Windspeed Analyses of April 3-4, 1974 Tornadoes

Comprehensive documentation damage has made it possible to estimate tornadic windspeeds from the analyses of structural failures, based on the damage of eight specific structural components. The reliability of the windspeed estimates depend on the complexity of the structural system involved. The more complex the structural system, the less reliable are the windspeed estimates. A free standing structure such as a light standard or chimney gives the best windspeed estimates whereas commercial buildings and residences give the least reliable values. Windspeed analyses of the tornadoes combined with evaluations of several previous windstorm incidents leads to: (1)Ground level windspeeds in most tornadoes are considerably less than 275 mph; (2)the appearance of damage does not correlate well with estimates of windspeeds that cause the damage; (3)wind rather than atmospheric pressure change is the principal damaging mechanism for conventional buildings; and (4)properly designed roof anchorages reduce damage to structures in tornadic winds.