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Wind Loads: The Nature of Wind. CE 694R – Fall 2007 T. Bart Quimby, P.E., Ph.D. UAA Civil Engineering Quimby & Associates. Methods of Research. Field Observations FEMA funds recognizance teams to visit disaster sites.

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wind loads the nature of wind

Wind Loads:The Nature of Wind

CE 694R – Fall 2007

T. Bart Quimby, P.E., Ph.D.

UAA Civil EngineeringQuimby & Associates

methods of research
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.

UAA Civil Engineering

the nature of wind
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.

UAA Civil Engineering

wind speed determination
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
    • Need annual maximum wind speed for 10 or more consecutive years.
    • Use Fisher-Tippett Type I simplified procedure given in Simiu & Scanlan (1986)

UAA Civil Engineering

alaska wind speeds
Alaska Wind Speeds

UAA Civil Engineering

measurement of wind speed
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
    • 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.

UAA Civil Engineering

converting fast mile to 3 sec gust

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:
    • 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

UAA Civil Engineering

variation of wind speed with height
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.

UAA Civil Engineering

topographic effect
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

UAA Civil Engineering

wind structure interaction
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
    • Requires wind tunnel testing

UAA Civil Engineering