Collaborative Research: Characterization, Modeling and Uncertainty Analysis of Tornado Wind and Its Effects on Buildings

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The devastation from recent tornadoes in Joplin, Missouri, and Tuscaloosa, Alabama, in 2011 and in Moore, Oklahoma, in 2013 highlight the national vulnerability to these windstorm events. Direct measurement of tornado wind speed near the ground level is difficult to obtain due to its unpredictable nature and destructive force. Current practice is to estimate wind speed based on observed damage to structures and non-structures using the Enhanced Fujita (EF) Scale, which is widely accepted in climatological study, risk analysis, and design of critical facilities. However, such damage-based methods have a great degree of uncertainty. Critical knowledge gaps exist about spatial and temporal distributions of wind flow near the ground level and how wind flow interacts with the terrain and structures. To address these knowledge gaps, this research will characterize, model, and analyze uncertainties in tornado wind and its effects on buildings. This research will lead to better understanding of the effects of tornado and terrain parameters on near-ground wind field structures, the transient aerodynamic force of tornado wind on building designs, and the uncertainties in building performance subject to tornado wind. This knowledge will contribute toward the foundation for developing performance-based building code provisions to mitigate the impact of tornado wind loads on buildings.This research aims to make the following three knowledge advances. First, knowledge for understanding the tornado wind field will be advanced through a systematic study of the effects of tornado and terrain parameters. This study will fill an important gap between a tornado's structure aloft and ground level damages and will provide the physics-based evidence critically needed for updating the EF Scale. Fragility functions will be developed to recalibrate the expected, upper bound, and lower bound wind speeds for Degree of Damage in the EF Scale. Second, understanding of pressure and load effects of non-synoptic winds, including tornadoes and thunderstorms, will be advanced with the development of transient aerodynamic force models. These models will not only enable better characterization of load effects under a non-stationary vortex but also will build a bridge to results accumulated from decades of research in stationary boundary layer wind. Third, a new framework for characterizing and quantifying uncertainties of the tornado wind load chain on buildings will be developed and validated with finite element models and post-storm damage surveys. This framework will permit the integration of uncertainties, including those of building properties and construction quality, in assessing building vulnerability, laying the foundation for performance-based building code provisions for tornadoes. This research is enabled by a confluence of latest advances in tornado simulation, data acquisition and modeling capabilities, full-scale studies of the tornado vortex, near-ground measurements of tornado wind, and theories in non-stationarity, many of which were not available a few years ago.