Chapter 6
WIND LOADS

€.1 GENERAL
'1.1 Scope. Buildings and other structures, including the Main
" r-j-Force Resisting System (MWFRS) and all components and
i.»:i.ting thereof, shall be designed and constructed to resist wind
Liianis as specified herein.
• 1-2 Allowed Procedures. The design wind loads for buildings
l: :'her structures, including the MWFRS and component and
-•OJing elements thereof, shall be determined using one of the
fl«?'*ing procedures: (1) Method 1—Simplified Procedure as
-«i",'ned in Section 6.4 for buildings meeting the requirements
»--. ";ed therein; (2) Method 2—Analytical Procedure as speci-
i--: -^ Section 6.5 for buildings meeting the requirements speci-
i-i -Terein; (3) Method 3—Wind Tunnel Procedure as specified
•:: aeoion 6.6.
«.'-* Wind Pressures Acting on Opposite Faces of Each
Itmuding Surface. In the calculation of design wind loads for
ir VA'FRS and for components and cladding for buildings, the
-:kt.uc sum of the pressures acting on opposite faces of each
nu—ng surface shall be taken into account.
- Minimum Design Wind Loading. The design wind load,
»•:.:•—ined by any one of the procedures specified in Sec-
"m"i - ; .2, shall be not less than specified in this section.
-A.1 Main Wind-Force Resisting System. The wind load to
i" &cij in the design of the MWFRS for an enclosed or partially
•JHOooed building or other structure shall not be less than 10 Ib/ft2
'"" k-\/m2) multiplied by the area of the building or structure
csed onto a vertical plane normal to the assumed wind direc-
The design wind force for open buildings and other struc-
Aall be not less than 10 Ib/ft2 (0.48 kN/m2) multiplied by the
.lima i; ^
• ; Components and Cladding. The design wind pressure
ponents and cladding of buildings shall not be less than
.«& .ressure of 10 Ib/ft2 (0.48 kN/m2) acting in either direction
1111—u to the surface.
- 3EFINITIONS
lowing definitions apply only to the provisions of
..*«,,»-: 6:
ndlTROVED: Acceptable to the authority having jurisdiction.
basic WIND SPEED, V: Three-second gust speed at 33 ft
iflftikr above the ground in Exposure C (see Section 6.5.6.3) as
———ned in accordance with Section 6.5.4.
•H-DING, ENCLOSED: A building that does not comply
-- requirements for open or partially enclosed buildings.
•.U-DING ENVELOPE: Cladding, roofing, exterior walls,
.iBiBi- door assemblies, window assemblies, skylight assem-
'•d other components enclosing the building.
BUILDING AND OTHER STRUCTURE, FLEXIBLE;
Slender buildings and other structures that have a fundamental
natural frequency less than 1 Hz.
BUILDING, LOW-RISE: Enclosed or partially enclosed
buildings that comply with the following conditions:
1. Mean roof height h less than or equal to 60 ft (18 m).
2. Mean roof height h does not exceed least horizontal dimen-
sion.
BUILDING, OPEN: A building having each wall at least
80 percent open. This condition is expressed for each wall by
the equation Ao >_ O.&Ag where
Ao = total area of openings in a wall that receives positive
external pressure, in ft2 (m2)
A, = the gross area of that wall in which Ay is identified, in
ft2 (m2)
BUILDING, PARTIALLY ENCLOSED: A building that
complies with both of the following conditions:
1. The total area of openings in a wall that receives positive
external pressure exceeds the sum of the areas of openings
in the balance of the building envelope (walls and roof) by
more than 10 percent.
2. The total area of openings in a wall that receives positive
external pressure exceeds 4 ft2 (0.37 m2) or 1 percent of the
area of that wall, whichever is smaller, and the percentage
of openings in the balance of the building envelope does not
exceed 20 percent.
These conditions are expressed by the following equations:
1. Ao > LlOAo,
2. Ao > 4-sq ft (0.37 m2) or >0.01A,, whichever is smaller,
and ao,/a,, 5 0.20
where
Ao, Ag are as defined for Open Building
Ao; = the sum of the areas of openings in the building en-
velope (walls and roof) not including Ao, in ft2 (m2)
Agi = the sum of the gross surface areas of the building en-
velope (walls and roof) not including Ag, in ft2 (m2)
BUILDING OR OTHER STRUCTURE, REGULAR-
SHAPED: A building or other structure having no unusual geo-
metrical irregularity in spatial form.
