An assessment of the inherent reliability of SANS 10162-2 for cold-formed steel columns using the direct strength method
Thesis (MEng)--Stellenbosch University, 2018.
ENGLISH ABSTRACT: With their thin and slender nature, Cold formed Steel (CFS) elements can be forged into a large variety of cross sections. It is recommended that the Direct Strength Method (DSM), a modern design method, replaces the effective width method for the design of CFS members. Research by Bauer (2016) revealed that, using the DSM, there is an insufficient level of safety provided by SANS 10162-2: design guide for Cold-Formed Steel Structures to achieve the target reliability prescribed by SANS 10160-1: the basis of structural design. This is partly due to a relatively high model uncertainty of the structural resistance. The main objective of this study is to determine whether the level of safety provided by SANS 10160-1 is enough to ensure an overall acceptable level of reliability for CFS members. A prequalified plain lipped C-section is considered for the investigation of the local and global buckling failure modes. A prequalified lipped C-section with a web stiffener is considered for the investigation of the distortional and global buckling failure modes. Various member lengths are considered to ensure that all buckling modes are induced. The Finite Strip Method (FSM) is performed to identify the three buckling modes. The members are subject to four load cases. Of which, each had different combinations of permanent, imposed and wind loads determined by partial factors and combination factors presented in SANS 10160-1. For the purposes of this study, only the ultimate limitstate was considered. The considered limit-state is analysed in two parts; the semi- and full probabilistic formulations of the limit-state. The former considers the structural resistance and the load effect according to codified design. The latter considers the structural resistance and the load effect as functions of random variables. The reliability analysis is conducted on the full probabilistic formulation of the limit-state to assess the reliability achieved by using the codified semi-probabilistic formulation of the limit-state. Model factors are considered for the structural resistance and the load effect. The structural resistance model factor is dependent on the buckling mode (Ganesan and Moen, 2010). A model factor for the permanent load is not considered and the model factor for wind load had been accounted for in the respective statistical moment parameters. A model factor for the imposed load is described by Holický (2009). The statistical moment parameters of the permanent and imposed loads are described by Holický (2009) and the statistical moment parameters of the wind load are described by Botha (2016). The global buckling mode yields the lowest reliability levels, ranging from β = 1.78 corresponding to an STR-P load combination to β = 2.87 corresponding to an STR load combination. The safety margin present in SANS 10160-1 partially compensates for the low level of reliability when the total load comprises of high proportions of imposed and wind load. The low levels of reliability, especially when there is significant permanent loading, are cause for concern. It is recommended that a different capacity reduction factor be applied to each dominating buckling mode for CFS compression members.
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