SESSION THREE

1
SESSION THREE
Minimum Design Loads for Buildings and Other
Structures ASCE 7-98 PROVISIONS AND BACKGROUND
2
ASCE 7 COMMITTEE

• 99 MEMBERS
• Producers (material and code group individuals
  and/or organizations)
• Consumers (consultants , practicing
  professionals, and federal agency
  representatives)
• General Interest (researchers, academicians, and
  public interest representatives)

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MEMBERSHIP OF WIND LOAD TASK COMMITTEE
Committee Member Affiliation
Location Dr. Kishor Mehta, Chair Natural Hazards
- Hurricane Lubbock, TX Dr. Jack Cermak Colorado
State University Ft Collins, CO Dr. Ahsan
Kareem University of Notre Dame Notre Dame,
IN Mr. Gill Harris CECO Building
Systems Columbus, MS Dr. Richard Marshall Natl
Inst of Stds and Tech Gaithersburg, MD Dr. Dale
Perry Texas A M University College Station,
TX Mr. Herb Saffir Herbert S. Saffir
Consulting Engrs Coral Gables, FL Mr. Thomas
Smith Natl Roofing Contractors
Assoc. Rosemont, IL Dr. Ted Stathopolous Concordia
University Montreal, Canada Mr. Rick Vognild
Southern Bldg Code Cong Intl,
Inc. Birmingham, AL
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Design Method -1

• Simplified Procedure
• enclosed or partially enclosed buildings
• roof slopes less than 10 degrees
• mean roof height less than or equal 30 ft
• regular shaped buildings
• no expansion joints or separations
• no topographic effects (over hills, ridges,...)

5
Design Method - 2

• Analytical Procedure
• regular shaped buildings
• no response characteristics that make it subject
  to instability due to aeroelastic phenomena
  (galloping of cable or bridge flutter of stop
  sign)

6
Design Methods - 3

• Wind Tunnel Procedure
• wind tunnel testing shall be permitted in lieu of
  Methods 1 and 2 for any building or structure.

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FIGURE 6.1 BASIC WIND SPEED
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Classification of Buildings and Other Structures
for Wind

• Description Category
• Storage facilities (low hazard to human life)
  I
• All others (not listed in category I, III, IV)
  II
• A subst. hazard to human life (schools) III
• Essential facilities (emergency shelters)
  IV

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IMPORTANCE FACTOR, I
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Exposure Categories

• Exposure A. Large city centers with at least 50
  of the buildings having a height in excess of 70
  ft.
• Exposure B. urban and suburban areas, or other
  terrain with numerous closely spaced obstruction
  having the size of single family dwellings or
  larger.
• Exposure C. Open terrain with scattered
  obstructions having heights generally less than
  30 ft (flat open country and grass lands).
• Exposure D. Flat unobstructed areas exposed to
  wind flowing over large bodies of water.

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Enclosure Classifications

• Open
• each wall at least 80 open
• Partially Enclosed
• the total area of openings in a wall that
  receives positive external pressure exceeds the
  sum of the areas of openings in the balance of
  the building (more than 4 sf)
• Enclosed

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ASCE 7-98
p q GC q 0.00256Kz Kzt V2 I p design
pressure in psf q velocity pressure in psf G
gust effect factor C pressure or force
coefficient 0.00256 constant for density of air
and dimensions Kz velocity pressure exposure
coefficient Kzt topographic factor V basic
wind speed in mph I importance factor
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WIND LOADING
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ASCE 7 - 98 FORMAT TABLE 6-1
BUILDINGS OTHER STRUCTURES and
OPEN BUILDINGS low-rise all MWFRS
p p F q GCA h 60 ft. h gt
60 ft. CC p p F q GCA Main
Wind Force Resisting System Components and
Cladding
15
CONSTANT 0.00256
16
PROBABILITY FOR STRUCTURE
17
IMPORTANCE FACTOR, I
18
Velocity pressure exposure coefficient Kz is a
function of height and exposure type.
19
Kz
Velocity Pressure Coefficients, Kz
20
GUST EFFECT FACTOR, G

• Rigid Structure Simplified Procedure
• G 0.80 for Exposures A and B
• G 0.85 for Exposures C and D

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Cp for MWFRS, All Heights
22
WIND TUNNEL RESULTS (GCp)
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TOPOGRAPHIC FACTOR Kzt

• Kzt (1 K1K2K3)2

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Example 1
100 ft X 200 ft X 160 ft Office
Building Location Houston,Texas Topography
Flat Terrain Suburban Framing R/C rigid frame
in both directions Floor slabs provide
diaphragm action
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Example 1
3' PARAPET
160 FT
100 FT
200 FT
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FOUR STEP SEQUENCE
STEP 1 Exposure and Building Classification STE
P 2 Basic Wind Speed STEP 3 Velocity
Pressures STEP 4 Design Pressures for MWFRS
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Step 1 Exposure and Building Classification

• Exposure B
• Classification Category II

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Step 2 Basic Wind Speed
Houston
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Step 3 Velocity Pressures
where qz velocity pressure at height z Kz
velocity pressure exposure coefficient
evaluated at ht z Kzt topographic factor V
basic wind speed 120 mph I importance factor
z height above ground
30
Step 3 Kz

• Values of Kz will be taken from Table 6-3 of
  the standard and intermediate values will be
  interpolated where required.