BUILDING OR OTHER STRUCTURES, RIGID: A build-
ing or other structure whose fundamental frequency is greater
than or equal to 1 Hz.
BUILDING, SIMPLE DIAPHRAGM: A building in which
both windward and leeward wind loads are transmitted through
floor and roof diaphragms to the same vertical MWFRS (e.g., no
structural separations).
COMPONENTS AND CLADDING: Elements of the build-
ing envelope that do not qualify as part of the MWFRS.

DESIGN FORCE, F: Equivalent static force to be used in
the determination of wind loads for open buildings and other
structures.
DESIGN PRESSURE, p: Equivalent static pressure to be used
in the determination of wind loads for buildings.
EAVE HEIGHT, h: The distance from the ground surface
adjacent to the building to the roof eave line at a particular wall.
If the height of the eave varies along the wall, the average height
shall be used.
EFFECTIVE WIND AREA, A: The area used to determine
GCp. For component and cladding elements, the effective wind
area in Figs. 6-11 through 6-17 and Fig. 6-19 is the span length
multiplied by an effective width that need not be less than one-
third the span length. For cladding fasteners, the effective wind
area shall not be greater than the area that is tributary to an indi-
vidual fastener.
ESCARPMENT: Also known as scarp, with respect to topo-
graphic effects in Section 6.5.7, a cliff or steep slope generally
separating two levels or gently sloping areas (see Fig. 6-4).
FREE ROOF: Roof with a configuration generally conform-
ing to those shown in Figs. 6-18A through 6-18D (monoslope,
pitched, or troughed) in an open building with no enclosing walls
underneath the roof surface.
GLAZING: Glass or transparent or translucent plastic sheet
used in windows, doors, skylights, or curtain walls.
GLAZING, IMPACT RESISTANT: Glazing that has been
shown by testing in accordance with ASTM El 886 and ASTM
El 996 or other approved test methods to withstand the impact of
wind-bome missiles likely to be generated in wind-bome debris
regions during design winds.
HILL: With respect to topographic effects in Section 6.5.7,
a land surface characterized by strong relief in any horizontal
direction (see Fig. 6-4).
HURRICANE PRONE REGIONS: Areas vulnerable to hur-
ricanes; in the United States and its territories defined as
1. The U.S. Atlantic Ocean and Gulf of Mexico coasts where
the basic wind speed is greater than 90 mi/h, and
2. Hawaii, Puerto Rico, Guam, Virgin Islands, and American
Samoa.
IMPACT RESISTANT COVERING: A covering designed
to protect glazing, which has been shown by testing in accor-
dance with ASTM El 886 and ASTM El 996 or other approved
test methods to withstand the impact of wind-bome debris mis-
siles likely to be generated in wind-bome debris regions during
design winds.
IMPORTANCE FACTOR, /: A factor that accounts for the
degree of hazard to human life and damage to property.
MAIN WIND-FORCE RESISTING SYSTEM (MWFRS):
An assemblage of structural elements assigned to provide sup-
port and stability for the overall structure. The system generally
receives wind loading from more than one surface.
MEAN ROOF HEIGHT, h: The average of the roof eave
height and the height to the highest point on the roof surface,
except that, for roof angles of less than or equal to 10°, the mean
roof height shall be the roof heave height.
OPENINGS: Apertures or holes in the building envelope that
allow air to flow through the building envelope and that are
designed as "open" during design winds as defined by these
provisions.
RECOGNIZED LITERATURE: Published research findings
and technical papers that are approved.
RIDGE: With respect to topographic effects in Section 6.5.7
an elongated crest of a hill characterized by strong relief in two
directions (see Fig. 6-4).
WIND-BORNE DEBRIS REGIONS: Areas within hurricane
prone regions located:
1. Within 1 mile of the coastal mean high water line where the
basic wind speed is equal to or greater than 110 mi/h and in
Hawaii, or
2. In areas where the basic wind speed is equal to or greater
than 120 mi/h.
