الفهرس 
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Abstract
Downburst wind loads pose great threats to many structural systems, especially large and extended systems such as transmission lines. Investigations of failures of transmission towers around the world have recorded that more than 80% are due to high intensity winds and the occurrence of downbursts and tornadoes. Downburst wind events generate extreme winds and recent reports of failures of transmission line systems in several regions around the world point to downburst events rather than other types of thunderstorms as the primary cause of failure.
The formation and propagation of downburst winds are different from those of boundary layer winds. The downburst wind speeds are more damaging than the boundary layer winds due to their higher intensity at low heights and the unbalanced horizontal distribution, as well as their upward wind component. In addition, downburst wind speeds and the associated loads vary with the different parameters associated with each downburst event.
There are many experimental and numerical models for simulating these types of loads. The investigation of structural systems under downburst wind loads is so complex that analytical and empirical models for simulating these types of loads are required to facilitate the analysis and design process. Since these models are too complex for routine design applications it is also necessary to develop simple charts for use in international standards and codes. There are disparities between the present analytical models and recorded field data, experimental and numerical simulation. Some of these can be explained by the fact that the effects of nonlinear growth of the boundary layer thickness are rarely included in these models, and that most of the available models are restricted to modelling the downburst event as a steady state case and do not consider the temporal changes in wind speeds. Downburst wind speeds continuously change with time through their life cycle and the importance of the continuous profile of the event arises during the investigation of large extended structural systems such as transmission line systems, where these structures can experience most of the downburst life period.
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In addition, the accumulated temporal profiles of downburst wind speeds are needed for dynamic analysis.
In the current study, an analytical model has been developed to simulate the steady state profile of downburst wind speeds including the nonlinear growth of the boundary layer. Following this, a parametric study was developed that was coupled with numerical CFD simulations to investigate several observed downbursts events. The different parameters of the downburst events such as the total age, intensity period, decay period, downburst diameter, initial location, path direction and parent storm translation speed have been estimated. Then, the estimated ages for downburst events have been combined with numerical simulation to establish a pair of intensity decay functions. This pair of functions has been added to the previous analytical steady state model to update it from steady state simulation to unsteady state simulation. Once the parent storm speed has been determined and added to the model, a full scale transient downburst wind speed model in the 4-dimensions has been developed. Then, several field cases are applied to show the application and accuracy of the presented model.
Finally, a case study has been undertaken of a 275 kV transmission line system that failed due to a downburst event on 20th November 2012 near Georgetown, South Australia. The developed parametric study was applied for estimating the different event parameters, the developed unsteady state model was applied for constructing the downburst wind speed in four dimensions on an extended failed transmission line system. The resultant wind speed profiles have been compared with the corresponding values in the current Australian standard (AS/NZS 7000:2010) and recommendations have been presented for modifications accordingly.