SECTION 204

DEAD LOADS

204.1 General
Dead loads consists of the weight of all
materials of construction incorporated into
the building or other structure, including but
not limited to walls, floors, roofs, ceilings,
stairways, built-in partitions, finishes,
cladding and other similarly incorporated
architectural and structural items, and fixed
service equipment, including the weight of
cranes.

204.2 Weights of Materials and

Constructions
The actual weights of materials and
constructions shall be used in determining
dead loads for purposes of design. In the
absence of definite information, it shall be
permitted to use the minimum values in
Tables 204-1 and 204-2.

204.3 Partition Loads

Floors in office buildings and other buildings
where partition locations are subject to
change shall be designed to support, in
addition to all other loads, a uniformly
distributed dead load equal to 1.0kPa.

Exception:
Access floor systems shall be designed to
support, in addition to all other loads, a
uniformly distributed dead load not less than
0.5kPa.
SECTION 205
LIVE LOADS 205.3.2 Concentrated Loads
Floor shall be designed to support safely the
uniformly distributed live loads given in
205.1 General Table 205-1 whichever produces the greatest
load effects. Unless otherwise specified the
Live loads shall be the maximum loads indicated concentration shall be assumed to
expected by the intended use or occupancy be uniformly distributed over an area 750-
but in no case shall be less than the loads mm square and shall be located so as to
required by this section. produce the maximum load effects in the
structural member.

205.2 Critical Distribution of Live

Loads Provision shall be made in areas where
Where structural members are arranged to vehicles are used or stored for concentrated
create continuity, members shall be designed loads. L, consisting of two or more loads
using the loading conditions, which would spaced 1.5m nominally on center without
cause maximum shear and bending moments. uniform live loads. Each load shall be 40
This requirement may be satisfied in percent of the gross weight of the maximum
accordance with the provisions of Section size vehicle to be accommodated. Parking
205.3.2 or 205.4.2, where applicable. garages for the storage of private or pleasure-
type motor vehicles with no repair or
refueling shall have a floor system designed
for a concentrated load of not less than 9 kN
205.3 Floor Live Loads acting on an area of 0.0015 m2 without
uniform live loads. The condition of
concentrated or uniform live load, combined
205.3.1 General in accordance with Section 203.3 or 203.4 as
appropriate, producing the greatest stresses
Floor shall be designed for the unit live loads
shall govern.
as set forth in Table 205-1. There loads shall
be taken as the minimum live loads of
horizontal projection to be used in the design
of buildings for the occupancies listed, and 205.3.4 Special Loads
loads at least equal shall be assumed for uses Provision shall be made for the special
not listed in this section but that creates or vertical and lateral loads as set forth in Table
accommodates similar loadings. 205-2.

Where it can be determined in designing

floors that the actual live load will be greater
than the value shown in Table 205-1, the
actual live load shall be used in the design of
such buildings or portions thereof. Special
provisions shall be made for machine and
apparatus loads.

205.3.2 Distribution of Uniform Floor

Loads
Where uniform floor loads are involved,
consideration may be limited to full dead load
on oil spans in combination with full live load
on adjacent spans and alternate spans.
205.4 Roof Live Loads 205.4.3 Unbalanced loading
Unbalanced loads shall be used where such
loading will result in larger members or
205.4.1 General connections. Trusses and arches shall be
designed to resist the stresses caused by unit
Roofs shall be designed for the unit live
live loads on one-half of the span if such
loads, L, set forth in Table 205-3. The live
loading results in reverse stresses, or stresses
loads shall be assumed to act vertically upon
the area projected on a horizontal place. greater in any portion than the stresses
produced by the required unit live load on the
entire span. For roofs whose structures are
composed of a stressed shell, framed or solid,
205.4.2 Distribution of Loads
wherein stresses caused by any point loading
Where uniform roof loads are involved in the are distributed throughout the area of the
design of structural members arranged to shell, the requirements for unbalanced unit
create continuity, consideration may be live load design may be reduced 50 percent.
limited to full dead loads on all spans in
combination with full roof live loads on
adjacent spans and on alternate spans. 205.4.4 Special Roof Loads
Roofs to be used for special purposes shall be
designed for appropriate loads as approved
Exception:
by the building official. Greenhouse roof
Alternate span loading need to be considered bars, purlins and rafters shall be designed to
where the uniform roof live load is 1.01kPa carry a 0.45 kN concentrated load, L, in
or more. addition in the uniform live load.

For those conditions where light-page metal 205.5. Reduction of Live Loads
performed structural sheets serve as the
The design live load determined using the
support and finish of roofs, roof structural
unit live loads as set forth in Table 205-1 for
members arranged to create continuity shall
roofs may be reduced on any member
be considered adequate if designed for full
supporting more than 15 m2, including flat
dead loads on all spans in combination with
slabs, except for floors in places of public
the most critical one of the following
superimposed loads: assembly and for live loads greater than 4.8
kPa, in accordance with the following
equation:
1. The uniform roof live load, L, set R=r(A-15) (205-1)
forth in Table 205-3 on all spans.
The reduction shall not exceed 40 percent for
2. A concentrated gravity load, L, of 9
members receiving load from one level, 60
kN placed on any span supporting a
percent for other members or R, as
tributary area greater than 18m2 to
determined by the following equation:
create maximum stresses in the
number, whenever this loading R= 23.1(1+ D/L) (205-2)
creates greater stresses than those
caused by the uniform live load. The where
concentrated load shall be placed on A = area of floor or roof supported by the
the member over a length of 0.75 m member, m2
along the span. The concentrated load
need not be applied to more than one D = dead load per square meter of area
span simultaneously. supported by the member, kPa
3. Water accumulation as prescribed in L = unit live load per square meter of area
Section 206.7. supported by the member, kPa
R = reduction in percentage.
r = rate of reduction equal to 0.08 for floors.
See table 205-3 for roofs.
For storage loads exceeding 4.8 kPa, no
reduction shall be made, except that design
live loads on columns may be reduced 20
percent.
The live load reduction shall not exceed 40
percent in garages for the storage of private
pleasure cars having a capacity of not more
than nine passengers per vehicle.

205.6 Alternate Floor Live Load

Reduction
As an alternate to Equation 205-1, the unit
live loads set forth in Table 205-1 may be
reduced in accordance with Equation 205-
3 on any member, including flat slabs,
having an influence area of 40 m2 or more.

