Wind Loads: The Nature of Wind

1. Wind Loads:The Nature of Wind CE 694R – Fall 2007 T. Bart Quimby, P.E., Ph.D. UAA Civil EngineeringQuimby & Associates

2. 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

3. 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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6. 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

7. Alaska Wind Speeds UAA Civil Engineering

8. 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

9. 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

10. 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

11. 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

12. 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