6.3 SYMBOLS AND NOTATION
The following symbols and notation apply only to the provisions
of Chapter 6:
A = effective wind area, in ft2 (m2)
Af = area of open buildings and other structures either nor-
mal to the wind direction or projected on a plane nor-
mal to the wind direction, in ft2 (m2)
Ag = the gross area of that wall in which Ao is identified,
in ft2 (m2)
Agi = the sum of the gross surface areas of the build-
ing envelope (walls and roof) not including A^, in
ft2 (m2)
Ao = total area of openings in a wall that receives positive
external pressure, in ft2 (m2)
An; = the sum of the areas of openings in the building enve-
lope (walls and roof) not including Ao, in ft2 (m2)
Agg = total area of openings in the building envelope in
ft2 (m2)
A, = gross area of the solid freestanding wall or solid sign,
in ft2 (m2)
a =• width of pressure coefficient zone, in ft (m)
5 = horizontal dimension of building measured normal to
wind direction, in ft (m)
5 = mean hourly wind speed factor in Eq. 6-14 from
Table 6-2
&~= 3-s gust speed factor from Table 6-2
Cf = force coefficient to be used in determination of wind
loads for other structures
cn = net pressure coefficient to be used in determination of
wind loads for open buildings
Cp = external pressure coefficient to be used in determina-
tion of wind loads for buildings
c = turbulence intensity factor in Eq. 6-5 from Table
6-2
D = diameter of a circular structure or member, in ft (m)
D' = depth of protruding elements such as ribs and spoilers,
in ft (m)
F = design wind force for other structures, in Ib (N)
G = gust effect factor
Gf = gust effect factor for MWFRSs of flexible buildings
and other structures
GCpn = combined net pressure coefficient for a parapet
GCp = product of external pressure coefficient and gust-
effect factor to be used in determination of wind loads

6.4 METHOD 1—SIMPLIFIED PROCEDURE
6.4.1 Scope. A building whose design wind loads are determined
in accordance with this section shall meet all the conditions of
GCpf = product of the equivalent external pressure coef-
ficient and gust-effect factor to be used in deter-
mination of wind loads for MWFRS of low-rise
buildings
GCpi = product of internal pressure coefficient and gust-
effect factor to be used in determination of wind
loads for buildings
sq == psols. factor for background response in Eqs. 6-4
and 6-8
gK = peak factor for resonant response in Eq. 6-8
gv = peak factor for wind response in Eqs. 6-4 and
6-8
H = height of hill or escarpment in Pig. 6-4, in ft (m)
h = mean roof height of a building or height of other
structure, except that eave height shall be used
for roof angle 0 of less than or equal to 10°, in
ft(m)
he = roof eave height at a particular wall, or the aver-
age height if the eave varies along the wall
/ = importance factor
/j = intensity of turbulence from Eq. 6-5
K\, kz, kt, = multipliers in Fig. 6-4 to obtain K,_,
Kd = wind directionality factor in Table 6-4
Kh = velocity pressure exposure coefficient evaluated
at height z = h
Kz = velocity pressure exposure coefficient evaluated
at height z
Kzt == topographic factor as defined in Section 6.5.7
L = horizontal dimension of a building measured
parallel to the wind direction, in ft (m)
Lh = distance upwind of crest of hill or escarpment
in Fig. 6-4 to where the difference in ground
elevation is half the height of hill or escarpment,
in ft (m)
Ls = integral length scale of turbulence, in ft (m)
Ly = horizontal dimension of return comer for a solid
freestanding wall or solid sign from Fig. 6-20,
in ft (m)
i = integral length scale factor from Table 6-2, ft (m)
n} == reduced frequency from Eq. 6-12
n\ •=- building natural frequency, Hz
p = design pressure to be used in determination of
wind loads for buildings, in Ib/ft2 (N/m2)
pl^ = wind pressure acting on leeward face in Fig. 6-9,
in Ib/ft2 (N/m2)
Pnei = net design wind pressure from Eq. 6-2, in Ib/ft2
(N/m2)
Pnei30 = net design wind pressure for Exposure B at h =
30 ft and / = 1.0 from Fig. 6-3, in Ib/ft2 (N/m2)
pp = combined net pressure on a parapet from
Eq. 6-20, in Ib/ft2 (N/m2)
ps = net design wind pressure from Eq. 6-1, in Ib/ft2
(N/m2)
Ps30 = simplified design wind pressure for Exposure B
at h = 30 ft and / = 1.0 from Fig. 6-2, in Ib/ft2
(N/m2)
pw = wind pressure acting on windward face in
Fig. 6-9, in Ib/ft2 (N/m2)
Q = background response factor from Eq. 6-6
q = velocity pressure, in Ib/ft2 (N/m2)
qit = velocity pressure evaluated at height z = h, in
Ib/ft2 (N/m2)
