Wind provisions

Wind Loads:
The ASCE 7 Provisions
CE 694R – Fall 2007
T. Bart Quimby, P.E., Ph.D.
Quimby & Associates
A Beginner's Guide to ASCE 7-05

Permitted Design Methods
 Method 1—Simplified Procedure
 (ASCE 7-05 Section 6.4)
 Low rise buildings. This is an outgrowth of work done
for/by the metal building industry.
 Method 2—Analytical Procedure
 (ASCE 7-05 Section 6.5)
 The typically used procedure. This is the main focus of
this presentation.
 Method 3—Wind Tunnel Procedure
 (ASCE 7-05 6.6)
See ASCE 7-05 6.1.2

Important Definitions
 Basic Wind Speed
 Building open, enclosed, partially enclosed
 Low-Rise Building
See ASCE 7-05 6.2

Exposure Categories
 Exposure A – Deleted in ASCE 7-02 and later
 Extremely sheltered. Large city centers with tall buildings.
 Exposure B
 Urban and suburban areas, wooded areas, areas with many
closely spaced obstructions.
 Exposure C
 Open terrain with scatter obstructions. Airports, areas that
are generally flat open country.
 Exposure D
 Flat, unobstructed areas and water surfaces outside
hurricane prone regions. This category includes smooth mud
flats, salt flats, and unbroken ice that extend 5,000 ft or 20
times the building height in the upwind direction.
See ASCE 7-05 6.5.6 & C6.5.6 (See images!)

Determining Exposure
 Wind Direction & Sectors (ASCE 7-05
6.5.6.1)
 the exposure of the building or structure shall
be determined for the two upwind sectors
extending 45o
either side of the selected wind
direction.
 the exposure resulting in the highest wind
loads shall be used to represent the winds
from that direction.

ASCE 7-05 Wind Pressures
 The basic form of the pressure equation:
p = qGC
 Where
 p = a wind pressure on a surface
 q = velocity pressure. This is the pressure due to a moving
fluid on a flat plate
 G = gust factor. The gust factor accounts for dynamic
interaction between the flowing air and the structure
 C = pressure coefficient. The pressure coefficient accounts
for varying pressure across a surface.

Velocity Pressure, q
 qz =Velocity Pressure = 0.00256KzKzt KdV2
I (lb/ft2
)
 Constant 0.00256
 V = Basic wind speed in mph
 I = Importance Factor (i.e. different MRI)
 Kz = Exposure Coefficient
 Kzt = Topographical Factor
 Kd = Wind Directionality Factor
 Evaluated at an elevation z:
 qz = 0.00256V2
IKzKztKd
 Evaluated at the building mean roof elevation, h:
 qh = 0.00256V2
I KhKhtKd
See ASCE 7-05 6.5.10

The Velocity Coefficient
 Based on the average density of air at sea level.
P
1
2
 V 2

1
2
[
0.0765
32.2
][
5280
3600
]2
V 2
 0.00256V 2
See ASCE 7-05 C6.5.10

Basic Wind Speed, V
 Obtained from Wind Speed maps in ASCE 7-
05 Figure 6-1.
 Determined by localized research using
approved probabilistic methods.
 “The basic wind speed shall be increased
where records or experience indicate that the
wind speeds are higher than those reflected
in Fig. 6-1.” (ASCE 7-05 6.5.4.1)
See ASCE 7-05 6.5.4

The Importance Factor, I
 Category I: I = .87
 MRI is 25 years
 Category II: I = 1.00
 MRI is 50 years
 Category III & IV: I = 1.15
 MRI is 100 years
 Building Categories are listed in ASCE 7-05
Table 1-1.
See ASCE 7-05 6.5.5, Table 6-1 and Commentary 6.5.5

Velocity Pressure Exposure
Coefficients, Kz and Kh
 Modifies basic wind pressure for heights other
than 33 ft and exposures other than exposure
C.
 Can compute K directly from equations in the
commentary for any height and/or exposure.
 Good for spreadsheet or computer
programming.
 For elevations less than 15 ft, use K15.
 For elevations above gradient height use Kg.
See ASCE 7-05 6.5.6.6, Tables 6-2 and 6-3, and C6.5.6.6

Kz & Kh Computation
Kz = 2.01(z/zg)2/a
K Computation
0.00
0.50
1.00
1.50
2.00
2.50
0 500 1000 1500 2000
Elevation, z (ft)
K
Exposure B
Exposure C
Exposure D
When z > zg use z = zg
When z < 15 use z = 15 ft

Topographical Factor, Kzt
 Kzt = 1.0 when:
 H/Lh < 0.2, or
 H < 15' for Exposures C & D,
or
 H < 60' for Exposure B.
 Kzt = (1+K1K2K3)2
See ASCE 7-05 6.5.7 & Commentary 6.5.7

Kzt
Constants

Kzt Multipliers by Equation
See ASCE 7-05 Figure 6.4

Directionality Factor, Kd
 This factor shall only be
applied when used in
conjunction with load
combinations specified
in Sections 2.3 and 2.4.
 The wind load factors
changed when the
directionality factor was
extracted.
See ASCE 7-05 6.5.4.4 and
Table 6-4

