Laboratory study of parameters influencing tornado flow field and tornado loading on low-rise buildings

Abstract

<p>Tornado is one of the deadliest natural hazards in the U.S. with a 10-year average fatality exceeding 100. The annual property-insured loss of more than $1 billion makes it even more problematic. To mitigate and address the loss caused by this natural hazard, it is important to (a) understand the tornado flow field and parameters that affect it, as well as (b) assess the tornado-induced wind loads on low-rise buildings and their components and the parameters that affect the peak wind loads for the purpose of improving their design. The above reasoning drives the two primary objectives of this research presented in this doctoral dissertation. The first objective is to study the parameters that influence the tornado flow field, and the second objective is to assess the peak tornado-induced wind loads for a building, considering its location and orientation with respect to the tornado’s mean path, as well as the maximum local loads on its components.</p> <p>To satisfy the first objective, the influence of tornado’s translation speed and swirl ratio, and the effects of ground roughness or terrain and topographic features on the tornado flow field were studied. For this purpose, a unique translating tornado simulator at Iowa State University was used. The terrain and topographic features of the ground on the tornado flow field were investigated by comparing the laboratory-simulated flow over a set of scaled ground roughness elements that represented a rough terrain, as well as scaled models of three types of topographies -2D-ridge, 2D-valley, and 2D-escarpment to the flow over a smooth flat ground surface. Topographies were set to have a constant geometric aspect ratio (height to along-wind length) following the latest edition of the ASCE7 standard for building design. Results show that a tornado passing over a smooth terrain out of the two terrains (smooth and rough) investigated, a tornado with a lower translation speed out of the two translations speeds used and a tornado passing over a 2D-ridge, amongst all the three topographies studied, produce a higher maximum wind speed which will result in a larger peak load on civil infra-structure. Investigation of swirl ratio shows that there is a larger surface area on the ground of minimum static pressure drop within a tornado core in a tornado with a higher swirl ratio and a more severe pressure drop at the tornado center in a tornado with a low swirl ratio which results in different wind damage potential in the two types of tornado.</p> <p>The second objective of this dissertation was to assess the tornado-induced wind loads, both overall and local, on low-rise buildings. Investigation of tornado-induced wind loads on a low-rise gable-roof building shows that the location of the building that results in the highest tornado-induced loads is about one core radius from the tornado mean path on the right side of the tornado’s translation direction which was identified as a region where the wind speeds intensify as a result of tornado’s translation compared to a stationary tornado. Orientation of a building with respect to the tornado mean path that produces the worst overall loads on a low-rise gable-roof building was also investigated. Finally, with regards to occurrence of the local peak tornado loads on low-rise building frames, the location of the most vulnerable building frames was identified. For design of buildings for the purpose of withstanding tornadoes, these frames should be strengthened.</p>