q, == velocity pressure for internal pressure determi-
nation, in Ib/ft2 (N/m2)
cfp = velocity pressure at top of parapet, in Ib/ft2
(N/m2)
q^ == velocity pressure evaluated at height z above
ground, in Ib/ft2 (N/m2)
R = resonant response factor from Eq. 6-10
rb, Rh, rl = values from Eq. 6-13
Ri = reduction factor from Eq. 6-16
Rn = value from Eq. 6-11
s = vertical dimension of the solid freestanding wall
or solid sign from Fig. 6-20, in ft (m)
r = rise-to-span ratio for arched roofs
V = basic wind speed obtained from Fig. 6-1, in mi/h
(m/s). The basic wind speed corresponds to a
3-s gust speed at 33 ft (10 m) above ground in
exposure Category C
V, == unpartitioned internal volume ft3 (m3)
V; = mean hourly wind speed at height z, ft/s (m/s)
W = width of building in Figs. 6-12 and 6-14A and
B and width of span in Figs. 6-13 and 6-15,
in ft (m)
X = distance to center of pressure from windward
edge in Fig. 6-18, in ft (m)
x = distance upwind or downwind of crest in
Fig. 6-4, in ft (m)
z = height above ground level, in ft (m)
z = equivalent height of structure, in ft (m)
Zg = nominal height of the atmospheric boundary
layer used in this standard. Values appear in
Table 6-2
Zmi, = exposure constant from Table 6-2
a = 3-s gust-speed power law exponent from Table
6-2
a = reciprocal of a from Table 6-2
a = mean hourly wind-speed power law exponent
in Eq. 6-14 from Table 6-2
ft = damping ratio, percent critical for buildings or
other structures
€ = ratio of solid area to gross area for solid free-
standing wall, solid sign, open sign, face of a
trussed tower, or lattice structure
>. = adjustment factor for building height and expo-
sure from Figs. 6-2 and 6-3
6 = integral length scale power law exponent in
Eq. 6-7 from Table 6-2
r] = value used in Eq. 6-13 (see Section 6.5.8.2)
6 = angle of plane of roof from horizontal, in de-
grees
v = height-to-width ratio for solid sign

6.4.1.1 or 6.4.1.2. If a building qualifies only under 6.4.1.2 for
design of its components and cladding, then its MWFRS shall be
designed by Method 2 or Method 3.
6.4.1.1 Main Wind-Force Resisting Systems. For the design of
MWFRSs the building must meet all of the following conditions:
1. The building is a simple diaphragm building as defined in
Section 6.2.
2. The building is a low-rise building as defined in Section 6.2.
3. The building is enclosed as defined in Section 6.2 and con-
forms to the wind-bome debris provisions of Section 6.5.9.3.
4. The building is a regular-shaped building or structure as
defined in Section 6.2.
5. The building is not classified as a flexible building as defined
in Section 6.2.
6. The building does not have response characteristics making
it subject to across wind loading, vortex shedding, instability
due to galloping or flutter; and does not have a site location
for which channeling effects or buffeting in the wake of
upwind obstructions warrant special consideration.
7. The building has an approximately symmetrical cross-
section in each direction with either a flat roof or a gable or
hip roof with0 ^45°.
8. The building is exempted from torsional load cases as in-
dicated in Note 5 of Fig. 6-10, or the torsional load cases
defined in Note 5 do not control the design of any of the
MWFRSs of the building.
6.4.1.2 Components and Cladding. For the design of compo-
nents and cladding the building must meet all the following
conditions:
1. The mean roof height h must be less than or equal to 60 ft
(h <. 60 ft).
2. The building is enclosed as defined in Section 6.2 and con-
forms to the wind-bome debris provisions of Section 6.5.9.3.
3. The building is a regular-shaped building or structure as
defined in Section 6.2.
4. The building does not have response characteristics making
it subject to across wind loading, vortex shedding, instability
due to galloping or flutter; and does not have a site location
for which channeling effects or buffeting in the wake of
upwind obstructions warrant special consideration.
5. The building has either a flat roof, a gable roof with 6 ^45°,
or a hip roof with 6 5 27°.
6.4.2 Design Procedure.
1. The basic wind speed V shall be determined in accordance
with Section 6.5.4. The wind shall be assumed to come from
any horizontal direction.
2. An importance factor I shall be determined in accordance
with Section 6.5.5.
3. An exposure category shall be determined in accordance
with Section 6.5.6.
4. A height and exposure adjustment coefficient, A,, shall be
determined from Pig. 6-2.
