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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

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UAA Civil Engineering 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. 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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Converting Fast Mile to 3 sec Gust V 3 = V fm (V3/V3600)/(V t /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 ASCE 7-95 Figure C6-1

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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, Z g, 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

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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

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