1
L = L0 [0.25 + 4.57 ( )] (205-3)
√𝐴1

where
A1 = influence area, m2
L = reduced design live load per square meter
of area supported by the member
L0 = unreduced design live load per square
meter of area supported by the member
(Table 205-1)

The influence are A1 is four times the

tributary area for a column, two times the
tributary for a beam, equal to the panel area
for a two-way slab, and equal to the product
of the span and the full flange width for a
precast T-beam.
The reduced live load shall not be less than
50 percent of the unit live load L0 for
members receiving load from one level only,
nor less than 40 percent of the unit live load
L0 for other members.
SECTION 206
OTHER MINIMUM LOADS 206.4 Anchorage of Concrete and
Masonry Walls
Concrete and masonry walls shall be
206.1 General anchored as required by Section 104.3.3.
Such anchorage shall be capable of resisting
In addition to the other design loads specified
the load combinations of Section 203.3 or
in this chapter, structures shall be designed to
203.4 using the greater of the wind or
resist the loads specified in this section and
earthquake loads required by this chapter or a
the special loads set forth in Table 205-2. See
minimum horizontal force of 4 kN/m of wall,
Section 207 for design wind loads and
substituted for E.
Section 208 for design earthquake loads.

206.5 Interior Wall Loads

206.2 Other Loads
Interior walls, permanent partitions and
Buildings and other structures and portions temporary partitions that exceed 1.8 m in
thereof shall be designed to resist all loads height shall be designed to resist all loads to
due to applicable fluid pressures, F, lateral which they are subjected but not less than a
soil pressures, H, ponding loads, P and self- load, L, of 0.25 kPa applied perpendicular to
straining forces. T. See Section 206.7 for the walls. The 0.25 kPa load need not be
ponding loads for roofs. applied simultaneously with wind or seismic
loads. The deflection of such walls under a
load of 0.25 kPa shall not exceed 1/240 of the
206.3 Impact Loads span for walls with brittle finishes and 1/120
of the span for walls with flexible finishes.
The live loads specified in Section 205.3 shall
See Table 208-13 for earthquake design
be assumed to include allowance for ordinary
requirements where such requirements are
impact conditions. Provisions shall be made
more restrictive.
in the structural design for uses and loads that
involve unusual vibration and impact forces.
See Section 206.9.3 for impact loads for
cranes, and Section 20.10 for heliport and Exception:
helistop landing areas. Flexible, folding or portable partitions are not
required to meet the load and deflection
criteria but must be anchored to the
206.3.1 Elevators supporting structure to meet the provisions of
this code.
All elevator loads shall be increased by 100%
for impact.

206.6 Retaining Walls

206.3.2 Machinery Retaining walls shall be designed to resist
For the purpose of design, the weight of loads due to the lateral pressure of retained
machinery and moving loads shall be materials in accordance with accepted
increased as follows to allow for impact: engineering practice. Walls retaining drained
soil, where the surface of the retained soil is
1. Elevator machinery 100% level, shall be designed for a load, H,
equivalent to that exerted by a fluid
2. Light machinery, shift-or motor-driven 20%
weighting not less than 4.7 kPa per meter of
3. Reciprocating machinery or power-driven units 50% depth and having a depth equal to that of the
retained soil. Any surcharge shall be in
4. Hangers for floors and balconies 33%
addition to the equivalent fluid pressure.
All percentages shall be increased where
Retaining walls shall be designed to resist
specified by the manufacturer.
sliding by at least 1.5 times the lateral force
and overturning by at least 1.5 times the
overturning moment, using allowable stress 206.9.3 Vertical Impact Force
design loads.
The maximum wheel loads of the crane shall
be increased by the percentages shown below
to determine the induced vertical impact or
206.7 Water Accumulation vibration force:
All roofs shall be designed with sufficient 1. Monorail cranes (powered) 25%
slope or camber to ensure adequate drainage
after the long-term deflection from dead load 2. Cab-operated or remotely operated bridge
or shall be designed to resist ponding load, P,
Cranes (powered) 25%
combined in accordance with Section 203.3
or 204.3. Pounding loaf shall include way 3. Pendant-operated bridge cranes (powered) 10%
water accumulation from any source due to
4. Bridge cranes or monorail cranes
deflection.
with hand-geared, trolley and hoist 0%

206.8 Uplift on Floors and Foundations

206.9.4 Lateral Force
In the design of basement floors and similar
approximately horizontal elements below The lateral force on crane runway beams with
grade, the upward pressure of water, where electrically powered trolleys shall be
applicable, shall be taken as the full calculated as 20% of the sum of the rated
hydrostatic pressure applied over the entire capacity of the crane and the weight of the
area. The hydrostatic load shall be measured hoist and trolley. The lateral force shall be
from the underside of the construction. Any assumed to act horizontally at the traction
other upward loads shall be included in the surface of a runway beam, in either direction
design. perpendicular to the beam, and shall be
distributed with due regard to the lateral
Where expansive soils are present under stiffness of the runway beam and supporting
foundations or slabs-on-ground, the structure.
foundations, slabs and other components
shall be designed to tolerate the movement or
resist the upward loads caused by the
206.9.5 Longitudinal Forces
expansive soils, or the expansive soil shall be
removed or stabilized around and beneath the The longitudinal force on crane runway
structure. beams, except for bridge cranes with hand-
geared bridges, shall be calculated as 10% of
the maximum wheel loads of the crane. The
206.9 Crane Loads longitudinal force shall be assumed to act
horizontally at the traction surface of a
206.9.1 General runway beam, in either direction parallel to
The crane load shall be the rated capacity of the beam.
the crane. Design loads for the runway
beams, including connections and support
brackets of moving bridge cranes and 206.10 Heliport and Helistop Landing
monorail cranes shall include the maximum Areas
wheel loads of the crane and the vertical
impact, lateral, and longitudinal forces
In addition to other design requirements
induced by the moving crane. of this chapter, heliport and helistop
landing or touchdown areas shall be
206.9.2 Maximum Wheel Load designed for the following loads,
The maximum wheel loads shall be the combined in accordance with Section
wheel loads produced by the weight of the 203.3 or 203.4:
bridge, as applicable, plus the sum of the
1. Dead load plus actual weight of the
rated capacity and the weight of the trolley
helicopter.
with the trolley positioned on its runway
2. Dead load plus a single
where the resulting load effect is maximum.
concentrated impact load, L,
covering 0.10 m2 of 0.75 times the
 fully loaded weight of the
helicopter if it is equipped with
hydraulic-type shock absorbers, or
1.5 times the fully loaded weight
of the helicopter if it is equipped
with a rigid or skid-type landing
gear.
The dead load plus a uniform live
load, L, of 4.8 kPa. The required live
load may be reduced in accordance
with Section 205.5 or 205.6.
SECTION 207 Partially Enclosed, and Open
Buildings of All Heights: The
WIND LOADS procedure is the former “buildings of
all heights method” in NSCP 2010
(ASCE 7-05), Method 2. A simplified
207.1 Specifications procedure, based on the Directional
Procedure, is provided for buildings
Buildings and other vertical structures shall up to 48m in height.
be designed and constructed to resist wind
loads as specified and presented in Sections  Section 207C discusses about
207A through 207F. Envelope Procedure for Enclosed and
Antenna towers and antenna supporting Partially Enclosed Low-Rise
structures shall be designed and constructed Buildings: This procedure is the
to resist wind loads as specified and former “low-rise buildings method” in
presented in ANSI/TIA-222-G-2005, entitled NSCP 2010 (ASCE 7-05), Method 2.
as ‘Structural Standards for Steel Antenna This section also incorporates NSCP
Tower and Antenna Supporting Structures 2010 (ASCE 7-05), Method 1 for
and ANSI/TIA-222-G-1-2007, entitled as MWFRS applicable to the MWFRS of
‘Structural Standards for Steel Antenna enclosed simple diaphragm buildings
Tower and Antenna Supporting Structures – less than 18m in height.
Addendum I.
 Section 207D discusses Other
Structures and Building
Appurtenances: A single section is
207A General Requirements
dedicated to determining wind loads
on non-building structures such as
signs, rooftop structures, and towers.
Commentary:
The format and layout of the wind load  Section 207E discusses about
provisions in this code have been Components ad Cladding. This code
significantly revised from NSCP 2001 and addresses the determination of
NSCP 2010 editions. The goal was to component and cladding loads in a
improve the organization, clarity, and use of single section. Analytical and
the wind load provisions by creating simplified methods are provided based
individual sub-sections organized according on the building height. Provisions for
to the applicable major subject areas. The open buildings and building
wind load provisions are now presented in appurtenances are also addressed.
Section 207A through 207F as opposed to
prior editions, where the provisions were  Section 207F discusses about Wind
contained in a single section. Tunnel Procedure.