6.4.2.1 Main Wind-Force Resisting System. Simplified design
wind pressures, ps, for the MWFRSs of low-rise simple dia-
phragm buildings represent the net pressures (sum of internal and
external) to be applied to the horizontal and vertical projections
of building surfaces as shown in Fig. 6-2. For the horizontal pres-
sures (zones A, B, C, D), ps is the combination of the windward
and leeward net pressures, ps shall be determined by the following
equation:
p, = \K»Ipsw                              (6-1) |
where
>. = adjustment factor for building height and exposure
from Fig. 6-2
kz, = topographic factor as defined in Section 6.5.7 evaluated |
at mean roof height, h                              |
/ = importance factor as defined in Section 6.2
p530 == simplified design wind pressure for Exposure B, at
h = 30 ft, and for / = 1.0, from Fig. 6-2
6.4.2.1.1 Minimum Pressures. The load effects of the design
wind pressures from Section 6.4.2.1 shall not be less than the
minimum load case from Section 6.1.4.1 assuming the pressures,
ps, for zones A, B, C, and D all equal to +10 psf, while assuming
zones E, F, G, and H all equal to 0 psf.
6.4.2.2 Components and Cladding. Net design wind pressures,
pnei, for the components and cladding of buildings designed using
Method 1 represent the net pressures (sum of internal and external)
to be applied normal to each building surface as shown in Fig. 6-3.
pnei ^all be determined by the following equation:
Pnet == >-K,tIpnet30                                        (6-2) |
where
k = adjustment factor for building height and exposure from
Fig. 6-3
Kit = topographic factor as defined in Section 6.5.7 evaluated I
at mean roof height, h                               |
/ = importance factor as defined in Section 6.2
PneW = net design wind pressure for exposure 5, at h = 30 ft,
and for / = 1.0, from Fig. 6-3
6.4.2.2.1 Minimum Pressures. The positive design wind
pressures, pnet, from Section 6.4.2.2 shall not be less than
+10 psf, and the negative design wind pressures, pnei, from
Section 6.4.2.2 shall not be less than -10 psf.
6.4.3 Air Permeable Cladding. Design wind loads determined
from Fig. 6.3 shall be used for all air permeable cladding unless
approved test data or the recognized literature demonstrate lower
loads for the type of air permeable cladding being considered.
6.5 METHOD 2—ANALYTICAL PROCEDURE
6.5.1 Scope. A building or other structure whose design wind
loads are determined in accordance with this section shall meet
all of the following conditions:
1. The building or other structure is a regular-shaped building
or structure as defined in Section 6.2.
2. The building or other structure does not have response char-
acteristics making it subject to across wind loading, vortex
shedding, instability due to galloping or flutter; or does not
have a site location for which channeling effects or buf-
feting in the wake of upwind obstructions warrant special
consideration.
6.5.2 Limitations. The provisions of Section 6.5 take into con-
sideration the load magnification effect caused by gusts in reso-
nance with along-wind vibrations of flexible buildings or other
structures. Buildings or other structures not meeting the require-
ments of Section 6.5.1, or having unusual shapes or response

-*»acteristics, shall be designed using recognized literature
Documenting such wind load effects or shall use the wind tunnel
•ocedure specified in Section 6.6.
<L5,2.1 Shielding. There shall be no reductions in velocity pres-
iar- due to apparent shielding afforded by buildings and other
aractures or terrain features.
<kSJ,2 Air Permeable Cladding. Design wind loads deter-
——ed from Section 6.5 shall be used for air permeable cladding
'•less approved test data or recognized literature demonstrate
h—cr loads for the type of air permeable cladding being
-awdered.
AJ-J Design Procedure.
; The basic wind speed V and wind directionality factor K^
shall be determined in accordance with Section 6.5.4.
1 An importance factor I shall be determined in accordance
»ith Section 6.5.5.
: An exposure category or exposure categories and velocity
pressure exposure coefficient K^ or K/,, as applicable, shall
be determined for each wind direction in accordance with
Section 6.5.6.
•'• A topographic factor Kz, shall be determined in accor-
dance with Section 6.5.7.
; A siist effect factor G or G f, as applicable, shall be deter-
auned in accordance with Section 6.5.8.
i- An enclosure classification shall be determined in accor-
dance with Section 6.5.9.
'ilernal pressure coefficient GCp, shall be determined in
A^-ordance with Section 6.5.11.1.
» Lcermil pressure coefficients Cp or GCp/, or force coeffi-
"-/nis Cf, as applicable, shall be determined in accordance
»iih Section 6.5.11.2 or 6.5.11.3, respectively.
* Velocity pressure qz or qh, as applicable, shall be deter-
•mned in accordance with Section 6.5.10.
i Design wind load p or F shall be determined in accor-
tece with Sections 6.5.12, 6.5.13, 6.5.14, and 6.5.15, as
^piicable.
