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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 l.jpg

Wind Loads:The Nature of Wind

CE 694R – Fall 2007

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

UAA Civil EngineeringQuimby & Associates

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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

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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.

UAA Civil Engineering

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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

    • 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

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Alaska Wind Speeds

UAA Civil Engineering

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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

    • 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

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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:

    • 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

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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

UAA Civil Engineering

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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

UAA Civil Engineering

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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

    • Requires wind tunnel testing

UAA Civil Engineering