 Section 207A provides the basic wind 207A.1 Procedures

design parameters that are applicable
207A.1.1 Scope
to the various wind load determination
methodologies outlined in Sections Buildings and other structures, including the
207B through 207F. Items covered in Main Wind-Force Resisting System
Section 207A include definitions, basic (MWFRS) and all components and cladding
wind speed, exposure categories, (C&C) thereof, shall be designed and
internal pressures, enclosure constructed to resist the wind loads
classification, gust-effects and determined in accordance with Section 207A
topographic factors, among others. A through 207F. The provisions of this section
general description of each section is define basic wind parameters for use with
provided below: other provisions contained in this code.

 Section 207B discusses about

Directional Procedure for Enclosed,
Commentary: including section references, is provided in
Figure 207A.1-1.
The procedures specified in this code provide
wind pressures and forces for the design of This version of the wind load standard
MWFRS and for the design components and provides several procedures (as illustrated in
cladding (C&C) of buildings and other Table 207A.1-1) from which the designer can
structures. The procedures involve the choose.
determination of wind directionally and
For MWFRS:
velocity pressure, the selection or
determination of an appropriate gust effect 1. Directional Procedure for Buildings of All
factor, and the selection of appropriate Heights (Section 207B)
pressure or force coefficients. The procedure
allows for the level of structural reliability 2. Envelope Procedure for Low-Rise
required, the effects of differing wind Buildings (Section 207C)
exposures, the speed-up effects of certain 3. Directional Procedure for Building
topographic features such as hills and Appurtenances and Other Structures (Section
escarpments, and the size and geometry of the 207D)
building or other structure under
consideration. The procedure differentiates 4. Wind Tunnel Procedure for All Buildings
between rigid and flexible buildings and and Other Structures (Section 207F)
other structures, and the result generally For Components and Cladding:
envelop the most critical load conditions for
the design of MWFRS as well as C&C. 1. Analytical Procedure for Buildings and
Building Appurtenances (Section 207E)
The pressure and for coefficients provided in
Sections 207B, 207C, 207D, and 207E have 2. Wind Tunnel Procedure for All Buildings
been assembled from the latest boundary- and Other Structure (Section 207F)
layer wind tunnel and full-scale tests and
from previously available literature. Because
the boundary-layer wind-tunnel results were A “simplified method” for which the designer
obtained for specific types of building, such can select wind pressures directly from a
as low- or high-rise buildings and buildings table without any calculation, when the
having specific types of structural framing building meets all the requirements for
system, the designer is cautioned against application of the method, is provided for
indiscriminate interchange of values among designing buildings using the Directional
the figures and tables. Procedure (Section 207B, Part 2), the
Envelope Procedure (Section 207C, Part 2)
and the Analytical Procedure for
207A.1.2 Permitted Procedures Components and Cladding (Section 207E).

