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Fujita Tornado Intensity Scale: A Reassessment From Structural Engineering Perspective.


pdf icon Fujita Tornado Intensity Scale: A Reassessment From Structural Engineering Perspective. (1157 K)
Phan, L. T.; Simiu, E.

Design for Wind and Wind Hazard Mitigation, 2nd U.S./Japan Workshop. Technical Memorandum of PWRI 3680. Proceedings. May 24-26, 1999, Tsukuba Science City, Japan, 123-130pp AND Wind Wind and Seismic Effects. U.S./Japan Natural Resources Development Program (UJNR). Joint Meeting, 31st. Technical Memorandum of PWRI 3653. Proceedings. May 11-14, 1999, Tsukuba, Japan, 71-78 pp, 1999, 1999.

Keywords:

wind effects; building construction; weather effects; meteorology; standards; structural engineering; wind engineering; damage; tornadoes

Abstract:

On May 27, 1997 the town of Jarrell in central Texas, and on May 30, 1998 the town of Spencer in South Dakota were hit by two violent tornadoes. We evaluated the structural damage caused by the Jarrell tornado and concluded that the worst damage can be explained by wind speeds corresponding to an F3 rating on the Fujita tornado intensity scale (wind speeds of 71 m/s to 92 m/s). An F4 (93 m/s to 116 m/s) rating, or the F5 (117 m/s to 142 m/s) rating officially issued by the National Weather Service (NWS), need not be assumed to explain that damage. We also present assessments of damage in the Spencer tornado, and suggest there is an inconsistency between the NWS ratings of the Jarrell and Spencer tornadoes. We ascribe the NWS ratings to the failure of the Fujita scale to account explicitly for the dependence of wind speeds causing specified types of damage upon two structural engineering factors: (1) the basic design wind speed at the geographical location of interest, and (2) quality of construction, defined as degree of conformity to applicable standards requirements. We address the need for a stronger involvement of the structural engineering profession in nationwide efforts to develop an improved tornado classification scale and a more realistic tornado windspeed database.