Master of Engineering Science
Civil and Environmental Engineering
Bitsuamlak, Girma T.
Building codes and standards have begun to incorporate tornado-specific loads to help protect community infrastructure against these potentially devastating storms, with a specific focus on low-rise buildings. The current tornado design loads are derived from atmospheric boundary layer (ABL) winds, even though it is relatively unclear to what extent tornado and ABL wind loads are equivalent for a given intensity. This research focuses on the tornado-resilient design of the main wind force resisting system (MWFRS) and component and cladding (C&C) elements of a low-rise gable roof building. Tornado loads derived from the results of experiments completed in a vortex simulator are compared to those obtained from both ABL wind tunnel tests and the proposed tornado-specific design provisions in the upcoming ASCE 7-22 standard. The external pressure datasets from the tornado and ABL wind tests are supplemented by a numerical model for internal pressure to represent enclosed and partially enclosed opening conditions. The analysis demonstrates the impact of the atmospheric pressure drop on enhancing the external tornado-induced MWFRS and C&C loads while showing that the tornado and ABL-induced loads are comparable once internal pressures are considered. Local variations attributed to increased tornado suction loads in high flow separation regions are also highlighted for the MWFRS and C&C. For the building geometry and simulated tornado studied, the peak normalized uplift forces, bending moments, and C&C pressures are found to be effectively enveloped by the ASCE 7-22 tornado design loads for the enclosed building configuration but generally exceed the ASCE loads for the perfectly sealed and partially enclosed cases. The lateral load coefficients computed from ASCE 7-22 are also found to be slightly unconservative irrespective of the opening condition. Further refinements in the internal and external pressure design values as well as the C&C loading zones may be required to better represent the tornado load conditions. Key uncertainties and limitations of the tornado data used in this study are outlined and discussed.
Summary for Lay Audience
Tornadoes are severe wind events that can result in enormous economic loss to communities and a significant loss of life. A large portion of tornado-induced damage is associated with low-rise buildings, which includes both residential and commercial structures. In response to this, building codes and standards in North America are beginning to incorporate clauses pertaining to tornado wind loads on buildings, which have been derived by applying adjustment factors to the design wind loads from typical storms. Since the characteristics of tornado winds differ significantly from normal straight-line winds, it is critical to understand how these variations carry through to the resulting tornado-induced loads on buildings, which can help to better inform the new tornado design provisions being included in engineering standards.
In this study, a scaled-down simulated tornado was produced at the WindEEE Dome facility at Western University. The tornado was translated past a low-rise building model, where pressure measurements were obtained on its external surface. These measurements were further supplemented by a numerical model used to estimate the internal pressures caused by the natural leakage or significant openings present in the building envelope. From the external and internal pressure data, net loads caused by the simulated tornado were derived for both the main structural system of the building (such as the primary frames) as well as for smaller component and cladding elements (such as roof and wall panels).
The experimental tornado-induced loads acting on the building were first compared to those calculated from straight-line wind tunnel data, showing that the net loads acting on the primary frames and building components behave quite similarly between the two flow regimes with some local variations. The same set of tornado loads were then compared to the proposed tornado design loads in the ASCE 7-22 standard from the United States, which revealed that the standard appears to perform well but can be slightly unconservative depending on the opening configuration of the building. The study showed that tornado wind loads are becoming more effectively understood by the engineering community, while highlighting some of the challenges that need to be addressed in future work.
Van Der Kooi, Cody J., "Considerations for the Tornado-Resilient Structural Design of Low-Rise Buildings" (2021). Electronic Thesis and Dissertation Repository. 8068.
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