The design wind loads for buildings and other

structures, including the MWFRS and C&C Limitations. The provisions given under
elements thereof, shall be determined using Section 207A1.2 apply to the majority of site
one of the procedures as specified in this locations and buildings and structures, but
article. An outline of the overall process for for some projects, these provisions may be
the determination of the wind loads, inadequate. Examples of site locations and
including section references, is provided in buildings and structures (or portions thereof)
Figure 207A.1-1. that may require other approved standards,
special studies using applicable recognized
literature pertaining to wind effects, or using
Commentary: the wind tunnel procedure of Section 207F
The design wind loads for buildings and include:
other structures, including the MWFRS and 1. Site locations that have channeling effects
C&C elements thereof, shall be determined or wakes from upwind obstructions.
using one of the procedures as specified in Channeling effects can be caused by
this article. An outline of the overall process topographic features (e.g., a mountain gorge)
for the determination of the wind loads, or buildings (e.g., a neighboring tall
buildings or a cluster of tall buildings).
Wakes can be caused by hills or by buildings established using the structural properties
or other structures. and deformational characteristics of the
resisting elements in a properly substantiated
2. Buildings with unusual or irregular
analysis, and not utilizing approximate
geometric shape, including barrel vaults, and
equations based on height.
other buildings whose shape (in plan or
vertical cross-section) differs significantly
from the shapes in Figures 207B.4-1, 207B.-
Shielding. Due to the lack of reliable
2, 207B.-7, 207C.4-1, and 207E.-1 to 207E.-
analytical procedures for predicting the
7. Unusual or irregular geometric shapes
effects of shielding provided by buildings and
include buildings with multiple setbacks,
other structures or by topographic features,
curved facades, or irregular plans resulting
reductions in velocity pressure due to
from significant indentations or projections,
shielding are not permitted under the
openings through the building, or multi-
provisions of this chapter. However, this does
tower buildings connected by bridges.
not preclude the determination of shielding
3. Buildings with response characteristics effects and the corresponding reductions in
that result in substantial vortex-induced velocity pressure by means of the wind tunnel
and/or torsional dynamic effects, or dynamic procedure in Section 207F.
effects resulting from aero-elastic
instabilities such as flutter or galloping. Such
dynamic effects are difficult to anticipate, 207A.1.2.1 Main Wind-Force Resisting
being dependent on many factors, but should System (MWFRS)
be considered when any one or more of the
following apply: Wind loads for MWFRS shall be determined
using one of the following procedures:
i. The height of the building is over 120m.
1. Directional Procedure for buildings of all
ii. The height of the building is greater than 4 heights as specified in Section 207B for
times its minimum effective width Bmin as buildings meeting the requirements specified
defined below: therein;
iii. The lowest natural frequency of the 2. Envelope Procedure for low-rise buildings
building is less than n1=0.25 Hz. as specified in Section 207C for buildings
meeting the requirements specified therein;
iv. The reduced velocity
̅̅̅ 3. Directional Procedure for Building
𝑽𝒛
>𝟓 Appurtenances (rooftop structures and
𝒏𝟏 𝑩𝒎𝒊𝒏 rooftop equipment) and Other Structures
𝒛̅ = 0.6h (such as solid freestanding walls and solid
freestanding signs, chimneys, tanks open
̅̅̅
𝑽𝟏̅= the mean hourly velocity at height 𝒛̅ signs, lattice frameworks and trussed towers)
The minimum effective width Bmin is defined as specified in Section 207D; or
as the minimum value of ∑ 𝒉𝒊 𝑩𝒊 /∑ 𝒉𝒊 4. Wind Tunnel Procedure for all buildings
considering all wind directions. The and all other structures as specified in Section
summations are performed over the height of 207F.
the building for each wind direction, hi, is the
height above grade of level i, and Bi is the
width at level i normal to the wind direction. 207A.1.2.2 Components and Cladding
Wind loads on components and cladding on
4. Bridges, cranes, electrical transmission all buildings and other structures shall be
lines, guyed masts, highway signs and designed using one of the following
lighting structures, telecommunication procedures:
towers, and flagpoles. 1. Analytical Procedures provided in Parts 1
When undertaking detailed studies of the through 6, as appropriate, of Section 207E; or
dynamic response to wind forces, the 2. Wind Tunnel Procedure as specified in
fundamental frequencies of the structure in Section 207F.
each direction under consideration should be
 BUILDING, PARTIALLY ENCLOSED is
a building that complies with both of the
207A.2 Definitions
following conditions:
The following definitions apply to the
1. The total area of openings in a wall that
provisions of Section 207:
receives positive external pressure exceeds
the sum of the areas of openings in the
balance of the building envelope (walls and
APPROVED is an acceptable to the roof) by more than 10 percent.
authority having jurisdiction.
2. The total area of openings in a wall that
receives positive external pressure exceeds
BASIC WIND SPEED, V is a three-second 0.37 m2.
gust speed at 10m above the ground in
Exposure C (see Section 207A.7.3) as
determined in accordance with Section These conditions are expressed by the
207A.5.1 following equations:
1. A0 > 1.10A0i

BUILDING, ENCLOSED is a building that 2. A0 > 0.37 m2 or 0.01Ag, whichever is

does not comply with the requirements for smaller, and A0i / Ag ≤ 0.20
open or partially enclosed buildings.
where:
A0, Ag = are as defined for Open Building
BUILDING ENVELOPE is a cladding,
A0i = the sum of the areas of openings in the
roofing, exterior walls, glazing, door
building envelope (walls and roof) not
assemblies, window assemblies, skylight
including A0, in m2
assemblies, and other components enclosing
the building. Agi = the sum of the gross surface areas of the
building envelope (walls and roof) not
including Agi, in m2
BUILDING AND OTHER STRUCTURE,
FLEXIBLE are slender buildings and other
structures that have a fundamental natural Components can be part of the MWFRS
frequency less than 1 Hz. when they act as shear walls or roof
diaphragms, but they may also be loaded as
individual components. The engineer needs
BUILDING, LOW-RISE are enclosed or to use appropriate loadings for design of
partially enclosed buildings that comply with components, which may require certain
the following conditions: components to be designed for more than one
type of loading, for example, long-span roof
1. Mean roof height h less than or equal to trusses should be designed for loads
18m. associated with MWFRS, and individual
2. Mean roof height h does not exceed least members of trusses should also be designed
horizontal dimension. for component and cladding loads (Mehta
and Marshall 1998). Examples of cladding
BUILDING, OPEN is a building having include wall coverings, curtain walls, roof
each wall at least 80 percent open. This coverings, exterior windows (fixed and
condition is expressed for each wall by the operable) and doors, and overhead doors.
equation A0 ≥0.8Ag.
where:
DIAPHRAGM in wind load applications has
A0 = total area of openings in a wall that been added in ASCE 7-10. This definition,
receives positive external pressure, in m2. for the case of untopped steel decks, differs
Ag = the gross area of that wall in which A0 somewhat from the definition used in Section
is identified, in m2. 12.3 of ASCE 7-10 because diaphragms
under wind loads are expected to remain
EFFECTIVE WIND ARE, A is an effective door or window system should be used in
wind area is the area of the building surface calculating the design wind pressure.
used to determine (GCp ). This area does not
necessarily correspond to the area of the
building surface contributing to the force MAIN WIND-FORCE RESISTING
being considered. Two cases arise. In the SYSTEM (MWFRS) can consist of a
usual case, the effective wind area does structural frame on an assemblage of
correspond to the area tributary to the force structural elements that work together to
component being considered. For example, transfer wind loads acting on the entire
for a cladding panel, the effective wind area structure to the ground. Structural elements
may be equal to the total area of the panel. such as cross-bracing, shear walls, roof
For a cladding fastener, the effective wind trusses, and roof diaphragms are part of the
area is the area of cladding secured by a Main Wind-Force Resisting System
single fastener. A mullion may receive wind (MWFRS) when they assist in transferring
from several cladding panels. In this case, the overall loads (Mehta and Marshall 1998).
effective wind area is the area associated with
the wind load that is transferred to the
mullion. WIND-BORNE DEBRIS REGIONS are
The second case arises where components defined to alert the designer to areas requiring
such as roofing panels, wall studs, or roof consideration of missile impact design. These
trusses are spaced closely together. The area areas are located within tropical cyclone
served by the component may become long prone regions where there is a high risk of
and narrow. To better approximate the actual glazing failure due to the impact of wind-
load distribution in such cases, the width of borne debris.
the effective wind area used to evaluate
(GCp ) need not be taken as less than one-
third the length of the area. This increase in 207A.3 Symbols and Notations
effective wind area has the effect of reducing The following symbols and notation apply
the average wind pressure acting on the only to the provisions of Section 207A
component. Note, however, that this effective through 207F:
wind area should only be used in determining
the (GCp ) in Figure 207E.4-1 through A = effective wind area, in m2
207E.4-6 and 207E.4-8. The induced wind Af = area of open buildings and other
load should be applied over the actual area structures either normal to the wind direction
tributary to the component being considered. or projected on a plane normal to the wind
For membrane roof systems, the effective direction, in m2
wind area is the area of an insulation board Ag = the gross area of that wall in which A0
(or deck panel if insulation is not used) if the is identified, in m2.
boards are fully adhered (or the membrane is
adhered directly to the deck). If the insulation Agi = the sum of the gross surface areas of the
boards or membrane are mechanically building envelope (walls and roof) not
attached or partially adhered, the effective including Agi, in m2
wind area is the area of the board or A0 = total area of openings in a wall that
membrane secured by a single fastener or receives positive external pressure, in m2
individual spot or row of adhesive.
A0i = the sum of the areas of openings in the
For typical door and window systems building envelope (walls and roof) not
supported on three or more sides, the including Ao, in m2
effective wind area is the area of the door or
window under consideration. For simple A0g = total area of openings in the building
spanning doors (e.g., horizontal spanning envelope in m2
section doors or coiling doors), large
As = gross area of the solid freestanding wall
specialty constructed doors (e.g., aircraft
or solid sign, in m2
hangar doors), and specialty constructed
glazing systems, the effective wind area of a = width of pressure coefficient zone, in m
each structural component composing the
B = horizontal dimension of building
measured normal to wind direction, in m be used for roof angle ᶿ less than or equal to
10o , in m
̅ = mean hourly wind speed factor in
𝒃
Equation 207A.9-1 from Table 207A.9-1 he = roof eave height at a particular wall, or the
average height if the eave varies along the wall
̂ = 3-s gust speed factor from Table 207A.9-
𝒃
1 hp = height to top of parapet in Figure 207B.6-
4 and 207E.7-1
Cf = force coefficient to be used in
determination of wind loads for other Iz = intensity of turbulence from Equation
structures 207A.9-7

