Wind Provisions - A Beginner`s Guide to Structural Engineering

Report
Wind Loads:
The ASCE 7 Provisions
CE 694R – Fall 2007
T. Bart Quimby, P.E., Ph.D.
Quimby & Associates
A Beginner's Guide to ASCE 7-05
Permitted Design Methods
See ASCE 7-05 6.1.2
 Method 1—Simplified Procedure
 (ASCE 7-05 Section 6.4)
 Low rise buildings. This is an outgrowth of work done
for/by the metal building industry.
 Method 2—Analytical Procedure
 (ASCE 7-05 Section 6.5)
 The typically used procedure. This is the main focus of
this presentation.
 Method 3—Wind Tunnel Procedure
 (ASCE 7-05 6.6)
A Beginner's Guide to ASCE 7-05
Important Definitions
See ASCE 7-05 6.2
 Basic Wind Speed
 Building open, enclosed, partially enclosed
 Low-Rise Building
A Beginner's Guide to ASCE 7-05
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.

A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
The Velocity Coefficient
See ASCE 7-05 C6.5.10
 Based on the average density of air at sea level.
1
5280 2 2
2 1 0.0765
P V  [
][
] V  0.00256V 2
2
2 32.2
3600
A Beginner's Guide to ASCE 7-05
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)
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
Kz & Kh Computation
Kz = 2.01(z/zg)2/a
When z > zg use z = zg
When z < 15 use z = 15 ft
K Com putation
2.50
2.00
Exposure B
K
1.50
Exposure C
1.00
Exposure D
0.50
0.00
0
500
1000
Elevation, z (ft)
A Beginner's Guide to ASCE 7-05
1500
2000
Topographical Factor, Kzt
See ASCE 7-05 6.5.7 & Commentary 6.5.7
 Kzt = 1.0 when:
 H/Lh < 0.2, or
 H < 15' for Exposures C & D,
or
 H < 60' for Exposure B.
 Kzt = (1+K1K2K3)2
A Beginner's Guide to ASCE 7-05
Kzt
Constants
A Beginner's Guide to ASCE 7-05
Kzt Multipliers by Equation
See ASCE 7-05 Figure 6.4
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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.”
A Beginner's Guide to ASCE 7-05
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.
C
P measured
1
V 2
2
A Beginner's Guide to ASCE 7-05
Enclosure Classifications
See ASCE 7-05 6.2 & 6.5.9
 A building is to be classified as one of the following:
 Open
 Ao > 0.8Ag for each wall
 Partially Enclosed
 Ao > 1.10 Aoi, and
 Ao > min[4 sqft , 0.01Ag], and
 Aoi/Agi < 0.20
 Enclosed
 A building that is neither open nor partially enclosed.
A Beginner's Guide to ASCE 7-05
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).
A Beginner's Guide to ASCE 7-05
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 it’s 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.
A Beginner's Guide to ASCE 7-05
Internal Pressure
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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


Leeward


Same regardless of building plan dimensions
Dependant on building plan dimensions
Side

Same regardless of building plan dimensions
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Domed Roofs
See ASCE 7-05 Figure 6-7
 Pressure distributions are fairly complex.
 Two load cases to be considered.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
Some Local Effects
 Wind
around a
corner
A Beginner's Guide to ASCE 7-05
Image from FEMA Multi
Hazard Seminar
Wind at a Corner
A Beginner's Guide to ASCE 7-05
Image from FEMA Multi
Hazard Seminar
Uplift on Roof
Images from FEMA Multi
Hazard Seminar
A Beginner's Guide to ASCE 7-05
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.
A Beginner's Guide to ASCE 7-05
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
A Beginner's Guide to ASCE 7-05
Typical
Roof
Chart
A Beginner's Guide to ASCE 7-05
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).
A Beginner's Guide to ASCE 7-05
Sample Problem
 V = 120 mph
 Exposure C
 Enclosed
A Beginner's Guide to ASCE 7-05

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