ASCE Wind Loads

ASCE SEMINAR

WIND LOADS ON BUILDINGS

AND OTHER STRUCTURES

Jon D. Raggett, PhD, PE, SEPresidentWest Wind Laboratory, Inc.2320 Del Monte Ave., Suite A-4Monterey, CA 93940831-883-1533raggett@westwindlaboratory.com

ASCE Wind Loads

ASCE SEMINARWIND LOADS ON BUILDINGS AND OTHER STRUCTURES

SESSIONS 1 AND 2 - TheoryDESCRIPTION OF WINDBLUFF BODY AERODYNAMICSSTRUCTURAL DYNAMICS

SESSIONS 3 AND 4 - ASCE 7-10, CHAPTERS 26-31DESCRIBE HOW ASCE 7-10 CHAPTERS 26-31 6 INCLUDES THEORY

SESSIONS 5 AND 6 - EXAMPLESHIGH-RISE BUILDINGLOW-RISE BUILDINGOPEN STRUCTURES SIGNSINDUSTRIAL STRUCTURES

SESSION 7 - EXAMPLESLOW-RISE BUILDINGS(COMPARISON OF METHODS)

SESSION 8 –WIND-BORNE DEBRIS AND EXPERIMENTAL METHODS

ASCE Wind Loads

Course Objectives

At the end of this course you will:

• Learn the fundamentals of bluff-body aerodynamics and structural dynamics;

• Learn how those fundamentals are included in ASCE 7-10

• Learn how to use ASCE 7-10 through a series of examples; and

• Have your questions answered.

ASCE Wind Loads

SESSION 1Wind Engineering Theory:

Wind Characteristics

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Snapshot of Wind Courtesy of Wind Science and Engineering, Texas Tech Univ.

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Nature of Winds

• Averaging time

• Wind speed profile

• Wind turbulence

• Topography

• Wind Hazard Risk

• Wind directionality

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TornadoesCourtesy of Wind Science and Engineering, Texas Tech Univ.

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Wind Speed Time Histories

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Wind Speed Time Histories

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Wind Characteristics As A Function of Averaging Time

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Wind Speed Characteristics As A Function of Time and Elevation

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Ratio of Averaging Time to Mean Hourly

Courtesy of Wind Science and Engineering, Texas Tech Univ.

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Power Law ProfilesCourtesy of Wind Science and Engineering, Texas Tech Univ.

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Power Law

Vz = Vg

Vz = Vg z > zg

zz

1

0 z zg g

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Wind Turbulence: IntensityCourtesy of Wind Science and Engineering, Texas Tech Univ.

Vmean

=19.7MPH

0

10

20

30

40

0 3 6 9 12 15

Time (Minutes)

Win

d S

pe

ed

(M

PH

)

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Wind Turbulence: Frequency Content (Spectrum)Courtesy of Wind Science and Engineering, Texas Tech Univ.

0.00

0.05

0.10

0.15

0.20

0.25

0.0001 0.001 0.01 0.1 1 10

Frequency (Hz, Cycles/Second)

n*S

u(n

)/

u2

0.1110100100010000

Period (Seconds)

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Logarithmic LawAtmospheric boundary layer (alternate used primarily by laboratories - similar results)Basic law for one-hour averaged mean wind speed

0

*2.5 lnz

U z uz

0

*

- mean wind speed at elevation above grade

- friction velocity

- surface roughness length

U z z

u

z

2 2* - Ratio of longitudinal variance to u

EXPOSURE Z0 (m)

Z0 (m)

IMPLIED IN ASCE 7-05

B 0.300 5.25 0.150

C 0.030 6.00 0.020

D 0.005 6.50 0.005

ASCE Wind Loads

Logarithmic Law (cont)

1

2

0

1

2

t 3600

0

Turbulence intensity

2.5ln

Mean wind speed as a function of averaging time

U 1

2.5ln

TI zU zz

z

c tz U z

zz

t(sec) c(t)

1 3.00

3 2.85

60 1.28

300 .54

600 .36

3600 .00

0.0706

01*1

*2 02

zu

u z

Change exposure

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Logarithmic Law (cont)

Longitudinal turbulence spectrum

nz

fU z

All consistent and derived from fundamental principles (Simiu, E., and Scanlan, R. H. Wind Effects on Structures, Third Edition, John Wiley & Sons, New York, 1996).

523*

, 200

1 50

nS z n f

u f

- frequency, Hz

, - power spectral density

n

S z n

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Escarpment Courtesy of Wind Science and Engineering, Texas Tech Univ.

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Topographic Effect

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DESIGN WIND SPEED• Design wind speed is based on probability• Objective is to obtain a maximum reference wind speed

with a specific mean recurrence interval (MRI), or mean return period, or probability that a specific wind speed is exceeded (equal to 1/MRI)

• Obtain from historical wind speed data a specific site• Obtain from hurricane simulation models• Smooth data over regions

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EXPECTED WIND SPEED(FROM HISTORICAL PEAK ANNUAL WIND SPEEDS)

0.78 .577V N LnNwhere

V(N) Peak wind speed with a return period of N years

Mean peak annual wind speed from n years of data

Standard deviation of the peak annual wind speeds fromn years of data

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Expected Wind Speed - Example (From Historical Peak Wind Speeds)

Courtesy of Wind Science and Engineering, Texas Tech Univ.

Year 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978

60 57 78 57 99 60 65 56 58 57 59 61

1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990

66 78 74 55 70 88 65 66 58 65 49 60

1991 1992 1993 1994 1995 1996 1997 1998

57 44 78 65 56 75 54 56

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Expected Wind Speeds - Example(From Historical Peak Annual Wind Speeds)

(Cont)Courtesy of Wind Science and Engineering, Texas Tech Univ.

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Expected Hurricane Wind Speeds(From Numerical Simulation Procedures)

Courtesy of Wind Science and Engineering, Texas Tech Univ.

TRACKS OF HURRICANES PASSING WITHIN 250 km OF GRAND CAYMAN ISLAND

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0

50

100

150

200

250

10 100 1000 10000

Return Period (Years)

Win

d S

pe

ed

(m

ph

)

Mean Hourly

One Minute

Fastest Mile

Peak Gust

Expected Hurricane Wind Speeds(From Numerical Simulation Procedures)

(Cont) Courtesy of Wind Science and Engineering, Texas Tech Univ.

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References for Hurricane Numerical Simulation Procedures

Dr. Peter Vickery Papers (that describe hurricane simulation procedures)

• Vickery, P. J., Skerlj, P.F., and Twisdale, L. A., "Simulation of Hurricane Risk in the U. S. Using Empirical Track Model", Journal of Structural Engineering, October 2000.

• Vickery, P. J. et al, "Hurricane Wind Field Model for Use in Hurricane Simulations", Journal of Structural Engineering, October 2000

• Vickery, P. J., et al, "Hurricane Gust Factors Revisited", Journal of Structural Engineering, May 2005

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Wind Directionality