Date of Award

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Civil Engineering

Committee Member

Dr. Weichiang Pang, Committee Chair

Committee Member

Dr. Shuyang Cao, Co-Chair

Committee Member

Dr. Charng Hsein Juang

Committee Member

Dr. Thomas Cousins

Abstract

Tornado is one of the most severe weather events that affect the United States and other countries every year. However, the research on tornado and its effects on structures is relatively scant compared with conventional boundary-layer wind events (e.g. hurricanes). With the increasing frequency of intensive tornadoes and high profile tornado disasters (e.g. 2011 Joplin Tornado and 2013 Moore, Oklahoma Tornado), the study on tornadoes has been one of the most pressing research needs in the wind engineering community in recent years. In this research, experimental studies on characteristics of tornado wind field and tornado-structure interactions were conducted using a laboratory-scale tornado vortex simulator. The obtained experimental results were later used in the development of a new three-dimensional (3D) tornado wind field model to characterize the multiple-cell tornado vortices. The developed 3D tornado wind field model was then used for risk assessment of tornado hazards. There were four major research tasks in this study. These inter-connected research tasks are briefly discussed and summarized as following: Task 1. Experimental simulation on tornado wind field: A series of experiments were conducted in the tornado vortex simulator in Tongji University in China, by manipulating the controlling parameters of tornado vortex simulator to simulate different tornado vortices with different swirl ratios. The cobra probe was used to capture three-dimensional wind speeds and wind pressure accompanying a tornado. In addition, the roughness effects on tornado vortices were considered by introducing the roughness elements. The obtained experimental results were used in Task 3 to develop a new three-dimensional tornado wind field model. Task 2. Tornado-structure interaction: Cooling tower is a wind sensitive structure due to its tall, flexible and thin shape characteristics. The hyperbolic-curve thin-shell rigid cooling tower model was modeled in this research. By adjusting the relative radial distance between the tornado vortex simulator and cooling tower model, the stationary tornado-induced wind loadings on cooling tower were studied. Additionally, the translation effects of tornado vortices were considered by controlling the translating speeds of the tornado vortex simulator. Note that all the tests in this task were conducted without considering the roughness effects. Low-rise buildings are frequently struck by tornadoes. Experimental simulation on tornado and low-rise building model interactions was conducted in tornado vortex simulator. Internal pressure distribution was considered and measured by varying the sizes of the opening holes on the building envelope (i.e. external wall surfaces) of the building model. By adjusting the relative radial distance between the tornado vortex simulator and low-rise building model, the stationary tornado-induced wind loadings on low-rise building were studied. The obtained tornado-structure interaction results were further used in Task 4 to conduct fragility analysis of low-rise building subjected to tornado hazards. Task 3. Three-dimensional tornado wind field model: Tornado wind field model is one of the fundamental elements required for risk assessment of tornado hazard. A parametric three-dimensional tornado wind field model was developed based on the experimental data obtained from Task 1. The least square method was utilized to determine the modeling parameters. Additionally, the wind speed errors were quantified by comparing the wind speeds between experimental data and the 3D wind field model. The comparison between the new wind field model and real tornado observation was made to validate the wind field model. Many existing tornado wind field models can only characterize single-cell vortices. The new wind field model can model the flow structure of multiple-cell tornado vortices and realistically simulate the velocity shear profile. Task 4. Fragility analysis of low-rise building: Risk assessment of tornado hazards is important for establishing building code design standards and estimating losses for insurance purpose. Using the tornado wind field model developed in Task 3, experimental data of tornado-structure interaction obtained from Tasks 1 and 2, and the capacity data from other research, a fragility framework for quantifying the failure probability of roof structure was developed. The fragility curves for roof sheathing panels and roof-to-wall connections were developed. Different damage levels for roof sheathing panels and roof-to-wall connections were considered, and the results were also compared with those obtained assuming the wind pressures of straight winds based on the ASCE 7-10 provision. The fragility curves developed may be used by the structural design committee to set design standards to achieve a balance between safety and cost.

Share

COinS