Wind%20Loads:%20The%20ASCE%207%20Provisions

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Wind LoadsThe ASCE 7 Provisions

• CE 694R Fall 2007
• T. Bart Quimby, P.E., Ph.D.
• Quimby Associates

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Permitted 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)

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Important Definitions
See ASCE 7-05 6.2

• Basic Wind Speed
• Building open, enclosed, partially enclosed
• Low-Rise Building

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Exposure 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.

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Determining 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.

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ASCE 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.

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Velocity 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

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The Velocity Coefficient
See ASCE 7-05 C6.5.10

• Based on the average density of air at sea level.

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Basic 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)

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The 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.

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Velocity 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.

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Kz Kh Computation
When z gt zg use z zg When z lt 15 use z 15 ft
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Topographical 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

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Kzt Constants
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Kzt Multipliers by Equation
See ASCE 7-05 Figure 6.4
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Directionality 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.

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The 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

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Gust 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.

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Pressure 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.

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Enclosure 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.

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Location 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).

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Internal 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.

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Internal Pressure
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External 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

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Buildings 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.

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Buildings 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

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Buildings 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.

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Wind 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.

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Wind 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

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Domed Roofs
See ASCE 7-05 Figure 6-7

• Pressure distributions are fairly complex.
• Two load cases to be considered.

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Arched Roofs
See ASCE 7-05 Figure 6-8

• Pressure coefficient depends on rate of rise of
  the arch.
• Pressure varies by along the arch.

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Components 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.

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Some Local Effects

• Wind around a corner

Image from FEMA Multi Hazard Seminar
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Wind at a Corner
Image from FEMA Multi Hazard Seminar
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Uplift on Roof
Images from FEMA Multi Hazard Seminar
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Wall 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.

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Roof 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

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Typical Roof Chart
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Finding 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).

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Sample Problem

• V 120 mph
• Exposure C
• Enclosed