Determination of rational tornado design wind speed for residential woodframe structures
Tornadoes are an extremely devastating natural hazard with significant consequences but are not considered in current structural design codes. This is due to their low probability of occurrence making them what has been termed a low-occurrence high-consequence event. This thesis aims to determine a rational design wind speed for tornadoes considering the current state-of-practice construction methods and is achieved through application of the fragility methodology. Five arche-type buildings are developed to be representative of the typical residential structures in tornado-prone regions in the United States. Wind analysis is performed using the ASCE 7-10 standard loading in combination with the existing literature which provides loading for a tornado on structures. Fragilities are developed for the main components along the vertical load path that included roof sheathing, roof-to-wall connection, and wall-to-foundation anchorage. The results obtained from the fragility analysis are validated with the EF scale ratings for residential construction and are then used to identify critical components that affect system performance. Based on the developed fragilities, (2)16d roof-to-wall connection and wall racking present the most critical components. Developing fragilities for all components along the vertical load path is essential in identifying critical components and deciding on improving a particular component. The effect of component improvement on system performance can range from minimal to significant depending on its fragility in comparison to the other components. Based on the analysis conducted in this thesis, it was found that using hurricane clips and improving the nail pattern will yield better system performance in an EF1 (86-110 mph) tornado. However, typical construction methods and products would not prove to be as effective in the case of an EF2 (111-135 mph) tornado with a high probability of failure of one or more components of the load path. The methodology presented in this thesis can be used in developing new design provisions that will essentially help in saving lives and preventing economic losses, assessment of the current building stock, post-disaster damage assessment, and provide a quantitative approach to more accurate evaluation of tornado ratings.