WIND LOADS: ASCE 7 PROVISIONS

Prepared by: Marcia C. Belcher, PE

Uplift on Roof

Wind at a Corner

Wind Around a Corner

Effects of Exposure on Wind VelocityA

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Typical Wind Pressure On A StructureA

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Method 1—Simplified Procedure

(ASCE 7-05 Section 6.4)

Low rise buildings w/mean roof height not to exceed

60 feet.

Method 2—Analytical Procedure

(ASCE 7-05 Section 6.5)

Most common procedure.

Applicable to all structure types

Method 3—Wind Tunnel Procedure

(ASCE 7-05 6.6)

Permitted Design Methods

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Mean Roof

Height

h

Simplified Method

Tables are provided by ASCE 7 for wind

pressures for exposure B at 30 ft.

An adjustment factor is provided for different

exposures and heights (hmax = 60 ft.)

The total wind pressure is the wind pressure

from the table multiplied by adjustment factors

and importance factor.

Simplified Method

Main wind force-resisting system:

The design wind pressure shall be determined as:

Ps = I Ps30

Where: = Adjustment factor for building height and

exposure

I = Importance factor

Ps30 = Simplified design wind pressure for

exposure B at h = 30 ft (from tables)

Application of Wind Pressures

Wind pressures are applied to the vertical and

horizontal projections of the structure.

+Sign indicates pressure ON surface

- Sign indicates pressure AWAY from surface (suction)

Zones on the Longitudinal Wall

Wind pressures on the longitudinal wall are calculated for

each ―zone‖ of a structure as follows:

Wall

Roof

OverhangLeeward

Roof

Windward Roof

Load Case #1: Wind on Longitudinal Wall

Zones on the End Wall

Wind pressures on the transverse wall are calculated for each

“zone” of a structure as follows:

Load Case #2: Wind on End Wall

Zone Definitions

Calculating the length of end zone X=2a:

The length, “a” shall be smaller 10% of least wall length or 0.4h,

BUT not less than 8% of least horizontal dimension or 6 ft.

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.

Exposure B

Exposure B and C

Exposure C

Exposure C

Exposure DA

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Importance Factor

Importance Factor for Wind

Example: Wind Pressure Determination Using Simplified Method

Wind blows on a small office building in a suburb of

Chicago. The terrain is flat and unobstructed. The

building is has a rectangular footprint with dimensions of

50’x100’. The eve height is 30 feet and the roof has an

angle of 20 degrees.

Determine wind load for main wind force resisting system

using the Simplified Method for Load Case #1.

Draw the pressure distribution on a typical interior section

thru the building.