The current design wind loads for buildings and other structures are based upon model tests in low-speed boundary-layer wind tunnels that generate straight-line winds. However, winds resulting from tornadoes and microbursts that could occur during storm events such as thunderstorms or hurricanes are far from being regular atmospheric boundary-layer type winds. In recent full-scale measurements using Doppler radar, it has been observed that tornadoes could produce intense winds in the region below 20 m from the ground. Microbursts are characterized by a strong localized down-flow and an outburst of strong winds near the surface. Thus, microburst winds have significant vertical velocity components and mean horizontal velocity distributions that are different from usual boundary-layer winds. This paper presents quasi-steady and transient wind load effects on a tall building in a laboratory-simulated tornado and microburst. Experiments were conducted in the Tornado/Microburst Simulator at Iowa State University. The microburst is simulated as a round jet, 1.83 m or 6 ft in diameter, impinging onto a flat ground plane. The tornado was simulated with a maximum vortex core diameter of 1.12 m or 3.7 ft. A 1/500 geometrically-scaled model of a tall building, 216 m (708 ft) in height with a square cross section of 54 m (177 ft), is used for this study. Comparisons of peak loads measured in this study with loads specified in ASCE 7- 02 showed that tornados of F2 strength or stronger would exceed the minimum design wind load provisions by a factor of 1.8 or greater.
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