Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Author "Botha, Jacques"

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    Probabilistic Models of Design Wind Loads in South Africa
    (Stellenbosch : Stellenbosch University, 2016-10) Botha, Jacques; Viljoen, Celeste; Retief, Johan Verster; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: The formulation of design wind loads on structures is subject to multiple sources of uncertainties,both inherent in the physical process itself and due to the engineering models used in the formulation. Accurate reliability modelling of this process is required to ensure that the desired reliability performance of structures is achieved when using wind loading standards. This dissertation presents an investigation of wind load uncertainties and the development of a probabilistic wind load model based on a rational and transparent reliability basis. A background investigation of the wind load formulation and relevant wind engineering models was performed in order to identify sources of uncertainties. Through this process the primary wind load components relevant to the South African wind load formulation were identified. These components are the time variant free-field wind pressure, and the time invariant pressure coefficients and terrain roughness factors. The traditional approach to probabilistic modelling of wind loads was investigated. Relevant existing probabilistic models were investigated and compared. The lack of substantiating background information relating to the existing models calls into question the trustworthiness of results obtained using those models. This serves as the primary motivation for the research presented in this dissertation. The reliability basis of the investigation was therefore developed in a rational and transparent manner. To this end the traditional reliability approach was extended through the use of hierarchical Bayesian models to quantify wind load uncertainties on the component level. The three primary wind load components were investigated independently. This investigation follows the investigation by Kruger (2011) of the South African strong wind climatology and the development of a new characteristic wind speed map. The results from that investigation were used to quantify the free-field wind pressure uncertainties due to the inherent variability of the strong wind climate and the conservative bias introduced through the use of a wind speed map. Lacking sufficient sources of information to quantify the time invariant component uncertainties, a rational reliability method for the use of comparison of wind loads standards as an indicator of wind load uncertainties was developed. This methodology was used in conjunction with direct sampling of limited available observed data to quantify the uncertainties inherent in pressure coefficients and terrain roughness factors. The new component models were compared to the corresponding distributions in the existing models. In all cases the new models resulted in higher reliability requirements than the existing models. Full probabilistic wind load models which include secondary factors were then developed using engineering judgement and Bayesian updating of the existing models. Multiple reliability assessments were performed using the new models to investigate the reliability performance of the South African wind load formulation. The results conclusively show that the current wind load partial factor of 1.3 does not provide adequate reliability performance. Finally, the effect on the average design wind load across the country due to the combined implementation of the new characteristic wind speed map and a potentially updated wind load partial factor was quantified.