Title: Wind%20Loads:%20The%20ASCE%207%20Provisions
1Wind LoadsThe ASCE 7 Provisions
- CE 694R Fall 2007
- T. Bart Quimby, P.E., Ph.D.
- Quimby Associates
2Permitted Design Methods
See ASCE 7-05 6.1.2
- Method 1Simplified Procedure
- (ASCE 7-05 Section 6.4)
- Low rise buildings. This is an outgrowth of work
done for/by the metal building industry. - Method 2Analytical Procedure
- (ASCE 7-05 Section 6.5)
- The typically used procedure. This is the main
focus of this presentation. - Method 3Wind Tunnel Procedure
- (ASCE 7-05 6.6)
3Important Definitions
See ASCE 7-05 6.2
- Basic Wind Speed
- Building open, enclosed, partially enclosed
- Low-Rise Building
4Exposure Categories
See ASCE 7-05 6.5.6 C6.5.6 (See images!)
- 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.
5Determining 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.
6ASCE 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.
7Velocity Pressure, q
See ASCE 7-05 6.5.10
- 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
8The Velocity Coefficient
See ASCE 7-05 C6.5.10
- Based on the average density of air at sea level.
9Basic Wind Speed, V
See ASCE 7-05 6.5.4
- 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)
10The Importance Factor, I
See ASCE 7-05 6.5.5, Table 6-1 and Commentary
6.5.5
- 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.
11Velocity Pressure Exposure Coefficients, Kz and Kh
See ASCE 7-05 6.5.6.6, Tables 6-2 and 6-3, and
C6.5.6.6
- 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.
12Kz Kh Computation
When z gt zg use z zg When z lt 15 use z 15 ft
13Topographical Factor, Kzt
See ASCE 7-05 6.5.7 Commentary 6.5.7
- Kzt 1.0 when
- H/Lh lt 0.2, or
- H lt 15' for Exposures C D, or
- H lt 60' for Exposure B.
- Kzt (1K1K2K3)2
14Kzt Constants
15Kzt Multipliers by Equation
See ASCE 7-05 Figure 6.4
16Directionality Factor, Kd
See ASCE 7-05 6.5.4.4 and Table 6-4
- 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.
17The 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
18Gust Factor, G
See ASCE 7-05 6.5.8
- 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.
19Pressure 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.
20Enclosure Classifications
See ASCE 7-05 6.2 6.5.9
- A building is to be classified as one of the
following - Open
- Ao gt 0.8Ag for each wall
- Partially Enclosed
- Ao gt 1.10 Aoi, and
- Ao gt min4 sqft , 0.01Ag, and
- Aoi/Agi lt 0.20
- Enclosed
- A building that is neither open nor partially
enclosed.
21Location 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).
22Internal Pressure Coefficients, GCpi
See ASCE 7-05 6.5.11.1 Figure 6-5
- Internal pressure is fairly easy because the air
is relatively stagnant and the shape of the
structure does not affect its 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.
23Internal Pressure
24External Pressure Coefficients, Cp
See ASCE 7-05 6.5.11.2 Figures 6-6, 6-7, and 6-8
- 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
25Buildings with Roofs Consisting of Flat Surfaces
See ASCE 7-05 Figure 6-6
- ASCE 7-05 Figure 6-6 gives the external
coefficients of wall and roof surfaces.
26Buildings with Roofs Consisting of Flat Surfaces
Wall Cp
See ASCE 7-05 Figure 6-6
- 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
27Buildings with Roofs Consisting of Flat Surfaces
Roof Cp
See ASCE 7-05 Figure 6-6
- 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.
28Wind Normal to Ridge
See ASCE 7-05 Figure 6-6
- 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.
29Wind Parallel to Ridge
See ASCE 7-05 Figure 6-6
- Parallel to ridge, flat or nearly flat
- Two different h/L ranges, both with stepped
pressures. - Interpolate between ranges
30Domed Roofs
See ASCE 7-05 Figure 6-7
- Pressure distributions are fairly complex.
- Two load cases to be considered.
31Arched Roofs
See ASCE 7-05 Figure 6-8
- Pressure coefficient depends on rate of rise of
the arch. - Pressure varies by along the arch.
32Components 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.
33Some Local Effects
Image from FEMA Multi Hazard Seminar
34Wind at a Corner
Image from FEMA Multi Hazard Seminar
35Uplift on Roof
Images from FEMA Multi Hazard Seminar
36Wall Components
See ASCE 7-05 Figure 6-11A
- For buildings under 60 ft
- See ASCE 7-05 Figure 6-17 for building greater
than 60 ft tall.
37Roof 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
38Typical Roof Chart
39Finding Net Pressure
See ASCE 7-05 6.5.12
- 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).
40Sample Problem
- V 120 mph
- Exposure C
- Enclosed