47.5 MB - The Whole Building Design Guide

Tornado: This is a violently rotating columnof air extending from the base of aThe majority of the tornadoes spawnedthunderstorm to the ground. The Fujitaduring hurricanes are classified as F2 orscale categorizes tornado severity based on weaker. However, a few F3 and at leastobserved damage. The six-step scale ranges two F4 tornadoes have been reported.from F0 (light damage) to F5 (incredibledamage). Weak tornadoes (F0 and F1) aremost common, but strong tornadoes (F2 and F3) frequently occur.Violent tornadoes (F4 and F5) are rare. Tornado path widths aretypically less than 1,000 feet; however, widths of approximately2.5 miles have been reported. Wind speed rapidly decreases withincreased distance from the center of a tornado. A critical facilityon the periphery of a strong or violent tornado could be subjectedto moderate to high wind speeds, depending upon the distancefrom the core of the tornado. However, eventhough the wind speed at a given facilitymight not be great, a facility on the peripherycould still be impacted by many largepieces of wind-borne debris. Tornadoes areresponsible for the greatest number of windrelateddeaths each year in the United States.Figure 3-2 shows the frequency of tornadooccurrence for a period between 1950 and1998, and Figure 3-3 shows the design windspeeds recommended by FEMA for designingcommunity shelters.Beginning in February 2007, the NationalWeather Service will use the EnhancedFujita Scale (EF-Scale) to categorizetornado severity. This new scale hassix steps, ranging from EF0 to EF5. Thenew scale was developed by Texas TechUniversity’s Wind Science and EngineeringCenter. See www.spc.noaa.gov/efscale/for further information on the EF-Scale.3.1.2 Probability of OccurrenceVia the importance factor, 2 ASCE 7 requires Category III and IVbuildings to be designed for higher wind loads than Category Iand II buildings (see Section 1.1.1). Hence, critical facilities designedin accordance with ASCE 7 have greater resistance tostronger, rarer storms. When designing a critical facility, designprofessionals should consider the following types of winds.Routine winds: In many locations, winds with low to moderatespeeds occur daily. Damage is not expected to occur duringthese events.2. The importance factor accounts for the degree of hazard to human life and damage to property. Importance factors are given in ASCE 7.MAKING CRITICAL FACILITIES SAFE FROM High Wind3-5