Wind Loads:The Nature of Wind

CE 694R – Fall 2007T. Bart Quimby, P.E., Ph.D.

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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.

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

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 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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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

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ASCE 7-95 Figure C6-1

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 Requires wind tunnel testing

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