CN = net pressure coefficient to be used in K1, K2, K3 = multiplier in Figure 207A.8-1to

determination of wind loads for other obtain Kzt
buildings Kd = wind directionality factor in Table
Cp = external pressure coefficient to be used 207A.6-1
in determination of wind loads for other Kh = velocity pressure exposure coefficient
buildings evaluated at height z = h.
c = turbulence intensity factor in Equation Kz = velocity pressure exposure evaluated at
207A.9-7 from Table 207A.9-1 height z.
D = diameter of a circular structure or Kzt = topographic factor as defined in Section
member, in m 207A.8
D’ = depth of protruding elements such as ribs L = horizontal dimension of a building
and spoilers, in m measured parallel to the wind direction, in
F = design wind for for other structures, in m Lh = distance upwind of crest of hill or
G = gust-effect factor escarpment in Figure 207A.8-1 to where the
difference in ground elevation is half the
Gf = gust-effect factor for MWFRS of flexible height of the hill or escarpment, in m
buildings and other structures
Lr = horizontal dimension of return corner for
(GCp) = product of external pressure a solid freestanding wall or solid sign from
coefficient and gust-effect factor to be used in Figure 207D.4-1, in m
determination of wind loads for buildings
Lz = integral length scale of turbulence, in m
(GCpf) = product of the equivalent external
pressure coefficient and gust-effect factor to l = integral length scale factor from Table
be used in determination of wind loads for 207A.9-1, m
MWFRS of low-rise buildings
N1 = reduced frequency from Equation
(GCpi) = product of internal pressure 207A.9-14
ᶿᶿᶿcoefficient and gust-effect factor to be used
na = approximate lower bound natural
in determination of wind loads for buildings
frequency (Hz) from Section 207A.9.2
(GCpn) = combined net pressure coefficient
n1 = fundamental natural frequency, Hz
for a parapet
P = design pressure to be used in
gQ = peak factor for background response in
determination of wind loads for buildings, in
Equation 207A.-6 and 207A.9-10
N/m2
gR = peak factor for resonant response in
PL = wind pressure acting on leeward face in
Equation 207A.9-10
Figure 207B.4-8, in N/m2
gv = peak factor for wind response in Equation
Pnet = net design wind pressure from Equation
207A.9-6 and 207A.9-10
207E.5-1, in N/m2
H = height of hill or escarpment in Figure
Pnet10 = net design wind pressure for Exposure
207A.8-1, in m
B at h = 10 m and I =1.0 from Figure 207E.5-
h = mean roof height of a building or height of 1, in N/m2
other structure, except that eave height shall
PP = combined net pressure on a parapet from zg = normal height of the atmospheric
Equation 207B.4-5, in N/m2 boundary layer used in this code. Values
appear in Table 207A.9-1
PS = net design wind pressure from Equation
207C.-1, in N/m2 zmin = exposure constant from Table 207A.9-
1
PS10 = simplified design wind pressure for
Exposure B at h = 10 m and I = 1.0from Figure a = 3-s gust-speed power law exponent from
207C.6-1, in N/m2 Table 207.A.9-1
PW = wind pressure acting on windward face â = reciprocal of α from Table 207A.9-1
in Figure 207B.4-8, in N/m2
𝒂̅ = mean hourly wind-speed power law
Q = background response factor from exponent in Equation 207A.9-16 from Table
Equation 207A.9-8 207A.9-1
q = velocity pressure in N/m2 ꞵ = damping ratio, percent critical for
qh = velocity pressure evaluated at height z = buildings or other structures
h, N/m2 ε = ratio of solid area to gross area for solid
qi = velocity pressure for internal pressure freestanding wall, solid sign, open sign, faces
determination, in N/m2 of a trussed tower, or lattice structure

