Engineering Advances for Damage Mitigation and Risk Reduction Relating to Tornadoes and Severe Convective Windstorms

Extreme weather phenomena such as hurricanes, tornadoes, downbursts, and gust fronts produce 36% of combined insured losses from all natural hazards annually in the United States. This presentation will primarily focus on the engineering advances and current state of understanding of straight-line, tornado and microburst winds near the ground surface and their wind loading effects on civil infrastructure, and steps that need to be taken for reduction of risk from wind hazards. Currently, civil structures are designed to resist only straight-line winds associated with a neutrally stable atmospheric boundary layer whereas tornadoes and downbursts produce winds that have distinctly different characteristics from the design wind specified in building codes. Tornadoes are vortices with significant tangential and vertical velocity components in the core region. Microbursts are intense downbursts characterized by a strong localized down-flow and an outburst of strong winds near the surface. Thus, the velocity fields in both of these wind phenomena are three dimensional and the structural loading effects they produce are transient in nature. The maximum wind speeds in a moderately intense tornado or a microburst reach magnitudes that can easily exceed the design wind speeds of 85-90 mph (3-sec gust at 10 ft height) specified for thunderstorm-prone regions outside the hurricane-zone in the United States. These phenomena of even moderate intensity can produce up to three times the design wind loads for structures located in these regions. Physical and numerical simulations of straight-line winds and structural modeling are routinely used to understand and predict wind damage of civil structures. In the past decade, these simulation techniques have been developed and applied to modeling of tornado and microburst winds. The speaker has developed unique wind tunnels that can physically simulate winds similar to those in tornadoes, microbursts and gust-fronts to study their wind loading effects on civil structures. While these laboratory simulations in a wind tunnel reproduced the most important features of these wind phenomena, it was found that computational (CFD) simulations are required to overcome their limitations. Examples of flow fields and wind loads on civil structures such as low-rise and high-rise buildings, and other types of structures in a tornado and a microburst and how these compare to those in a straight-line wind event are presented. Wind loads on a structure in a tornado were found to be influenced by the terrain surrounding the structure, geometry and shape of the structure, interference from surrounding structures, openings in the building, and flow characteristics of the tornado. It is shown how a combination-approach of numerical and physical simulations, such as Finite Element Modeling of a building and laboratory simulations of a tornado's interaction with the building, was used in a sequential manner to understand the interaction of a low-rise residential building with an EF5 tornado to predict its structural damage observed during a post-storm damage survey. This technique has not only helped to calibrate the surface winds resulting from a tornado and the damage scale associated with it like the EF-scale but also helped to understand the load paths in a building and its components that will eventually result in improvements in its wind-resistance capacity in the future. The importance of wind-borne debris in causing structural damage during a storm is well known. Engineering models exist to predict the path of simplified wind-borne debris such that its impact on the integrity of the building envelope can be predicted. A technique to predict the trajectory of geometrically simplified debris in a tornado is presented. Finally, a few examples of mitigation measures that can be taken to reduce the risk to building damage from severe wind storms and future engineering advances that are needed to improve wind resilience are presented.