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As an alternative to the ASCE 7 topographic wind provisions, the provisions of Section R301.2.1.5.1 shall be permitted to be used to design for wind speed-up effects, where required by Section R301.2.1.5.
Structures located on the top half of isolated hills, ridges or escarpments meeting the conditions of Section R301.2.1.5 shall be designed for an increased basic wind speed as determined by Table R301.2.1.5.1. On the high side of an escarpment, the increased basic wind speed shall extend horizontally downwind from the edge of the escarpment 1.5 times the horizontal length of the upwind slope (1.5L) or 6 times the height of the escarpment (6H), whichever is greater. See Figure R301.2.1.5.1(2) for where wind speed increase is applied.
TABLE R301.2.1.5.1
ULTIMATE DESIGN WIND SPEED MODIFICATION FOR TOPOGRAPHIC WIND EFFECTa, b
ULTIMATE DESIGN WIND SPEED FROM FIGURE R301.2(2)(mph) AVERAGE SLOPE OF THE TOP HALF OF HILL, RIDGE OR ESCARPMENT (percent)
0.10 0.125 0.15 0.175 0.20 0.23 0.25
Required ultimate design wind speed-up, modified for topographic wind speed-up (mph)
95 114 119 123 127 131 137 140
100 120 125 129 134 138 144 147
105 126 131 135 141 145 151 154
110 132 137 142 147 152 158 162
115 138 143 148 154 159 165 169
120 144 149 155 160 166 172 176
130 156 162 168 174 179 NA NA
140 168 174 181 NA NA NA NA
150 180 NA NA NA NA NA NA
For SI: 1 mile per hour = 0.447 m/s, 1 foot = 304.8 mm.
NA = Not Applicable.
  1. Table applies to a feature height of 500 feet or less and dwellings sited a distance equal or greater than half the feature height.
  2. Where the ultimate design wind speed as modified by Table R301.2.1.5.1 equals or exceeds 140 miles per hour, the building shall be considered as "wind design required" in accordance with Section R301.2.1.1.
FIGURE R301.2.1.5.1(1)
TOPOGRAPHIC FEATURES FOR WIND SPEED-UP EFFECT
FIGURE R301.2.1.5.1(2)
ILLUSTRATION OF WHERE ON A TOPOGRAPHIC FEATURE, WIND SPEED INCREASE IS APPLIED
FIGURE R301.2.1.5.1(3)
UPWIND OBSTRUCTION
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