The ASCE 7 Provisions

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Nov 16, 2013 (3 years and 11 months ago)

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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


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)


See ASCE 7
-
05 6.1.2

A Beginner's Guide to ASCE 7
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05

Important Definitions


Basic Wind Speed


Building open, enclosed, partially enclosed


Low
-
Rise Building

See ASCE 7
-
05 6.2

A Beginner's Guide to ASCE 7
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05

Exposure Categories


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.

See ASCE 7
-
05 6.5.6 & C6.5.6 (See images!)

A Beginner's Guide to ASCE 7
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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 45
o

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
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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
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05

Velocity Pressure, q


q
z

=Velocity Pressure = 0
.
00256
K
z
K
zt

K
d
V
2

I
(lb/ft
2
)


Constant 0.00256


V = Basic wind speed in mph


I = Importance Factor (i.e. different MRI)


K
z

= Exposure Coefficient


K
zt

= Topographical Factor


K
d

= Wind Directionality Factor


Evaluated at an elevation z:



q
z

= 0.00256V
2
IK
z
K
zt
K
d


Evaluated at the building mean roof elevation, h:



q
h

= 0.00256V
2
I K
h
K
ht
K
d

See ASCE 7
-
05 6.5.10

A Beginner's Guide to ASCE 7
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05

The Velocity Coefficient


Based on the average density of air at sea level.

P

1
2

V
2

1
2
[
0
.
0765
32
.
2
]
[
5280
3600
]
2
V
2

0
.
00256
V
2
See ASCE 7
-
05 C6.5.10

A Beginner's Guide to ASCE 7
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05

Basic Wind Speed, V


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)

See ASCE 7
-
05 6.5.4

A Beginner's Guide to ASCE 7
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05

The Importance Factor, I


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.

See ASCE 7
-
05 6.5.5, Table 6
-
1 and Commentary 6.5.5

A Beginner's Guide to ASCE 7
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05

Velocity Pressure Exposure
Coefficients, K
z

and K
h


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 K
15
.


For elevations above gradient height use K
g
.

See ASCE 7
-
05 6.5.6.6, Tables 6
-
2 and 6
-
3, and C6.5.6.6

A Beginner's Guide to ASCE 7
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05

K
z

& K
h

Computation

K
z
= 2.01(z/z
g
)
2/a
K Computation
0.00
0.50
1.00
1.50
2.00
2.50
0
500
1000
1500
2000
Elevation, z (ft)
K
Exposure B
Exposure C
Exposure D
When z > z
g

use z = z
g

When z < 15 use z = 15 ft

A Beginner's Guide to ASCE 7
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05

Topographical Factor, K
zt


K
zt

= 1.0 when:



H/L
h

< 0.2, or



H < 15' for Exposures C & D,
or



H < 60' for Exposure B.



K
zt

= (1+K
1
K
2
K
3
)
2

See ASCE 7
-
05 6.5.7 & Commentary 6.5.7

A Beginner's Guide to ASCE 7
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05

K
zt

Constants

A Beginner's Guide to ASCE 7
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05

K
zt

Multipliers by Equation

See ASCE 7
-
05 Figure 6.4

A Beginner's Guide to ASCE 7
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05

Directionality Factor, K
d


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.


See ASCE 7
-
05 6.5.4.4 and
Table 6
-
4

A Beginner's Guide to ASCE 7
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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
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05

Gust Factor, G


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
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05

See ASCE 7
-
05 6.5.8

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
2

V
2
A Beginner's Guide to ASCE 7
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05

Enclosure Classifications


A building is to be classified as one of the following:


Open


A
o

>

0.8A
g

for each wall


Partially Enclosed


A
o

> 1.10 A
oi
, and


A
o

> min[4 sqft , 0.01A
g
], and


A
oi
/A
gi

<

0.20


Enclosed


A building that is neither open nor partially enclosed.

A Beginner's Guide to ASCE 7
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05

See ASCE 7
-
05 6.2 & 6.5.9

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
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05

Internal Pressure Coefficients, GC
pi


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.


GC
pi


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.

See ASCE 7
-
05 6.5.11.1 & Figure 6
-
5

A Beginner's Guide to ASCE 7
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05

Internal Pressure

A Beginner's Guide to ASCE 7
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05

External Pressure Coefficients, C
p


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

See ASCE 7
-
05 6.5.11.2 & Figures 6
-
6, 6
-
7, and 6
-
8

A Beginner's Guide to ASCE 7
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05

Buildings with Roofs Consisting of
Flat Surfaces


ASCE 7
-
05 Figure 6
-
6 gives the external
coefficients of wall and roof surfaces.

See ASCE 7
-
05 Figure 6
-
6

A Beginner's Guide to ASCE 7
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05

Buildings with Roofs Consisting of
Flat Surfaces


Wall C
p


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


See ASCE 7
-
05 Figure 6
-
6

A Beginner's Guide to ASCE 7
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05

Buildings with Roofs Consisting of
Flat Surfaces


Roof C
p


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.

See ASCE 7
-
05 Figure 6
-
6

A Beginner's Guide to ASCE 7
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05

Wind Normal to Ridge


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.


See ASCE 7
-
05 Figure 6
-
6

A Beginner's Guide to ASCE 7
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05

Wind Parallel to Ridge


Parallel to ridge,
flat or nearly flat


Two different
h/L ranges,
both with
stepped
pressures.


Interpolate
between
ranges


See ASCE 7
-
05 Figure 6
-
6

A Beginner's Guide to ASCE 7
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05

Domed Roofs


Pressure distributions are fairly complex.


Two load cases to be considered.

See ASCE 7
-
05 Figure 6
-
7

A Beginner's Guide to ASCE 7
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05

Arched Roofs


Pressure coefficient depends on rate of rise
of the arch.


Pressure varies by along the arch.

See ASCE 7
-
05 Figure 6
-
8

A Beginner's Guide to ASCE 7
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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
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05

Some Local Effects


Wind
around a
corner

A Beginner's Guide to ASCE 7
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05

Image from FEMA Multi
Hazard Seminar

Wind at a Corner

A Beginner's Guide to ASCE 7
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05

Image from FEMA Multi
Hazard Seminar

Uplift on Roof

A Beginner's Guide to ASCE 7
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05

Images from FEMA Multi
Hazard Seminar

Wall Components


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
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05

See ASCE 7
-
05 Figure 6
-
11A

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
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05

Typical
Roof
Chart

A Beginner's Guide to ASCE 7
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05

Finding Net Pressure


The net pressure is the vector sum of the
internal and external pressures.


Typical form:

p = qGC
p



q
i
(GC
pi
)


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

See ASCE 7
-
05 6.5.12

Sample Problem


V = 120 mph


Exposure C


Enclosed

A Beginner's Guide to ASCE 7
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05