'•lit* 1—c Wind Speed. The basic wind speed, V, used in
ik —enination of design wind loads on buildings and other
———b shall be as given in Fig. 6-1 except as provided in
W—— 6-5.4.1 and 6.5.4.2. The wind shall be assumed to come
ii— — horizontal direction.
i«fc*J Special Wind Regions. The basic wind speed shall be in-
——— ••ere records or experience indicate that the wind speeds
•Rilllkr dun those reflected in Fig. 6-1. Mountainous terrain,
JfLJnd special regions shown in Fig. 6-1 shall be examined for
———I »«»d conditions. The authority having jurisdiction shall,
ii! '--.aBary adjust the values given in Fig. 6-1 to account for
.wwms suai »ind speeds. Such adjustment shall be based on me-
al information and an estimate of the basic wind speed
• accordance with the provisions of Section 6.5.4.2.
—•ation of Basic Wind Speeds from Regional
Dta. In areas outside hurricane-prone regions, re-
-:ic data shall only be used in lieu of the basic
^ a\ en in Fig. 6-1 when (1) approved extreme-value
•lysis procedures have been employed in reducing
I (2) the length of record, sampling error, averaging
er height, data quality, and terrain exposure of
the anemometer have been taken into account. Reduction in basic |
wind speed below that of Fig. 6-1 shall be permitted.           |
In hurricane-prone regions, wind speeds derived from simula-
tion techniques shall only be used in lieu of the basic wind speeds
given in Fig. 6-1 when (1) approved simulation and extreme value
statistical analysis procedures are used (the use of regional wind
speed data obtained from anemometers is not permitted to define
the hurricane wind-speed risk along the Gulf and Atlantic coasts,
the Caribbean, or Hawaii) and (2) the design wind speeds result-
ing from the study shall not be less than the resulting 500-year
return period wind speed divided by \/1.5.
In areas outside hurricane-prone regions, when the basic wind
speed is estimated from regional climatic data, the basic wind
speed shall be not less than the wind speed associated with an
annual probability of 0.02 (50-year mean recurrence interval),
and the estimate shall be adjusted for equivalence to a 3-s gust
wind speed at 33 ft (10 m) above ground in exposure Category
C. The data analysis shall be performed in accordance with this
chapter.
6.5.4.3 Limitation. Tornadoes have not been considered in de-
veloping the basic wind-speed distributions.
6.5.4.4 Wind Directionality Factor. The wind directionality
factor, K^, shall be determined from Table 6-4. This factor shall
only be applied when used in conjunction with load combinations
specified in Sections 2.3 and 2.4.
6.5.5 Importance Factor. An importance factor, 7, for the build-
ing or other structure shall be determined from Table 6-1 based
on building and structure categories listed in Table 1-1.
6.5.6 Exposure. For each wind direction considered, the upwind
exposure category shall be based on ground surface roughness
that is determined from natural topography, vegetation, and con-
structed facilities.
6.5.6.1 Wind Directions and Sectors. For each selected wind
direction at which the wind loads are to be evaluated, the expo-
sure of the building or structure shall be determined for the two
upwind sectors extending 45° either side of the selected wind di-
rection. The exposures in these two sectors shall be determined
in accordance with Sections 6.5.6.2 and 6.5.6.3 and the exposure
resulting in the highest wind loads shall be used to represent the
winds from that direction.
6.5.6.2 Surface Roughness Categories. A ground surface
roughness within each 45° sector shall be determined for a dis-
tance upwind of the site as defined in Section 6.5.6.3 from the
categories defined in the following text, for the purpose of assign-
ing an exposure category as defined in Section 6.5.6.3.
Surface Roughness B: Urban and suburban areas, wooded ar-
eas, or other terrain with numerous closely spaced obstructions
having the size of single-family dwellings or larger.
Surface Roughness C: Open terrain with scattered obstructions
having heights generally less than 30 ft (9.1 m). This category
includes flat open country, grasslands, and all water surfaces in
hurricane prone regions.
Surface Roughness D: Flat, unobstructed areas and water sur-
faces outside hurricane prone regions. This category includes
smooth mud flats, salt flats, and unbroken ice.
6.5.6.3 Exposure Categories
Exposure B: Exposure B shall apply where the ground surface
roughness condition, as defined by Surface Roughness B, prevails
in the upwind direction for a distance of at least 2,600 ft (792 m)
or 20 times the height of the building, whichever is greater.