Empirical Approach to Evaluating the Tornado Fragility of Residential Structures

Tornado-induced wind load modeling has advanced significantly in recent years, but comparison of the experimental or numerical models to observed tornado damage is limited. This paper describes the development of empirically derived tornado fragility functions for residential structures following the 22 May 2011 Joplin, Missouri tornado. The fragility functions were created by combining a residential structure damage assessment of the Joplin tornado using the enhanced Fujita (EF) scale and a tornado wind field model conditioned to tree-fall patterns. The damage states for the fragility functions used the degrees of damage (DOD) for one- and two-story residential structures from the EF scale. The empirical tornado fragility functions were compared to analytically derived fragility functions for straightline winds from the FEMA Hazus hurricane model to provide a first attempt at ascertaining how different or similar failure wind speeds are for residential structures under tornado-induced and straightline wind loads. Median failure wind speeds from the empirical tornado fragility functions increased monotonically with DOD, from 33.4 m/s for DOD1 to 85.6 m/s for DOD9. Median failure wind speeds of the empirical tornado fragility functions and Hazus hurricane straightline wind fragility functions differed by 5% or less when suburban terrain was assumed throughout the damage path, and 25% or less when open terrain was assumed. The results suggest that tornado load amplification factors are at worst no more than 55% for residential structures, and may be lower than 10%.