Page 77 - 2018_IRC

BUILDING PLANNING

TABLE R301.2.1.5.1
ULTIMATE DESIGN WIND SPEED MODIFICATION FOR TOPOGRAPHIC WIND EFFECT a, b
ULTIMATE DESIGN AVERAGE SLOPE OF THE TOP HALF OF HILL, RIDGE OR ESCARPMENT (percent)
WIND SPEED FROM 0.10 0.125 0.15 0.175 0.20 0.23 0.25
FIGURE R301.2(5)A
(mph) Required ultimate design wind speed-up, modified for topographic wind speed-up (mph)
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.
a. Table applies to a feature height of 500 feet or less and dwellings sited a distance equal or greater than half the feature height.
b. 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.

H H/2

Lh H/2 Lh

L = 2Lh L= 2Lh

ESCARPMENT RIDGE OR HILL

Note: H/2 determines the measurement point for Lh. L is twice Lh.
FIGURE R301.2.1.5.1(1)
TOPOGRAPHIC FEATURES FOR WIND SPEED-UP EFFECT

GREATER OF 1.5L OR 6H
APPLY INCREASED
WIND SPEED TO
TOP HALF OF HILL OR
RIDGE
H H/2

Lh

L

HILL OR RIDGE ESCARPMENT
FIGURE R301.2.1.5.1(2)
ILLUSTRATION OF WHERE ON A TOPOGRAPHIC FEATURE, WIND SPEED INCREASE IS APPLIED

52 2018 INTERNATIONAL RESIDENTIAL CODE ®
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