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ASCE 7 Wind Loading Provisions
CE 6999c
Lecture 2
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Wind Loads
When structures block the flow of wind,the winds kinetic energy is convertedinto potential energy of pressure.
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Wind Loads
The effect of wind on structuredepends:
Density of the air
Velocity of the air
Angle of incidence of the wind
Shape and stiffness of the structure
Roughness of its surface.
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Wind Loads
For design purposes, wind loading canbe treated using:
Static Approach: Most common
approach.
Dynamic Approach: Building or other
structures not meeting requirements ofsection 6.5.1 (ASCE 7-05), or havingunusual shapes or response characteristic .
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Wind Loads: Static Approach
The fluctuating pressure of constantlyblowing wind is approximated by amean velocity pressure:
is air density
- V is air velocity
2V
2
1 =q
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Wind load: Static Approach
Previous equation is modified toaccount for:
Structure Importance (I)
Structure Height (Kz) Structure Location (terrain)
Considering the factors previously
mentioned, the mean velocity pressurebecomes:
)lb/ft(V00256.022IKKKq dztzz =
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Wind load: ASCE 7-05 (cont.) Where:
V= Velocity in mi/h of a 3-second gust of wind measured
at 33 ft above the ground during a 50-year recurrenceperiod.
I = The importance factor that depends upon the natureof the building occupancy.
Buildings with low hazard to human life, in a non-hurricaneprone region:
Agriculture facilities have I = 0.87 Hospital facilities have I = 1.15
Kz = The velocity pressure exposure coefficient, which isa function of height and depends upon the groundterrain.
Kzt = A factor that accounts for wind speed increases dueto hills and escarpments. Flat ground Kzt = 1.
Kd = factor that accounts for the direction of the wind. Itis used only when the structure is subjected tocombinations of loads. If wind alone is to be consider, Kd= 1
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Figures and images
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Table for KD and I
Table 6-1: ITable 6-4: KD
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Table 1-1: Occupancy Category of building and other structures
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Table 1-1: Occupancy Category of building and other structures(cont.)
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Topographic Factor, Kzt
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Exposure B: Example
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Exposure B: Example
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Exposure B and C: Example
ExposureC
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Exposure C: Example
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Exposure C: Example
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Exposure D: Example
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ASCE/SEI 7-05 Method 1 Simplified Procedure (permitted
for regular shaped low-rise buildings) Method 2 Analytical Procedure (permitted
for re ular sha ed buildin s and structures
not susceptible to across wind loading orother special considerations due to location)
Method 3 Wind Tunnel Procedure (for all
structures)
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Method 1 Simplified Procedure
Main Wind-Force Resisting System(MWFRS): An assemblage of structuralelements assigned to provide support
and stability for the overall structure.The system generally receives windloading from more than one surface.
Components and Cladding: Elements ofthe building envelope that do not
qualify as part of the MWFRS.
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Mean Roof Height
h2
1
21
then,10If
2
then,10If
hh
hhh
o
o
=
+=>
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Example
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Method 2 Analytical Procedure
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Example
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