qp = velocity pressure at top parapet, in N/m2 𝛆̅ = integral length scale power law exponent
in Equation 27A.9.9 from Table 207A.9-1
qr = velocity pressure evaluated at height z
above ground, in N/m2 λ = adjustment factor for building height and
exposure from Figures. 207C.-1 and 207E.51
R = resonant response factor from Equation
207A.9-12 ὴ = value send I Equation 207A9-15
Rh, RL = values from Equations 207A.9-15 θ = angel of plane of roof from horizontal, in
degrees.
Ri = reduction response factor from Equation
207A.11-1
Rn = value from Equation 207A.9-13 207A.4 General
s = vertical dimension of the solid 207A.4.1 Sign Convention
freestanding wall or solid sign from Figure
Positive pressure acts toward the surface and
207D.4-1, in m
negative pressure acts away from the surface.
r = rise-to-span ratio for arched roofs
v = height-to-width ratio solid sign
207A.4.2 Critical Load Condition
V = basic wind speed obtained from Figure
Values of external and internal pressure shall
207A.5-1A through 207A.5-IC in m/s. The
be combined algebraically to determine the
basic wind spend corresponds to a 3-s gust
most critical load.
speed at 10 m above the ground in Exposure
Category C,
Vi = unpartitioned internal volume m3 207A.4.3 wind Pressure Acting on
Opposite Faces of Each Building Surface
̅̅̅
𝑽𝒛 = mean hourly wind speed at height 𝒛̅ m/s
In the calculation of design wind loads for the
W = width of building in Figures 207E.-3 and
MWFRS and for components and cladding
207E.4-5A and 207E.4-5B and width of span
for buildings, the algebraic sum of the
in Figures 207E.4-4 and 207E.4-6
pressures acting on opposite faces of each
x = distance upwind or downwind of crest in building surface shall be taken into account.
Figure 207A.8-1, in m
z = height above ground level, in m
Commentary:
𝒛̅= equivalent height of structure, in m Section 207A.4.3 is included in the code to
ensure that internal and external pressures
acting on a building surface area taken into on several factors important to an accurate
account by determining a net pressure from wind specification.
the algebraic sum of those pressures. For
1. A strength design wind speed map brings
additional information on the application of
the wind loading approach in line with that
the net components and cladding wind
used for seismic loads in that they both
pressure acting across a multilayered
essentially eliminate the use of a load factor
building envelope system, including air-
for strength design.
permeable cladding, refer to Section
C207E.1.5. 2. Multiple maps remove inconsistencies in
the use of importance factors that actually
should vary with location and between
207A.5 Wind Hazard Map tropical cyclone-prone regions for
Occupancy Category III, IV and V structures
207A.5.1 Basic Wind Speed and acknowledge that the demarcation
The basic wind speed, V, used in the between tropical cyclone and non-tropical
determination of design wind loads on cyclone winds change with the recurrence
buildings and other structures shall be interval.
determined from Figure 207A.5-1 as follows, 3. The new maps establish uniformity in the
except as provided in Section 207A.5.2 and return period of the design-basis winds, and
207A.5.3: they more clearly convey that information.
For Occupancy Category III, IV and V 4. The new maps, by providing the design
buildings and other structures-use Figure wind speed directly, more clearly inform
207A.5-1A. owners and their consultations about the
For Occupancy Category II buildings and storm intensities for which designs are
other structures-use Figure 207A.5-1B. performed.

For Occupancy Category I buildings and

other structures-use Figure 207A.5-1C. Selection of Return Periods. In the
The wind shall be assumed to come from any development of the design wind speed map
horizontal direction. The basic wind speed used in Section 207 NSCP 2010, the Wind
shall be increased where records or Load Subcommittee evaluated the wind
experience indicate that the wind speeds are importance factor, Iw, that had been in use
higher than those reflected in Figure 207A.5- since 1982. The task committee recognized
1. that using a uniform value of the wind
importance factor probably was not
appropriate because risk varies with location
Commentary: along the coast.

This edition of NSCP departs from prior To determine the return periods to be used in
editions by providing wind maps that are the new mapping approach, the task
directly applicable for determining pressures committee needed to meet with PAGASA
for strength design approaches. Rather than scientists, gather historical records and
using a single map with importance factors evaluate representative return period for
and a load factor for each building wind speeds determined in accordance with
occupancy category in this edition there are Section 207 NSCP 2010 and earlier, wherein
different maps for different categories of determination of pressures appropriate for
building occupancies. The updated maps are strength design started with mapped wind
based on a new and more complete analysis speeds, but involved multiplication by
of tropical cyclone characteristics (Vickery et importance factors and a wind load factor to
al. 2008a, 2008b and 2009) performed over achieve pressures that were appropriate for
the past 10 years. strength design. Furthermore, it was
assumed that the variability of the wind speed
The decision to move to multi-strength design dominated the calculation of the wind load
maps in conjunction with a wind load factor factor. The strength design wind load, WT, is
of 1.0 instead of using a single map used with given as:
an importance and a load factor of 1. Relied
WT, = CF,(V50 I )2 WLF (c207A.5-1)
Where CF is a building, component, or years. In the development of Equation
structure specific coefficient that includes the C207A.5-6, the term (V100 /V50) WLF replaces
effects of the things like building height, the WLF used in Equation C207A.5-5,
building geometry, terrain, and gust factor as effectively resulting in a higher load factor
computed using the procedures outlined in for Occupancy Category I, II and III
NSCP 2010. VLF is the 50-year return period structures equal to WLF(V100 /V50)2. Thus for
design win speed. WLF is the wind load occupancy Category I and II structures, the
factor, and I is the importance factor. basic wind speed is associated with a return
period of 1,700 years, or an annual
Starting with the nominal return period of 50
exceedance probability of 0.000588.
years, the ratio of the wind speed for any
Similarly, the 25-year return period wind
return period to the 50-year return period
speed associated with Occupancy Category
wind speed can be computed from Peterka
III, IV and V buildings equates to a 300-year
and Shahid (1998):
return period wind speed with a wind load
VT/V50 = [0.3+0.11n(12T)] (c207A.5-2) factor of 1.0.

where T is the return period in years and VT Wind Speed. The wind speed maps of Figure
is the T- year return period wind speed. The 207A.5-1 present basic wind speeds for the
strength design wind load, WT, occurs when: entire archipelago of the Philippines. The
wind speeds correspond to a 3-sec gust
WT = CF𝑽𝟐𝑻 = 𝐂𝐅𝑽𝟐𝟓𝟎 WLF (c207A.5-3) speeds at 1om above ground for exposure
Thus, category C.

