Course: GFM 103 Exterior Environmental Management

Reading and Audio-Visual Assignments

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Probability of Wind Occurence

Most office buildings are designed for a 50-year mean recurrence interval wind event (2 percent annual probability). A 50-year storm would be expected to happen about once every 50 years; however, a 50-year storm could occur more or less frequently. A 50-year storm might not occur within any 50-year interval, but two 50-year storms could occur within 1 year.

ASCE 7 requires office buildings where more than 300 people congregate in one area to be designed for a 100-year mean recurrence interval wind event (1 percent annual probability); therefore, these particular office buildings are designed to resist stronger, rarer storms than most office buildings. Other office buildings that must be designed for a 100-year mean recurrence interval wind event include: 1) office buildings that will be used for hurricane or other emergency shelter; 2) office buildings housing a day care center with capacity greater than 150 occupants, 3) office buildings designated for emergency preparedness, communication, or emergency operation center or response, 4) office buildings housing critical national defense functions, and 5) office buildings containing sufficient quantities of hazardous materials. See ASCE 7 Table 1-1 for specific criteria.

The importance factor is used to adjust the mean recurrence interval. For a 50-year interval, the importance factor is 1.00. For a 100-year interval, the importance factor is 1.15. Hence, for a 100-year interval, the design loads are increased by 15%.

When designing an office building, designers should consider the following:

Regular winds. In many locations, winds with low to moderate speeds occur daily. Damage is not expected to occur during these events.

Stronger winds. At a given site, stronger winds (e.g., winds with a basic wind speed in the range of 70 to 80 mph peak gust) may occur from several times a year to only once a year or less frequently. 70 to 80 mph is the threshold at which damage normally begins to occur to building elements that have limited wind resistance due to problems associated with inadequate design, strength, application, or material deterioration.

Design level winds. Office buildings that experience design level events and events that are somewhat in excess of design level should experience little, if any damage; however, design level storms frequently cause extensive building envelope damage. Structural damage also occurs, but less often. Damage experienced with design level events is typically associated with inadequate design, application, or material deterioration. The exceptions are wind-driven water infiltration and windborne debris (missiles) damage.

Note: Missile damage is very common during hurricanes and tornadoes. Missiles can puncture roof coverings, many types of exterior walls, and glazing. The IBC does not address missile-induced damage, except for glazing in windborne debris regions. (Windborne debris regions are limited to portions of hurricane-prone regions.) In hurricane-prone regions, significant office building damage should be expected even during design level hurricane events, unless special enhancements are incorporated into the building's design (see Applications section).

Tornadoes. Although more than 1,200 tornadoes typically occur each year in the U.S., the probability of a tornado occurring at any given location is quite small. The probability of occurrence is a function of location. As shown in Figure 2, only a few areas of the U.S. frequently experience tornadoes, and tornadoes are very rare in the west. The Oklahoma City area is the most active location in the U.S., with 106 recorded tornadoes between the years 1890 and 2000.

Except for window breakage, well designed, constructed, and maintained office buildings should experience little if any damage from weak tornadoes. However, because many office buildings have wind-resistance deficiencies, weak tornadoes often cause building envelope damage. Most office buildings experience significant building envelope damage and damage to interior partitions and ceilings if they are in the path of a strong or violent tornado.