14B-16-1609  Wind loads.
The provisions of Section 1609 of IBC are adopted by reference with the following modifications:
1.   Revise Section 1609.1.1 to read:
1609.1.1 Determination of wind loads.
Wind loads on every building or structure shall be determined in accordance with Chapters 26 to 30 of ASCE 7. The basic design wind speed, V, and the exposure category for a site shall be determined in accordance with Section 1609. Wind shall be assumed to come from any horizontal direction and wind pressures shall be assumed to act normal to the surface considered.
Exceptions:
1.   [Reserved.]
2.   Buildings of Group R-3, R-4 or R-5 occupancy using the provisions of AWC WFCM (including buildings containing up to 3 dwelling units).
3.   Buildings of Group R-3, R-4 or R-5 occupancy using the provisions of AISI S230 (including buildings containing up to 3 dwelling units).
4.   Designs using NAAMM FP 1001.
5.   Designs using TIA-222 for antenna-supporting structures and antennas, provided that the horizontal extent of Topographic Category 2 escarpments in Section 2.6.6.2 of TIA-222 shall be 16 times the height of the escarpment.
6.   Wind tunnel tests in accordance with ASCE 49 and Sections 31.4 and 31.5 of ASCE 7.
7.   Wind loads determined in accordance with Chapter 28 of ASCE 7 shall only be allowed for buildings containing only Group F, S or U occupancies.
8.   Wind loads determined in accordance with Section 1609.6.
The wind speeds in Section 1609.3 are basic design wind speeds, V. Allowable stress design wind speeds, Vasd, from Section 1609.3.1 shall be used when the provisions of the standards referenced in Exceptions 4 and 5 are used.”
2.   Delete Section 1609.1.1.1.
3.   Add a new Section 1609.1.2 to read:
1609.1.2 Special investigations.
Special investigations shall be required to determine wind loads for buildings and other structures with a mean roof height exceeding 400 feet (122 m). Special investigations shall be required to determine wind loads for buildings and other structures with a mean roof height of 400 feet (122 m) or less and having irregular shapes, response characteristics or site locations with shielding or channeling effects that warrant special consideration.”
4.   Delete Section 1609.2 and its subsections and tables.
5.   Revise Section 1609.3 to read:
1609.3 Basic design wind speed.
The basic design wind speed, V, for nominal design 3-second gust wind speeds at 33 feet (10 m) above ground for exposure category C for the determination of the wind loads shall be as follows:
1.   The basic design wind speed, V, for use in the design of Risk Category I structures shall be 101 mph (45 m/s).
2.   The basic design wind speed, V, for use in the design of Risk Category II structures shall be 107 mph (48 m/s).
3.   The basic design wind speed, V, for use in the design of Risk Category III structures shall be 114 mph (51 m/s).
4.   The basic design wind speed, V, for use in the design of Risk Category IV structures shall be 119 mph (53 m/s).”
6.   Delete all figures and tables included in Section 1609.3.
7.   Revise Section 1609.3.1 to read:
1609.3.1 Wind speed conversion.
Where required, the basic design wind speeds of Section 1609.3 shall be converted to allowable stress design wind speeds, Vasd, using Equation 16-33.”
8.   Revise the Description of V in Equation 16-33 to read:
V = Basic design wind speed determined from Section 1609.3.”
9.   Delete Table 1609.3.1.
10.   Revise Section 1609.4.2 by deleting the second sentence describing Surface Roughness D.
11.   Insert a new Section 1609.6 to read:
1609.6 Alternative wind loads for enclosed structure with mean roof height of 60 feet or less.
Where the mean roof height of a structure is 60 feet (18.3 m) or less, wind loads on the main windforce-resisting system and on components and cladding shall be permitted to be determined in accordance with Section 1609.6, provided the following conditions are met:
1.   The structure is classified as “enclosed” in accordance with Chapter 26 of ASCE 7.
2.   Both the windward and leeward wind loads are transmitted by roof and vertically spanning wall assemblies, through continuous floor and roof diaphragms, to the main windforce-resisting system.
3.   The longest horizontal dimension of the structure is no more than five times the shortest horizontal dimension of the structure.
4.   The structure has no unusual geometrical irregularity in spatial form.
5.   The structure does not have response characteristics that make it subject to across-wind loading, vortex shedding, or instability caused by galloping or flutter.
6.   The structure does not have a site location for which channeling effects or buffeting in the wake of upwind obstructions warrant special consideration.
1609.6.1 Main windforce-resisting system.
The load due to wind pressure, W, on the main windforce-resisting system shall be based on exposure category and risk category as indicated in Table 1609.6.1.
TABLE 1609.6.1
LOAD DUE TO WIND PRESSURE ON MAIN WINDFORCE-RESISTING SYSTEM c, d, e, f, g
 
