Transcript Nature of Wind - A Beginner's Guide to Structural

Wind Loads:
The Nature of Wind
CE 694R – Fall 2007
T. Bart Quimby, P.E., Ph.D.
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Quimby & Associates
Methods of Research
 Field Observations
 FEMA funds recognizance teams to visit disaster sites.
 Data collected from field observations has improved with the
advent of video cameras!
 Experimental
 Wind Tunnel Studies.
 Computational
 Computation Fluid Mechanics requires huge amounts of
computing capacity.
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The Nature of Wind
 Wind is chaotic

Wind speed varies considerably at any given
instant in time.
 Wind speed generally increases with height
 Gust size varies along wind, across wind,
and vertical
 We try to make sense out of this chaos with
general approximations.
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Wind Speed Determination
 Wind Maps
Contour maps of basic wind speed expressed in terms of 3
second gust.
 The last change to the Alaska map was in ASCE 7-05 when
it was adjusted for change to 3 second gust. No effort was
made to incorporate new Alaskan data.
 Generated using probabilistic methods.
 Probabilistic methods
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Need annual maximum wind speed for 10 or more consecutive
years.
Use Fisher-Tippett Type I simplified procedure given in Simiu &
Scanlan (1986)
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Alaska Wind Speeds
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Measurement of Wind Speed
 Old methods measured wind in terms of “fastest-mile”
at 10m (33') above ground at Exposure C.
 Current methods of determining wind speed is in
terms of “3- second gust” speed.
 Important to know basis for wind speed
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Wind speeds used in designs prior to ASCE 7-95 are not directly
comparable to wind speeds in current designs.
75 mph “fastest-mile” = 90 mph “3-second gust”
Hurricane 120 mph “fastest-mile” = 152 mph “3-sec. gust”
See ASCE 7-95 Commentary 6.5.2.
 ASCE 7-95 and later uses 3 second gust speeds.
 Basic Wind Speed is determined for a 50-yr mean recurrence
interval (MRI).
 Can convert to other MRI using ASCE 7-05 Table C6-7.
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ASCE 7-95 Figure C6-1
Converting
Fast Mile
to
3 sec Gust
 V3 = Vfm(V3/V3600)/(Vt /V3600)
 Convert 90 mph fastest mile to 3 sec gust:
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Averaging time, t = (3600 s/hr)/(90 mph) = 40 s/mi
From Chart: V40/V3600 = 1.29
From Chart: V3/V3600 = 1.53
V3 = 90 mph (1.53/1.29) = 107 mph
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Variation of Wind Speed with
Height
 Ground obstructions retard the movement of air close
to the ground surface, reducing wind speed
 At some height above ground, the movement of air is
no longer affected by ground obstruction. This is
called Gradient Height, Zg, which is function of
surface roughness.
 ASCE 7 use an empirical power law equation to
compute the variation in wind speed with height and
surface roughness.
 See ASCE 7-05 Commentary 6.5.6.6.
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Topographic Effect
 Local abrupt
topography affects
wind near the ground.
 Wind speed depends
on shape of hill,
location of building,
and height above
ground
 The current procedure
was first presented in
ASCE 7-95
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Wind/Structure Interaction
 Aerodynamics: Pressure and Force
Coefficients
 Buffeting: Along-Wind Resonance

Only important for flexible structures.
 Vortex Shedding

Not included in ASCE 7
 Aeroelastic: Galloping, Flutter
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Requires wind tunnel testing
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