The Gust Factor, G
 Factor accounting for:
 Gustiness and turbulence
 Gust frequency
 Gust size
 Integral scale longitudinal and lateral
 Frequency of structure
 Structural damping
 Aerodynamic admittance
 Gust correlation

Gust Factor, G
 For stiff buildings and stiff structures
 G = 0.85
 For flexible buildings and other structures
 Calculate “by a rational analysis that
incorporates the dynamic properties of the
main wind-force resisting system.”
See ASCE 7-05 6.5.8

Pressure Coefficients, C
 The pressure coefficients are based on
 The enclosure category of the structure
 The location on a structure for which a pressure is to
be computed.
 The pressure coefficients have been determined
experimentally from wind tunnel studies done on
regular shaped structures
 The coefficient represents the ratio between measured
pressure and the computed basic velocity pressure.
C
Pmeasured
1
2
 V 2

Enclosure Classifications
 A building is to be classified as one of the following:
 Open
 Ao > 0.8Ag for each wall
 Partially Enclosed
 Ao > 1.10 Aoi, and
 Ao > min[4 sqft , 0.01Ag], and
 Aoi/Agi < 0.20
 Enclosed
 A building that is neither open nor partially enclosed.
See ASCE 7-05 6.2 & 6.5.9

Location of Pressure
 ASCE 7 provides means for computing forces on
various surfaces.
 The building envelope surfaces experience pressure
on both sides (i.e. external and internal).

Internal Pressure Coefficients, GCpi
 Internal pressure is fairly easy because the air is
relatively stagnant and the shape of the structure
does not affect it’s magnitude.
 As gusting is not a concern internally, the gust factor
and the pressure coefficient are combined.
 GCpi
 The magnitude of the internal pressure coefficient is
strictly dependent on the enclosure classification.
 The pressure can be both positive or negative (i.e.
suction) depending on the direction of the wind
relative to opening for partially enclosed or enclosed
buildings.
 Both internal pressures must be considered.
See ASCE 7-05 6.5.11.1 & Figure 6-5

Internal Pressure

External Pressure Coefficients, Cp
 As external surfaces are subject to “flowing” air, the
pressure varies considerably on the building surface
depending on structural configuration and direction of
the wind.
 Coefficients also depend on whether the resulting
forces are to be used to design/analyze:
 Main Wind-Force Resisting Systems
 Structural elements that support large areas exposed
to the wind
 Components & Cladding
 Structural elements that support small areas exposed
to the wind
See ASCE 7-05 6.5.11.2 & Figures 6-6, 6-7, and 6-8

Buildings with Roofs Consisting of
Flat Surfaces
 ASCE 7-05 Figure 6-6 gives the external
coefficients of wall and roof surfaces.
See ASCE 7-05 Figure 6-6

Flat Surfaces – Wall Cp
 Wall pressure depends on whether the wall is
 Windward
 Same regardless of building plan dimensions
 Leeward
 Dependant on building plan dimensions
 Side
 Same regardless of building plan dimensions

Flat Surfaces – Roof Cp
 Dependent on direction of wind relative to
ridge
 Coefficients are given for various conditions.
Interpolation is used to find values of
conditions between those given.

Wind Normal to Ridge
 Wind NORMAL to ridge
 Values given for different
building height to length
ratios and roof slope
angles.
 Windward roof surfaces
 Can be both positive
and negative on some
slopes. Both need
consideration as
separate load cases.
 Leeward roof surfaces
 All negative.

Wind Parallel to Ridge
 Parallel to ridge,
flat or nearly flat
 Two different
h/L ranges,
both with
stepped
pressures.
 Interpolate
between
ranges

Domed Roofs
 Pressure distributions are fairly complex.
 Two load cases to be considered.

Arched Roofs
 Pressure coefficient depends on rate of rise
of the arch.
 Pressure varies by along the arch.

Components & Cladding
 Elements of the structure that support local
peak loads need to be designed for these
pressures.
 The magnitude of the force is dependent on
the wind area tributary to the component
 The smaller the tributary area of a component
the more likely to see relatively high pressures
on their tributary areas.

Some Local Effects
 Wind
around a
corner
Image from FEMA Multi
Hazard Seminar

Wind at a Corner
A Beginner's Guide to ASCE 7-05 Image from FEMA Multi
Hazard Seminar

Uplift on Roof
Images from FEMA Multi
Hazard Seminar

Wall Components
 For buildings
under 60 ft
 See ASCE
7-05 Figure
6-17 for
building
greater than
60 ft tall.
See ASCE 7-05 Figure 6-11A

Roof Components
 Lots of different roof types with different
requirements.
 Gable Roofs of various angles
 Gable/Hip Roofs
 Stepped Roofs
 Multispan Gable Roofs
 Monoslope Roofs
 Sawtooth Roofs

Typical
Roof
Chart

Finding Net Pressure
 The net pressure is the vector sum of the
internal and external pressures.
 Typical form:
p = qGCp – qi(GCpi)
 Note the sign… positive pressure externally
opposes positive pressure internally (i.e. they
act in opposite directions).
See ASCE 7-05 6.5.12

Sample Problem
 V = 120 mph
 Exposure C
 Enclosed

Wind provisions

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