Page 56 - 2018_IRC

BUILDING PLANNING

The text of Section 6.2.2 of ASTM E1996 shall or structure is to be constructed. For a site located in the
be substituted as follows: transition zone between categories, the category result-
6.2.2 Unless otherwise specified, select the wind ing in the largest wind forces shall apply. Account shall
zone based on the ultimate design wind speed, be taken of variations in ground surface roughness that
V , as follows: arise from natural topography and vegetation as well as
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6.2.2.1 Wind Zone 1–130 mph  ultimate design from constructed features. For a site where multiple
detached one- and two-family dwellings, townhouses or
wind speed, V < 140 mph. other structures are to be constructed as part of a subdi-
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6.2.2.2 Wind Zone 2–140 mph  ultimate design vision or master-planned community, or are otherwise
wind speed, V < 150 mph at greater than 1 mile designated as a developed area by the authority having
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(1.6 km) from the coastline. The coastline shall be jurisdiction, the exposure category for an individual
measured from the mean high-water mark. structure shall be based on the site conditions that will
6.2.2.3 Wind Zone 3–150 mph (58 m/s)  ulti- exist at the time when all adjacent structures on the site
mate design wind speed, V  170 mph (76 m/s), have been constructed, provided that their construction
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or 140 mph (54 m/s)  ultimate design wind is expected to begin within 1 year of the start of con-
speed, V  170 mph (76 m/s) and within 1 mile struction for the structure for which the exposure cate-
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(1.6 km) of the coastline. The coastline shall be gory is determined. For any given wind direction, the
measured from the mean high-water mark. exposure in which a specific building or other structure
6.2.2.4 Wind Zone 4–ultimate design wind speed, is sited shall be assessed as being one of the following
V > 170 mph (76 m/s). categories:
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TABLE R301.2.1.2 1. Exposure B. Urban and suburban areas, wooded
WINDBORNE DEBRIS PROTECTION FASTENING areas or other terrain with numerous closely
SCHEDULE FOR WOOD STRUCTURAL PANELS a, b, c, d spaced obstructions having the size of single-
FASTENER SPACING (inches) a, b family dwellings or larger. Exposure B shall be
FASTENER TYPE Panel 4 feet < 6 feet < assumed unless the site meets the definition of
span ≤ panel span panel span another type exposure.
4 feet ≤ 6 feet ≤ 8 feet
No. 8 wood-screw-based 2. Exposure C. Open terrain with scattered obstruc-
anchor with 2-inch embedment 16 10 8 tions, including surface undulations or other
length irregularities, having heights generally less than
No. 10 wood-screw-based 30 feet (9144 mm) extending more than 1,500
anchor with 2-inch embedment 16 12 9 feet (457 m) from the building site in any quad-
length rant. This exposure shall apply to any building
1 / -inch lag-screw-based located within Exposure B type terrain where the
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anchor with 2-inch embedment 16 16 16 building is directly adjacent to open areas of
length Exposure C type terrain in any quadrant for a dis-
For SI: 1 inch = 25.4 mm, 1 foot = 304.8 mm, 1 pound = 4.448 N, tance of more than 600 feet (183 m). This cate-
1 mile per hour = 0.447 m/s. gory includes flat, open country and grasslands.
a. This table is based on 180 mph ultimate design wind speeds, V , and a 45-
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foot mean roof height. 3. Exposure D. Flat, unobstructed areas exposed to
b. Fasteners shall be installed at opposing ends of the wood structural panel. wind flowing over open water, smooth mud flats,
Fasteners shall be located not less than 1 inch from the edge of the panel. salt flats and unbroken ice for a distance of not
c. Anchors shall penetrate through the exterior wall covering with an less than 5,000 feet (1524 m). This exposure shall
embedment length of not less than 2 inches into the building frame. apply only to those buildings and other structures
Fasteners shall be located not less than 2 / 2 inches from the edge of exposed to the wind coming from over the unob-
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concrete block or concrete.
d. Panels attached to masonry or masonry/stucco shall be attached using structed area. Exposure D extends downwind
vibration-resistant anchors having an ultimate withdrawal capacity of not from the edge of the unobstructed area a distance
less than 1,500 pounds. of 600 feet (183 m) or 20 times the height of the
R301.2.1.3 Wind speed conversion. Where referenced building or structure, whichever is greater.
documents are based on nominal design wind speeds R301.2.1.5 Topographic wind effects. In areas desig-
and do not provide the means for conversion between nated in Table R301.2(1) as having local historical data
ultimate design wind speeds and nominal design wind documenting structural damage to buildings caused by
speeds, the ultimate design wind speeds, V , of Figure wind speed-up at isolated hills, ridges and escarpments
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R301.2(5)A shall be converted to nominal design wind that are abrupt changes from the general topography of
speeds, V , using Table R301.2.1.3. the area, topographic wind effects shall be considered
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R301.2.1.4 Exposure category. For each wind direc- in the design of the building in accordance with Section
tion considered, an exposure category that adequately R301.2.1.5.1 or in accordance with the provisions of
reflects the characteristics of ground surface irregulari- ASCE 7. See Figure R301.2.1.5.1(1) for topographic
ties shall be determined for the site at which the building features for wind speed-up effect.

2018 INTERNATIONAL RESIDENTIAL CODE ® 31
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