VT/V50 = [0.3+0.11n(12T)] (c207A.5-) Serviceability Wind Speed. For applications

of serviceability, design using maximum
= √ WLF likely events, or other applications, it may be
and from Equation C207A.5-4, the retur desired to use wind speeds associated with
period T associated with the strength design mean recurrence intervals other than those
wind speed is: given in Figures 207A.5-1 to 207A.5-1C. To
accomplish this, previous editions of NSCP
T = 0.00228 exp(10√ WLF ) (c207A.5-5) 2010 provided tables with factors that
Using the wind load factor of 1. as specified enabled the user to adjust the basic design
in Section 207 NSCP 2010, from Equation wind speed (previously having a return
period of 50 years to wind speeds associated
C207A.5-5 we get T = 709 year, and
with other return periods.
therefore Vdesign = V709 / √ WLF ᴝ V700 / √ WLF.
Thus for Occupancy Category IV structures, For applications of serviceability, design
the basic wind speed is associated with a using maximum likely events, or other
applications, Appendix C presents maps of
return period of 700 years, or an annual
exceedance probability of 0.0014. peak gust wind speeds 10m above ground in
Exposure C conditions for return periods of
The importance factor used in Section 207 10, 25, 50, and 100 years.
NSCP 2010 and earlier for the computation
The probability Pn that the wind speed
of wind loads for the design of Occupancy
associated with a certain annual probability
Category I and II structures is defined so that
the nominal 50-year return period non- Pa will be equalled or exceeded at least once
tropical cyclone wind speed is increased to during an exposure period of n years is give
by:
be representative of a 100-year return period
value. Following the approach used above to Pn = 1 – (1- Pa)n (c207A.5-7)
estimate the resulting effective strength
design return period associated with a 50- As an example, if a wind speed is based upon
year basic design speed, in the case of the Pa =0.02 (50-year mean recurrence interval),
100-year return period basic wind sped in the there exists a probability of 0.40 that this
non-tropical cyclone-prone regions, we find speed will be equalled or exceeded during a
that: 25-year period, and a 0.4 probability of being
equalled or exceeded in a 50-year period.
T = 00228 exp[10(V100 /V50) √ WLF (c207A.5-6)
Similarly, if a wind speed is based upon Pa =
where for V100 /V50 computed from Equation 0.00143 (700-year mean recurrence
C207A.5-4 with WLF = 1.6, we find T = 1,697 interval), there exist a 3.5% probability that
this speed will be equalled or exceeded wind speeds should be made at the
during a 25-year period, and a 6.9% micrometeorological scale on the basis of
probability of being equalled or exceeded in wind engineering or meteorological advice
a 50-year period. and used in accordance with the provision of
Section 207A.5.3 when such adjustments are
Some products have been evaluated and test
warranted. Due to the complexity of
methods have been developed based on
mountainous terrain and valley gorges in
design wind speeds that are consistent with
Hawaii, there are topographic wind speed-up
the unfactored load effects typically used in
effects that cannot be addressed solely by
Allowable Stress Design. Table C207A.5-6
Figure 207A.8-1 (Applied Research
provides conversion from the strength
Associated 2001).
design-based design wind speed used in the
ASCE 7-10 design wind speed maps and the
Section 207 NSCP 2010 design wind speed
207A.5.3 Estimation of Basic Wind Speeds
used in these product evaluation reports and
from Regional Climatic Data
test methods. A column of values is also
provided to allow coordination with ASCE 7- In areas outside tropical cyclone-prone
93 design wind speeds. regions, regional climatic data shall only be
used in lieu of the basic wind speeds given in
Figure 207A.5-1 when (1) approved extreme-
207A.5.2 Special Wind Regions value statistical-analysis procedures have
been employed in reducing the data; and (2)
Mountainous terrain, gorges, and special
the length of record, sampling error,
wind regions shown in Figure 207A.5-1 shall
averaging time, anemometer height, data
be examined for unusual wind conditions.
quality, and terrain exposure of the
The authority having jurisdiction shall, if
anemometer have been taken into account.
necessary, adjust the values given in Figure
Reduction in basic wind speed below that of
207A.5-1 to account for higher local wind
Figure 207A.5-1 shall be permitted.
speeds. Such adjustment shall be based on
meteorological information and an estimate In tropical cyclone-prone regions, wind
of the basic wind speed obtained in speeds derived from simulation techniques
accordance with the provisions of Section shall only be used in lieu of the basic wind
207A.5.3. speed given in Figure 207A.5-1 when
approved simulation and extreme value
statistical analysis procedures are used.
Commentary:
In areas outside tropical cyclone-prone
Although the wind speed map of Figure regions, when the basic wind speed is
207A.5-1is valid for most regions of the estimated from regional climatic data, the
country, there are special regions in which basic wind speed shall not be less than the
wind sped anomalies are known to exist. wind speed associated with the specified
Some of these special regions are noted in mean recurrence interval, and the estimate
Figure 207A.5-1. Winds blowing over shall be adjusted for equivalence to a 3-s gust
mountain ranges through gorges or river wind speed at 10m above ground in Exposure
valleys in these special regions can develop C. The data analysis shall be performed in
speeds that are substantially higher than the accordance with this section.
values indicated on the map. When selecting
basic wind speeds in these special regions
use of regional climatic data and Commentary:
consultation with a wind engineer or
When using climatic data in accordance with
meteorologist is advised.
the provisions of Section 207A.5.3 and in lieu
It is also possible that anomalies in wind of the basic wind speed given in Figure
speeds exists o a micrometeorological scale. 207A.5-1, the user is cautioned that the gust-
For examples, wind speed-up over hills and factors, velocity pressure exposure
escarpments is addressed in Section 207A.8. coefficients, gust effect factors, pressure
Wind speeds over complex terrain may be coefficients, and force coefficients of this
better determined by wind-tunnel studies as code are intended for use with the 3-s gust
described in Section 207F. Adjustments of speed at 10m above ground in pen country. It
is necessary, therefore, that regional climatic winds coming from any given direction and
data based on a different averaging time, for (2) the reduced probability of the maximum
example, hourly mean or fastest mile, be pressure coefficients occurring for any given
adjusted to reflect peak gust speeds at 10m direction. The wind directionality factor
above ground in open country. (identified as Kd, in the code) is tabulated in
Table 207A.-1 for different structure types.
In using local data, it should be emphasized
As new research became available, this
that sampling errors can lead to large
factor can be directly modified. Values for the
uncertainties in specification of the wind
factor were established from references in the
speed. Sampling errors are the errors
literature and collective committee judgment.
associated with the limited size of the
The Kd, value for round chimneys, tanks and
climatological data samples (years of record
similar structures is given as 0.95 in
of annual extremes). It is possible to have a
recognition of the fact that the wind load
8.9m/s error in wind speed at an individual
resistance may not be exactly the same in all
station with a record length of 30 years.
directions as implied by a value of 1.0. A
While local records of limited extent often
value of 0.85 might be more appropriate if a
must be used to define wind speed in special
triangular trussed frame is shrouded in a
wind areas, care and conservatism should be
round cover. A value of 1.0 might be more
exercised in their use.
appropriate for a round chimney having a
If meteorological data are used to justify a lateral load resistance equal in all directions.
wind speed lower than 177-km/h 700-yr peak The designer is cautioned by footnote to
gust at 10m, an analysis of sampling error is Table 207A.6-1 and the statement in Section
required to demonstrate that the wind record 207A.6, where reference is made to the fact
could not occur by chance. This can be that this factor is only to be used in
accomplished by showing that the difference conjunction with the load combinations
between predicted speed and 177 km/h specified in Section 2.3 and 2.4 of ASCE 7-
contains two to three standard deviations of 10.
sampling error (Simiu and Scanlan 199).
Other equivalent methods may be used.
207A.5.7 Exposure
For each wind direction considered, the
207A.5.4 Limitation
upwind exposure shall be based on ground
Tornadoes have not been considered in surface roughness that is determined from
developing the basic wind-speed natural topography, vegetation and
distributions. constructed facilities.

