2 Wind Load Procedures ASCE 7 05

```STRUCTURE INSTITUTE Lectures and Hands-On Series
Wind Load Procedures: ASCE 7 -05
Method 2: Analytical Procedure
i.
Regular-shaped building as defined in section 6.2
ii.
shedding, instability due to galloping or flutter and channeling
effects.
Automatic Wind Loads based on section 6.5
1. Exposure and Pressure Coefficients
1.1.
Exposure from Extents of Rigid Diaphragms
i.
Wind loads are applied on the vertical projected area of
the building determined from story heights and input
diaphragm exposure widths.
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1.2.
Exposure from Area Objects
i.
Wind loads are applied using the exposure from area
objects method, are applied on the vertical and horizontal
surfaces of the user selected area objects as described in
section 6.5.12.
2. Wind Exposure Parameters
2.1.
Wind Direction Angle
i.
Angle measured in degrees.
ii.
Angle is always measured counterclockwise from the
positive global X-axis.
iii.
An angle “0” means, the wind is blowing to the positive
global X-direction.
iv.
An angle “90” means, the wind is blowing to the positive
global Y-direction.
v.
An angle “180” means, the wind is blowing to the negative
global X-direction.
vi.
An angle “270” means, the wind is blowing to the negative
global Y-direction.
2.2.
Windward Coefficient, Cp
i.
Used in calculating pressure in the windward side.
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2.3.
Leeward Coefficient, Cp
i.
2.4.
Used in calculating pressure in the leeward side.
Wind Cases (ASCE 7 05 figure 6-9)
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2.5.
Eccentricity Ratio e1 (from figure 6-9)
2.6.
Eccentricity Ratio e2 (from figure 6-9)
3. Wind Coefficients
3.1.
Wind Speed (mph)
i.
Is described in section 6.5.4 (refer to NSCP for local wind
speed). A typical range of the basic wind speed is 85 to 150
mph.
ii.
Say 200 kph / 1.609 = 125 mph
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3.2.
Exposure Type
i.
Exposure A – deleted in ASCE 7-02 and later
Extremely sheltered. Large city centers with tall buildings.
ii.
Exposure B
Urban and suburban areas, wooded areas, areas with
many closely spaced obstructions.
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iii.
Exposure C
Open terrain with scattered obstructions. Airports, areas
that are generally flat open country.
iv.
Exposure D
Flat, unobstructed areas and water surfaces outside
hurricane prone regions.
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3.3.
Wind Importance Factor, I
i.
Is described in table 6-1. A typical range of values for I is 0.77
to 1.15.
3.4.
Topographical Factor, Kzt
i.
Is described in section 6.5.7.2. the default value for Kzt is 1.0
and cannot be < 1.0.
ii.
For flat terrain Kzt = 1.0
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3.5.
Gust Effect Factor, G
i.
Is described in section 6.5.8.
ii.
G = 0.85 for stiff buildings and stiff structures.
iii.
For building f < 1 hertz, flexible buildings and other structures
Calculate “G” by rational analysis that incorporates the
dynamic properties of the main wind-force resisting system.
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3.6.
Directionality Factor, Kd
i.
Is described in section 6.5.4.4. a typical value for Kd is 0.85
to 0.95
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4. Exposure Height
4.1.
Top Story
i.
4.2.
Bottom Story
i.
Lowest story level that is exposed to wind loading. It may be
advantageous to specify a higher level as the bottom story
5. ASCE 7 -05 Wind Load Cases
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