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Step 3 Kzt
32
Step 3 Importance Factor

• Used to adjust the level of structural
  reliability to be consistent with the building
  classifications in Table 1-1.
• I1.0 for a Category II building
• Probability of Exceedence 0.02
• Probability that the design wind speed will be
  exceeded during a 50 yr life is 0.64

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PROBABILITY FOR STRUCTURE
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Step 3 Velocity Pressures
for this building Kzt 1.0 V 120 mph I
1.0 qz 0.00256 Kz (1.0) (120)2 (1) qz
36.9 Kz psf
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Kz
Velocity Pressure Coefficients, Kz
36
Step 3 Velocity Pressures
qz 36.9 Kz psf
37
Step 4 Design Pressures for the MWFRS
Where p pressure on surface q velocity
pressure G gust effect factor Cp external
pressure coefficient qh velocity pressure at
mean roof height GCpi product of gust effect
factor and internal pressure
coefficient
38
Step 4 Design Cases For MWFRS

• Wind normal to 200 ft face
• Positive and negative internal pressure
• Wind parallel to 200 ft face
• Positive and negative internal pressure

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Step 4 Gust Effect Factor, G

• G 0.80 for Exposure B

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Step 4 Wall External Pressure Coefficients
Figure 6-3
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Step 4 Wall Cps
42
Step 4 Roof Cps
Figure 6-3
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Step 4 GCpi

• Assume the openings are evenly distributed in the
  walls and roof
• Houston, TX is in a hurricane-prone region,
  however wind-borne debris resistant glazing is
  used
• GCpi 0.18

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Step 4 MWFRS Net Pressures
45
Step 4 MWFRS Net Pressures(Wind Normal to 200
ft Face)
internal pressure 7.5 psf
46
Step 4 Pressures(Wind Normal to the 200 ft Face
External Only)
34.7 psf
23.3 psf
160 ft
26.7 psf
80 ft
120 ft
24.5 psf
Internal pressure 7.5 psf
80 ft
16.7 psf
21.9 psf
50 ft
19.1 psf
30 ft
16.5 psf
15 ft
13.4 psf
100 ft
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Step 4 MWFRS Net Pressures(Wind Parallel to
200 ft Face)
internal pressure 7.5
48
Step 4 Pressures(Wind Parallel to the 200 ft
Face External Only)
32.8 psf
26.0 psf
20.7 psf
160 ft
26.7 psf
80 ft
160 ft
120 ft
24.5 psf
Internal pressure 7.5 psf
80 ft
10.0 psf
21.9 psf
50 ft
19.1 psf
30 ft
14.5 psf
15 ft
13.4 psf
200 ft
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EXAMPLE 2
Low-Rise Building MWFRS Location Memphis,
Tennessee Topography Flat Dimensions 200 ft x
250 ft in plan Eave height of 20 ft
Roof slope 412 (18.43 deg.) Framing Rigid frame
spans 200 ft _at_25 OC Braced frame in 250
ft direction
50
Ridge
20'
Eave
Cross
Frame
Bracing
Girt
250'
4
12
100'
100'
200'
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Step 1 Exposure and Building Classification

• Exposure C
• Classification Category II

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Step 2 Basic Wind Speed
Memphis
53
Step 3 Velocity Pressures
where qz velocity pressure at height z Kz
velocity pressure exposure coefficient
evaluated at ht z Kzt topographic factor V
basic wind speed I importance factor z height
above ground
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Step 3 Kz

• Values of Kz will be taken from Table 6-3 of
  the standard and intermediate values will be
  interpolated where required.

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Step 3 Kzt
56
Step 3 Importance Factor

• Used to adjust the level of structural
  reliability to be consistent with the building
  classifications in Table 1-1.
• I1.0 for a Category II building
• Probability of Exceedence 0.02
• Probability that the design wind speed will be
  exceeded during a 50 yr life is 0.64

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Kz
Velocity Pressure Coefficients, Kz
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Step 3 Velocity Pressures
59
Step 4 Design Pressures for the MWFRS
Where p pressure on surface q velocity
pressure G gust effect factor Cp external
pressure coefficient qh velocity pressure at
mean roof height GCpi product of gust effect
factor and internal pressure
coefficient
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Step 5 Design Cases

• Frame
• Wind normal to ridge
• Positive and negative internal pressure
• Positive and negative windward roof pressure
• Wind parallel to ridge
• Positive and negative internal pressure
• Cross Bracing
• Wind parallel to ridge
• Positive and negative internal pressure

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Step 4 G

• G 0.85 (Use)

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Step 4 Design PressuresWind Normal to Ridge,
(-) Windward Roof Pressure, (-) Internal Pressure
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Step 4 Design Pressures Wind Normal to Ridge,
() Windward Roof Pressure, (-) Internal Pressure
64
Step 4 Design Pressures Wind Normal to Ridge,
(-) Windward Roof Pressure, () Internal Pressure
65
Step 4 Design Pressures Wind Normal to Ridge,
() Windward Roof Pressure, () Internal Pressure
66
Step 4 Design Pressures Frame Load Wind
Parallel to Ridge, () Internal Pressure
67
Step 4 Design Pressures Frame Load Wind
Parallel to Ridge, (-) Internal Pressure
68
Step 4 Design Pressures Cross Bracing Load
Wind Parallel to Ridge, () Internal Pressure
-19.9psf
-12.8psf
-9.2psf
36.7'
36.7'
53.3'
11.7psf
40'
10.8psf
30'
-11.7psf
10.0psf
20'
8.9psf
15'
8.2psf
69
Step 4 Design Pressures Cross Bracing Load
Wind Parallel to Ridge, (-) Internal Pressure
-12.3psf
-5.2psf
-1.6psf
36.7'
36.7'
53.3'
19.3psf
40'
18.4psf
30'
-4.1psf
17.6psf
20'
16.4psf
15'
15.8psf