207A.6 Wind Directionality Commentary:

The wind directionality factor, Kd, shall be The descriptions of the surface roughness
determined from Table 207A.-1. This categories and exposure categories in
directionality factor shall only be included in Section 207A.7 have been expressed as far as
determining wind loads when the load possible in easily understood verbal terms
combinations specified in Section 2.3 and 2.4 that are sufficiently precise for most practical
are used for the design. The effect of wind applications. Upwind surface roughness
directionality in determining wind loads in conditions required for Exposure B and D
accordance with Section 207F shall be based are shown schematically in Figures
on an analysis for wind speeds that conforms C207A.7-1 and C207.7-2, respectively. For
to the requirements of Section 207A.5.3 cases where the designer wishes to make a
more detailed assessment of the surface
roughness category and exposure category,
Commentary: the following more mathematical description
is offered for guidance (Irwin 2006). The
The wind load factor 1.3 in ASCE 7-95 ground surface roughness is best measured in
include a “wind directionality factor” of 0.85 terms of a roughness length parameter called
(Ellingwood 1981 and Ellingwood et al. z0. Each of the surface roughness categories
1982). This factor accounts for two effects: B through D correspond to a range of values
(1) The reduced probability of maximum
of this parameter, as does the even rougher contains open patches, such as highways,
category A used in previous versions of the parking lots ad playing fields. These cause
code in heavily built-up urban area but local increases in the wind speeds at their
removed in the present edition. The range of edges. By using an expose coefficient
z0 in meters (m) for each terrain category is corresponding to a lower than typical value
given in Table C207A.7-1. Exposure A has of z0. Some allowance is made for this. The
been included in C207A.7-1 as a reference alternative would be to introduce a number
that may be useful when using the Wind of exceptions to use of Exposure B in
Tunnel Procedure. Further information on suburban areas, which would add an
values of z0 in different types of terrain can be undesirable level of complexity.
found in Simiu and Scanlan (1996) and Table
The value of z0 for a particular terrain can be
C207A.7-2 based on Davenport et al. (2000)
estimated from the typical dimensions of a
and Wieringa et al. (2001). The roughness
surface roughness elements and their spacing
classifications in Table C207A.7-2 are not
on the group area using an empirical
intended to replace the use of exposure
relationship, due to Lettqu (1969), which is:
categories as required in the code for
structural design purposes. However, the 𝑺𝒐𝒃
z0 = 0.5Hob (C207A.7-1)
terrain roughness classification in Table 𝑨𝒐𝒃
C207A.7-2 may be related to ASCE 7
Hob = the average height of the roughness in
exposure categories by comparing z0 values
the upwind terrain
between Table C207A.7-1 and Table
C207A.7-2 fall within the range of z0 values Sob = the average vertical frontal area per
for Exposure C in Table C207A.7-1. obstruction presented to the wind
Similarly, the z0 values for Classes 5 and 6 in
Aob = the average area of ground occupied by
Table C207A.7-2 fall within the range of z0
each obstruction, including the open area
values for Exposure B in Table C207A.7-1.
surrounding it

Research described in Powell et al. (2003),

Vertical frontal area is defined as the area of
Donelan et al. (2004), and Vickery et al.
the projection of the obstruction onto a
(2008b) showed that the drug coefficient over
vertical plane normal to the wind direction.
the ocean high winds in tropical cyclones did
The area Sob may be estimated by summing
not continue to increase with increasing wind
the approximate vertical frontal areas of al
speed as previously believed (e.g., Powell
obstructions within a selected area of upwind
1980). These studies showed that the sea
fetch and dividing the sum by the number of
surface drag coefficient, and hence the
obstructions in the area. The average height
aerodynamic roughness of the ocean,
Hob may be estimated in a similar way by
reached a maximum at mean wind speeds of
averaging the individual heights rather than
about 30m/s. There is some evidence that the
using the frontal areas. Likewise Aob may be
drag coefficient actually decreases (i.e., the
estimated by dividing the size of the selected
sea surface becomes aerodynamically
area of upwind fetch by the number of
smoother) as the wind speed increases
obstruction init.
further (Powell et al. 2003) or as the tropical
cyclone radius decreases (Vickery et al.
2008b). The consequences of these studies
are that the surface roughness over the ocean As an example, if the upwind fetch consists
primarily of single family homes with typical
in a tropical cyclone is consistent with that of
height Hob = 6m, vertical frontal area
exposure D rather than exposure C.
Consequently the use of exposure D along the (including some trees on each lot) of 100m2,
tropical cyclone coastline is now required. and ground area per home of 1,000m2, , then
zo is calculated to be zo = 0.5 x 100/1,000 =
For Exposure B the tabulated values of K2 0.3m, which falls into exposure category B
correspond to z0 = 0.2 m, which is below the according to Table C207A.7-1.
typical value of 0.3 m, whereas for Exposure
C and D they correspond to the typical value Trees and bushes are porous and are
of z0. The reason for the difference in deformed by strong winds, which reduce their
Exposure B is that this category of terrain, effective frontal areas (ESDU, 1993). For
which is applicable to suburban areas, often conifers and other evergreens no more than
50 percent of their gross frontal areas can be one or more wind direction due to future
taken to be effective in obstructing the wind. demolition and/or development.
For deciduous trees and bushes no more than
207A.7.1 Wind Directions and Sectors
15 percent of their gross frontal area can be
taken to be effective in obstructing the wind. For each selected wind direction at which the
Gross frontal area is defined in the context as wind loads are to be determined, the exposure
the projection onto a vertical plane (normal of the building or structure shall be
to the wind) of the area enclosed by the determined for the two upwind sectors
envelope of the tree or bush. extending 450 either side of the selected, wind
direction. The exposure in these two sectors
Ho (1992) estimated that the majority of
shall be determined in accordance with
buildings (perhaps as much as 60 percent to
Sections 207A.7.2 and 207A.7.3, and the
80 percent) have an exposure category
exposure whose use would result in the
corresponding to Exposure B. While the
highest wind loads shall be used to represent
relatively simple definition in the code will
the winds from that direction.
normally suffice for most practical
applications, oftentimes the designer is in
need of additional information, particularly
with regard to the effect of large openings or
clearings (e.g., large parking lots, freeways,
or tree clearings) in the otherwise “normal”
ground surface roughness B. The following is
offered as guidance for these situations:
The simple definition of Exposure B given in
the body of the code, using the surface
roughness category definition, is shown
pictorially in Figure C207A.7-1. This
definition applies for the surface roughness B
condition prevailing 800 m upwind with
insufficient “open patches” as defined in the
following text to disqualify the use of
Exposure B.
An opening in the surface roughness B large
enough to have a significant effect on the
exposure category. Determination is as an
“open patch” An open patch is defined as an
opening greater than or equal to
approximately 50 m on each side (i.e.,
greater than 50 m by 50 m). Openings
smaller than this need not be considered in
the determination of the exposure category.
The effect of open patches of surface
roughness C or D on the use of exposure
category B is shown pictorially in Figures
C207A.7-3 and C207A.7-4. Note that the
plan location of any open patch may have a
different effect for different wind directions.
Aerial photographs, representative of each
exposure type, are included in the
commentary to aid the user in establishing
the proper exposure for a given site.
Obviously, the proper assessment of exposure
is a matter of good engineering judgment.
This fact is particularly true in light of the
possibility that the exposure could change in