Browsing by Author "Van Nierop, Stacey C."
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- ItemTarget reliability of concrete structures governed by serviceability limit state design(Stellenbosch : Stellenbosch University, 2018-03) Van Nierop, Stacey C.; Viljoen, Celeste; Lenner, Roman; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Structures are typically designed for the ultimate limit state (ULS) while the serviceability limit state (SLS) is checked. However, in many cases the design is governed by SLS requirements. The target reliability for the ULS ßt,ULS is recommended based on cost optimisation and back calibration of existing practice. The current recommendation in ISO2394 is ßt,SLS = 1:5 for the irreversible SLS, although, it is unclear how this value was obtained. The aim of this research is to determine reasonable ßt,SLS values for all serviceability requirements with a focus on concrete structures governed by SLS design. The approach taken, by this thesis, is based on reliability-based economic optimisation in a generalised context, taking into account a range of failure consequences and cost classes. A generalised framework is established to perform a reliability analysis. The limit state function g for the SLS is generalised to g = 1 -πE. This equation is based on the assumption that the design limit is deterministic and the action-resistance effect E is the only random variable. From the generic limit state equation, a generalised decision parameter d is determined, and the reliability level ß(d) is determined through a FORM analysis. Through economic optimisation of the generalised ß(d) values, a parametric table of ßt,SLS values is obtained. The table accounts for a range of cost ratios C1=Cf = [0:5 - 100] (relating the failure costs Cf to the costs per unit of the decision parameter C1) and coefficient of variation of the action-resistance effect VE = [0:05 - 0:50]. ßt,SLS is observed to systematically increase as the cost ratio increases and VE decreases. The current recommendation of 1.5 seems reasonable from a practical perspective for relatively low cost ratios. Two examples of the application of ßt,SLS, from the generic development,are provided. ßt,SLS of a water retaining structure, governed by cracking, and ßt,SLS of a simply supported beam, for de ections, are investigated. E is the product of either the mean-maximum crack width wm;max or mean-maximum deflections δm,max and the model factor Ø VE is then determined through Monte Carlo analysis and Cf/C1 is quantified for the two structures. Once VE and Cf-C1 are known, ßt,SLS is obtained from the generic development. Through economic optimisation of the specific ß(d) values, ßt,SLS is obtained for the same costs. These target values are compared to ßt;SLS from the generic development. The discrepancy identified in the ßt,SLS values for the examples is due to the inefficiency of the generic decision parameter to increase the reliability level of the structure compared to the efficiency of the specific decision parameters.