Quantitative Insight into Rational Tornado Design Wind Speeds for Residential Wood-Frame Structures Using Fragility Approach

Tornadoes are an extremely devastating natural hazard that have significant consequences but are currently not considered in the structural design codes due to their low probability of occurrence. These low-occurrence high-consequence events have been studied but not to the extent of other natural hazards. This paper presents the results of a study whose objective was to provide quantitative insight into a rational design wind speed for tornadoes considering the current state-of-practice in construction methods. Five archetype buildings were selected to be representative of the typical residential structure design space in tornado-prone regions of the United States. Wind analysis was performed using the ASCE standard loading in combination with modification factors for tornadic winds developed from the available literature. Fragilities were developed for the main components along the vertical load path that included roof sheathing and roof-to-wall connection. Then, system fragilities for the combined load path of these two potential failure modes were formulated as a series system. The results of the fragility analysis were compared with the Enhanced Fujita (EF) scale ratings for residential construction and were then used to identify which critical component had a greater effect on system performance. Finally, it was concluded that, if proper foundation anchorage is provided, it is possible to design a residential wood-frame structure for winds from an EF1 tornado, possibly an EF2, but not an EF3, using conventional construction methods including hurricane clips.