, B is the default exposure category in both ASCE 7 and IBC. Exposure C consists primarily of open terrain with scattered obstructions but also includes shoreline in hurricane-prone regions. Exposure D applies to shore lines (excluding those in hurricane-prone regions) with wind flowing over open water for a distance of at least one mile.

Buildings must also be classified based on their importance. The wind importance factor Iw specified in the codes is used to adjust the return period for a structure based on its relative level of importance. For example, the importance factor for structures housing critical national defense functions is 1.15, while the importance factor for an agricultural building not as critical as a defense facility, is 0.87.

The applicable wind speeds for the United States and some tropical islands specified in the wind speed maps are three-second gusts at 33 feet above ground for Exposure Category C. In the model codes that preceded the IBC (the National Building Code. Standard Building Code, and Uniform Building Code) and versions of ASCE 7 prior to 1995, wind speeds were shown as "fastest-mile winds," which is defined as the average speed of a one-mile column of air passing a reference point.

While the designated 3-sec gust wind speed for a particular site is higher than values on the fastest-mile map, the averaging times are also different. The averaging time for a fastest-mile wind speed is different for each wind speed, while the averaging time for the 3-sec gust speeds varies from 3 to 8 sec, depending upon the sensitivity of the instruments.

Wind load provisions given in three nationally and internationally recognized standards are discussed in this section. These are the

1. Uniform Building Code (UBC) 1997.

2. ASCE Minimum Design Loads for Buildings and Other Structures (ASCE 7-02).

3. National Building Code of Canada (NBCC) 1995.

1.4.1. Uniform Building Code, 1997: Wind Load Provisions

Wind load provisions of UBC 1997 are based on the ASCE 7-88 standard with certain simplifying assumptions to make calculations easier. The design wind speed is based on the fastest-mile wind speed as compared to the 3-sec gust speeds of the later codes. The prevailing wind direction at the site is not considered in calculating wind forces on the structures: The direction that has the most critical exposure controls the design. Consideration of shielding by adjacent buildings is not permitted because studies have shown that in certain configurations, the nearby buildings can actually increase the wind speed through funneling effects or increased turbulence. Additionally, it is possible that adjacent existing buildings may be removed during the life of the building being designed.

To shorten the calculation procedure, certain simplifying assumptions are made. These assumptions do not allow determination of wind loads for flexible buildings that may be sensitive to dynamic effects and wind-excited oscillations such as vortex shedding. Such buildings typically are those with a height-to-width ratio greater than 5, and over 400 ft (121.9 m) in height. The general section of the UBC directs the user to an approved standard for the design of these types of structures. The ASCE 7-02, adopted by IBC 2003 (discussed later in this chapter), is one such standard for determining the dynamic gust response factor required for the design of these types of buildings.

UBC provisions are not applicable to buildings taller than 400 ft (122 m) for normal force method, Method 1, and 200 ft (61 m) for projected area method, Method 2. Any building, including those not covered by the UBC, may be designed using wind-tunnel test results.

The minimum basic wind speed at any site in the United States is shown in Fig. 1.8. The wind speed represents the fastest-mile wind speed in an exposure C terrain at 33 ft (l0 m) above grade, for a 50-year mean recurrence interval. The probability of experiencing a wind speed faster than the value indicted in the map, in any given year is 1 in 50, or 2%.

1.4.1.2. Special Wind Regions

Although basic wind speeds are constant over hundreds of miles, some areas have local weather or topographic characteristics that affect design wind speeds. These special wind regions are defined in the UBC map. Because some jurisdictions prescribe basic wind speeds higher than the map, it is prudent to contact local building officials before commencing with the wind design.

1.4.1.3. Hurricanes and Tornadoes

The wind speeds shown in the UBC map come from data collected by meterological stations throughout the continental United States, Alaska, Hawaii, Puerto, Rico, and Virgin Islands. However, coastal regions did not have enough statistical measurements to predict hurricane wind speeds. Therefore data generated by computer simulations have been used to formulate basic hurricane wind speeds.

Asce Basic Wind Speeds Maps
Figure 1.8. Minimum basic wind speeds in miles per hour ( X 1.61 for km/h). (From UBC 1997.)

Tornado level winds are not included in the map because the mean recurrence intervals of tornadoes are in the range of 400-500 years, as compared to the 50 years interval typically used in wind design.

1.4.1.4. Exposure Effects

Every building site has its own unique characteristics in terms of surface roughness and length of upwind terrain associated with the roughness. Simplified code methods cannot account for the uniqueness of the site. Therefore the code approach is to assign broad exposure categories for design purposes.

Similar to the ASCE method, the UBC distinguishes between three exposure categories; B, C, and D. Exposure B is the least severe, representing urban, suburban, wooded, and other terrain with numerous closely spaced surface irregularities; Exposure C is for flat and generally open terrain with scattered obstructions; and the most severe, Exposure D, is four unobstructed coastal areas directly exposed to large bodies of water. Discussion of the exposure categories follows.

It should be noted that Exposure A (centers of large cities where over half the buildings have a height in excess of 70 feet), included in some standards, is not recognized in the UBC. The UBC considers this type of terrain as Exposure B, allowing no further decrease in wind pressure.

Exposure B has terrain with buildings, forest, or surface irregularities, covering at least 20% of the ground level area extending 1 mile (1.61 km) or more from the site.

Exposure C has terrain that is flat and generally open, extending one-half mile (0.81 km) or more from the site in any full quadrant.

Exposure D represents the most severe exposure in areas of basic wind speeds of 80 mph (129 km/h) or greater, and has terrain that is flat and unobstructed facing large bodies of water over one mile (1.61 km) or more in width relative to any quadrant of the building site. Exposure D extends inland from shoreline one-fourth mile (0.4 km) or 10 times the building height, whichever is greater.

1.4.1.5. Site Exposure

Even though a building site may have different exposure categories in different directions, the most severe exposure is used for all wind-load calculations regardless of building orientation or direction of wind.

Exposure D is perhaps the easiest to determine because it is explicitly for unobstructed coastal areas directly exposed to large bodies of water. It is not as easy to determine whether a site falls into Exposure B or C because the description of these categories is somewhat ambiguous. Morevoer, the terrain surrounding a site is usually not uniform and can be composed of zones that would be classified as Exposure B while others would be classified as Exposure C. When such a mix is encountered, the more severe exposure governs. The UBC classifies a site as Exposure C when open terrain exists for one full 90° quadrant extending outward from the building for at least one-half mile. If the quadrant is less than 90° or less than one-half mile, then the site is classified as Exposure B. It is essential to select the appropriate category because force levels could differ by as much as 65% between Exposure B and C. It is advisable to contact the local building official before embarking on a building design with a questionable site exposure category. If the site has a view of a cliff or hill, it may be prudent to assign Exposure C to D to account for higher wind velocity effects.

1.4.1.6. Design Wind Pressures

The design wind pressure p is given as a product of the combined height, exposure, and gust factor coefficient Ce; the pressure coefficient Cq; the wind stagnation pressure qs; and building Importance Factor Iw.

The pressure qs manifesting on the surface of a building due to a mass of air with density p, moving at a velocity v is given by Bernoulli's equation:

The density of air p is 0.0765 pcf, for conditions of standard atmosphere, temperature (59 °F), and barometric pressure (29.92 in. of mercury).

Since velocity given in the wind map is in mph, Eq. (1.6) reduces to

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