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
A Beginner's Guide to ASCE 7-05
Important Definitions
Basic Wind Speed
Building open, enclosed, partially enclosed
Low-Rise Building
See ASCE 7-05 6.2
A Beginner's Guide to ASCE 7-05
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!)
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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 = 0.00256V2IKzKztKd
Evaluated at the building mean roof elevation, h: qh = 0.00256V2I KhKhtKd
See ASCE 7-05 6.5.10
A Beginner's Guide to ASCE 7-05
The Velocity Coefficient
Based on the average density of air at sea level.
P1
2V
2
1
2[
0.0765
32.2][
5280
3600]
2V
2 0.00256V
2
See ASCE 7-05 C6.5.10
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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)
KExposure B
Exposure C
Exposure D
When z > zg use z = zg
When z < 15 use z = 15 ft
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Kzt
Constants
A Beginner's Guide to ASCE 7-05
Kzt Multipliers by Equation
See ASCE 7-05 Figure 6.4
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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.”
A Beginner's Guide to ASCE 7-05
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.
CP
measured
1
2V 2
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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).
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Internal Pressure
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Buildings with Roofs Consisting of
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
See ASCE 7-05 Figure 6-6
A Beginner's Guide to ASCE 7-05
Buildings with Roofs Consisting of
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.
See ASCE 7-05 Figure 6-6
A Beginner's Guide to ASCE 7-05
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.
See ASCE 7-05 Figure 6-6
A Beginner's Guide to ASCE 7-05
Wind Parallel to Ridge
Parallel to ridge,
flat or nearly flat
Two different
h/L ranges,
both with
stepped
pressures.
Interpolate
between
ranges
See ASCE 7-05 Figure 6-6
A Beginner's Guide to ASCE 7-05
Domed Roofs
Pressure distributions are fairly complex.
Two load cases to be considered.
See ASCE 7-05 Figure 6-7
A Beginner's Guide to ASCE 7-05
Arched Roofs
Pressure coefficient depends on rate of rise
of the arch.
Pressure varies by along the arch.
See ASCE 7-05 Figure 6-8
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
Some Local Effects
Wind
around a
corner
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Typical
Roof
Chart
A Beginner's Guide to ASCE 7-05
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).
A Beginner's Guide to ASCE 7-05
See ASCE 7-05 6.5.12
Sample Problem
V = 120 mph
Exposure C
Enclosed
A Beginner's Guide to ASCE 7-05