Tornado Hazard Analysis and Effect of Structure Size

Abstract

The United States of America experiences more than 1000 tornadoes every year. Different from other large scale natural hazards such as earthquake and hurricane, the impact of a tornado is relatively small. The effect of structure size on tornado risk assessment is very important. Neglecting the structure size may lead to underestimation of tornado strike probability. This study presents the development of size-dependent tornado design maps for the United States. Using a stochastic tornado simulation model and a wind field model, tornado hazard maps for EF0 to EF5 wind speeds are developed for four different target structure sizes, namely point target, small (0.08 mi2), medium (0.03 mi2) and large (0.5 mi2) circular targets. A model to quantify the relationship between tornado strike probability and target size is proposed. Using this relationship, the tornado hazard for a given location and structure size can be interpolated from the four size dependent hazard maps. This quantitative tornado hazard estimation method considering size effect can be used by engineers to determine the design wind speed.