Abstract
Tornadoes are a particularly devastating natural hazard that affect communities across the United States, particularly the Midwest and South. They are unique from an engineering point-of-view due to their very low probability of occurrence but often highly destructive consequences. The 2011 season was particularly devastating to the Southeastern portion of the U.S. This paper presents a single case study of a 2012 tornado that struck a single large rural light-frame wood house with an unconventional roof system. A fragility methodology was used as a tool to probabilistically study the loss of the roof system, and bound an Enhanced Fujita (EF) scale rating of the tornado. The tornado was initially rated as an EF3 tornado by the U.S. National Weather Service. However, following a detailed site inspection verified with numerical structural models, the tornado was downgraded to an EF2 tornado. As expected, the use of nail connections in a roof-to-wall connection resulted in a weaker link compared to a hurricane clip. The approach presented in this paper can be used as a supplement to the EF rating provided by U.S. National Weather Service meteorologists when unusual conditions in either the structure or surroundings exists.
Highlights
On average, more than 1000 tornadoes are reported annually nationwide; regions in the Midwest and South experience a very high frequency of tornadoes [1]
This paper presented a systematic assessment of the loss of a roof system in a two-story residential wood frame residential building that was impacted by a winter tornado in Tuscaloosa County, Alabama
This provided an insight into the tornado wind speed that resulted in the observed failure and formed the basis for a more quantitative analysis of the Enhanced Fujita (EF) rating for this particular case study
Summary
More than 1000 tornadoes are reported annually nationwide; regions in the Midwest and South experience a very high frequency of tornadoes [1]. Light-frame wood construction comprises the majority (approximately 90%) of the residential building stock in the U.S [12] These types of structures are extremely susceptible to natural hazards, especially tornadoes, due to their light weight. There is a very low probability of a structure being directly impacted by a tornado when considering the area of a tornado compared to the area of the U.S buildings in the U.S are typically designed using loads from ASCE Standard 7 which were developed for straight line winds. After the building was hit by the tornado, a very thorough site investigation was conducted by the authors to determine the manner in which the roof system failed This structure is used as an illustrative example in this paper to demonstrate the application of the fragility methodology to probabilistically study loss of a roof system in a residential wood-frame structure under tornado loads.
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