Page 78 - 2018_IRC

BUILDING PLANNING

CHECK FOR OBSTRUCTION IN ACCORDANCE
WITH SECTION R301.2.1.5 IF DISTANCE IS
LESS THAN 100 H, OR 2 MILES

UPWIND TOPOGRAPHIC ESCARPMENT
FEATURE
H

RIDGE OR HILL

H = HEIGHT OF EQUIPMENT
FIGURE R301.2.1.5.1(3)
UPWIND OBSTRUCTION

In these designated areas, topographic wind effects R301.2.2 Seismic provisions. Buildings in Seismic
shall apply only to buildings sited on the top half of an Design Categories C, D , D , and D shall be constructed
2
1
0
isolated hill, ridge or escarpment where all of the fol- in accordance with the requirements of this section and
lowing conditions exist: other seismic requirements of this code. The seismic pro-
1. The average slope of the top half of the hill, ridge visions of this code shall apply as follows:
or escarpment is 10 percent or greater. 1. Townhouses in Seismic Design Categories C, D , D 1
0
2. The hill, ridge or escarpment is 60 feet (18 288 and D .
2
mm) or greater in height for Exposure B, 30 feet 2. Detached one- and two-family dwellings in Seismic
(9144 mm) or greater in height for Exposure C, Design Categories, D , D and D .
2
0
1
and 15 feet (4572 mm) or greater in height for Buildings in Seismic Design Category E shall be
Exposure D. designed to resist seismic loads in accordance with the
3. The hill, ridge or escarpment is isolated or unob- International Building Code, except where the seismic
structed by other topographic features of similar design category is reclassified to a lower seismic design
height in the upwind direction for a distance mea- category in accordance with Section R301.2.2.1. Compo-
sured from its high point of 100 times its height nents of buildings not required to be designed to resist
or 2 miles (3.2 km), whichever is less. See Figure seismic loads shall be constructed in accordance with the
R301.2.1.5.1(3) for upwind obstruction. provisions of this code.
4. The hill, ridge or escarpment protrudes by a fac- R301.2.2.1 Determination of seismic design cate-
tor of two or more above the height of other gory. Buildings shall be assigned a seismic design cate-
upwind topographic features located in any quad- gory in accordance with Figure R301.2(2).
rant within a radius of 2 miles (3.2 km) measured R301.2.2.1.1 Alternate determination of seismic
from its high point. design category. The seismic design categories and
R301.2.1.5.1 Simplified topographic wind speed- corresponding short-period design spectral response
up method. As an alternative to the ASCE 7 topo- accelerations, S , shown in Figure R301.2(2), are
DS
graphic wind provisions, the provisions of Section based on soil Site Class D, used as an assumed
R301.2.1.5.1 shall be permitted to be used to design default, as defined in Section 1613.2.2 of the Interna-
for wind speed-up effects, where required by Sec- tional Building Code. If soil conditions are deter-
tion R301.2.1.5. mined by the building official to be Site Class A, B,
Structures located on the top half of isolated or D, the seismic design category and short-period
hills, ridges or escarpments meeting the conditions design spectral response accelerations, S , for a site
DS
of Section R301.2.1.5 shall be designed for an shall be allowed to be determined in accordance with
increased basic wind speed as determined by Table Figure R301.2(3), or Section 1613.2 of the Interna-
R301.2.1.5.1. On the high side of an escarpment, the tional Building Code. The value of S determined in
DS
increased basic wind speed shall extend horizontally accordance with Section 1613.2 of the International
downwind from the edge of the escarpment 1.5 Building Code is permitted to be used to set the seis-
times the horizontal length of the upwind slope mic design category in accordance with Table
(1.5L) or 6 times the height of the escarpment (6H), R301.2.2.1.1, and to interpolate between values in
whichever is greater. See Figure R301.2.1.5.1(2) for Tables R602.10.3(3), R603.9.2(1) and other seismic
where wind speed increase is applied. design requirements of this code.

2018 INTERNATIONAL RESIDENTIAL CODE ® 53
Copyrighted © 2017 by, or licensed to, ICC (ALL RIGHTS RESERVED); licensed to UL, LLC pursuant to License Agreement with ICC. No further reproductions authorized or distribution authorized.
ANY UNAUTHROIZED REPRODUCTION OR DISTRIBUTION IS A VIOLATION OF THE FEDERAL COPYRIGHT ACT AND THE LICENSE AGREEMENT; AND SUBJECT TO CIVIL AND CRIMINAL PENALTIES THEREUNDER.