Wind Provisions -

Prepared by: Marcia C. Belcher, PE
Uplift on Roof
Wind at a Corner
Wind Around a Corner
Effects of Exposure on Wind Velocity
A Beginner's Guide to ASCE 7-05
Typical Wind Pressure On A Structure
A Beginner's Guide to ASCE 7-05
Permitted Design Methods
We will
study this
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)
Mean Roof
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
Adjustment factor for building height and
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:
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
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 D
A Beginner's Guide to ASCE 7-05
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.