ASCE7 Wind Provisions

Wind Loads:The ASCE 7 Provisions

CE 694R – Fall 2007T. 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 = 0.00256V2IKzKztKd

Evaluated at the building mean roof elevation, h: qh = 0.00256V2I KhKhtKd

See ASCE 7-05 6.5.10


The Velocity Coefficient

Based on the average density of air at sea level.

P 12 V 2 1

2[ 0.0765

32.2][ 5280

3600]2V 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)

KExposure 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.

CPmeasured

12 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


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



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.



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


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ASCE7 Wind Provisions