An assessment of the inherent reliability of SANS 10162-2 for cold-formed steel columns using the direct strength method

[摘要] 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 ofCFS members.Research by Bauer (2016) revealed that, using the DSM, there is an insufficient level ofsafety provided by SANS 10162-2: design guide for Cold-Formed Steel Structures toachieve 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. Themain objective of this study is to determine whether the level of safety provided by SANS10160-1 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 andglobal buckling failure modes. A prequalified lipped C-section with a web stiffener isconsidered 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 ofpermanent, imposed and wind loads determined by partial factors and combinationfactors presented in SANS 10160-1. For the purposes of this study, only the ultimate limitstatewas considered.The considered limit-state is analysed in two parts; the semi- and full probabilisticformulations of the limit-state. The former considers the structural resistance and theload effect according to codified design. The latter considers the structural resistance andthe load effect as functions of random variables. The reliability analysis is conducted onthe full probabilistic formulation of the limit-state to assess the reliability achieved byusing the codified semi-probabilistic formulation of the limit-state.Model factors are considered for the structural resistance and the load effect. Thestructural 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 byHolický (2009) and the statistical moment parameters of the wind load are described byBotha (2016).The global buckling mode yields the lowest reliability levels, ranging from β = 1.78corresponding to an STR-P load combination to β = 2.87 corresponding to an STR loadcombination. The safety margin present in SANS 10160-1 partially compensates for thelow level of reliability when the total load comprises of high proportions of imposed andwind load. The low levels of reliability, especially when there is significant permanentloading, are cause for concern. It is recommended that a different capacity reduction factorbe applied to each dominating buckling mode for CFS compression members.

[发布机构] Stellenbosch University