EXPOSURE CATEGORY
LOAD DUE TO WIND PRESSURE (pounds per square foot)
Risk Category
I or II
III
IV
B
25
28
31
C
32
37
40
D
38
43
46
 
For SI: 1 pound per square foot = 0.0479 kN/ m 2
a.   Exposure category shall be determined in accordance with Section 1609.4.3.
b.   Risk category shall be determined in accordance with Section 1604.5.
c.   Wind loads shall use the following load cases (see ASCE 7, Chapter 27):
   Case 1: 100 percent of the design wind pressure acting on the projected area perpendicular to each principal axis of the structure, considered separately for each principal axis.
   Case 2: 75 percent of the design wind pressure acting on the projected area perpendicular to each principal axis of the structure, in conjunction with a torsional moment resulting from this wind pressure acting at an eccentricity equal to 15 percent of the building width, considered separately for each principal axis.
   Case 3: Wind loading as defined in Case 1 but considered to act simultaneously on each principal axis at 75 percent of the value specified in Case 1.
   Case 4: Wind loading as defined in Case 2 but considered to act simultaneously on each principal axis at 75 percent of the value specified in Case 2.
d.   For buildings that include parapets, additional parapet load shall be applied in accordance with ASCE 7, Section 27.3.4.
e.   For the main windforce-resisting system of a roof, the values shall be multiplied by 1.0 for negative (uplift) pressure and 0.3 for positive (downward) pressure. (See Section 1609.6.2 for components and cladding.)
f.   All wind loads are ultimate wind loads (W) and shall be utilized in load combinations using strength or load and resistance factor design per Section 1605.2. Where required, service level loads may be obtained by dividing the values in the table by 1.6, however these service level loads shall not be used for either method of load combination prescribed in Section 1605.2 or 1605.3.1.
g.   Projecting elements such as flag poles, sign structures, tanks and towers shall be designed in accordance with ASCE 7.
1609.6.2 Components and cladding.
The load due to wind pressure, W, on components and cladding shall be the values in Table 1609.6.1 multiplied by the multiplier provided in Table 1609.6.2.
TABLE 1609.6.2
MULTIPLIERS TO DETERMINE COMPONENTS AND CLADDING WIND PRESSURE
 
CONDITION
MULTIPLIERk
Flat Roof (less than 7° slope)
Sloped Gable Roofj
Wall (field) a
1.2
1.2
Wall (corner) a, c
1.4
1.4
Roof (field) b
1.0 h
2.0
Roof (edge) b
2.3 d, h
3.0 f, i
Roof (corner) b
3.2 e, h
3.6 g, i
 
For SI: 1 foot = 304.8 mm, 1 degree = 0.017 rad.
a.   Wall pressures shall act toward (positive) or away (negative) from the surface.
b.   Roof pressures shall be considered as acting away (negative) from the surface.
c.   Wall corners shall be defined as a distance from the corner equal to the lesser of 0.1 times the least horizontal dimension or 0.4 times the height in both directions (minimum 3 feet).
d.   Flat roof edges shall be defined as a distance from the roof edge of 1.2 times the height.
e.   Flat roof corners shall be defined as a distance from each roof corner of 0.6 times the height (for a width of 0.2 times the height).
f.   Sloped gable roof edges shall be defined as a distance from the roof edge equal to the lesser of 0.1 times the least horizontal dimension or 0.4 times the height (minimum 3 feet) from the roof edge.
g.   Sloped gable roof corners shall be defined as a distance from any roof corner or peak equal to the lesser of 0.1 times the least horizontal dimension or 0.4 times the height (minimum  3 feet) in both directions.
h.   For overhangs on flat roofs utilize the flat roof multipliers above (includes pressure contribution from both upper and lower surface).
i.   For overhangs on sloped gable roofs, increase the sloped gable roof multipliers above by 30 percent (includes pressure contribution from both upper and lower surface).
j.   Loads above shall not apply to hip roofs (Refer to Chapter 30 of ASCE 7).
k.   Resulting wind loads are ultimate wind loads and shall be utilized in load combinations using strength or load and resistance factor design per Section 1605.2 or converted in accordance with Section 1609.3.1.
(Amend Coun. J. 2-19-20, p. 14473, Art. II, § 74; Amend Coun. J. 6-25-21, p. 32180, Art. II, § 24)