47.5 MB - The Whole Building Design Guide

Exposure: The characteristics of the terrainFor additional exposure information,(i.e., ground roughness and surface irregularitiesin the vicinity of a building) influencesee the Commentary of ASCE 7, whichincludes several aerial photographs thatthe wind loading. ASCE 7 defines three exposurecategories, Exposures B, C, and D.illustrate the different terrain conditionsassociated with Exposures B, C, and D.Exposure B is the roughest terrain categoryand Exposure D is the smoothest. ExposureB includes urban, suburban, and woodedareas. Exposure C includes flat open terrain with scattered obstructionsand areas adjacent to water surfaces in hurricane-proneregions (which are defined below under “basic wind speed”).Exposure D includes areas adjacent to water surfaces outside hurricane-proneregions, mud flats, salt flats, and unbroken ice.Because of the wave conditions generated by hurricanes, areasadjacent to water surfaces in hurricane-prone regions are consideredto be Exposure C rather than the smoother Exposure D. Thesmoother the terrain, the greater the wind pressure; therefore,critical buildings located in Exposure C would receive higher windloads than those located in Exposure B, even at the same basicwind speed.Basic wind speed: ASCE 7 specifies the basic wind speed for determiningdesign wind loads. The basic wind speed is measured at 33feet above grade in Exposure C (flat open terrain). If the buildingis located in Exposure B or D, rather than C, an adjustment forthe actual exposure is made in the ASCE 7 calculation procedure.Since the 1995 edition of ASCE 7, the basic wind speed measurementhas been a 3-second peak gust speed. Prior to that time,the basic wind speed was a fastest-mile speed (i.e., the speed averagedover the time required for a mile-long column of air to passa fixed point). 3 Most of the United States has a basic wind speed(peak gust) of 90 mph, but much higher speeds occur in Alaskaand in hurricane-prone regions. The highest speed, 170 mph, occursin Guam.Hurricane-prone regions include Atlantic and Gulf coastal areas(where the basic wind speed is greater than 90 mph), Hawaii, and theU.S. territories in the Caribbean and South Pacific (see Figure 3-1).3. Peak gust speeds are about 15 to 20 mph higher than fastest-mile speeds (e.g., a 90-mph peak basic wind speed is equivalent to a 76-mphfastest-mile wind speed). IBC Chapter 16 provides a table of equivalent basic wind speeds.MAKING CRITICAL FACILITIES SAFE FROM High Wind3-9