Inelastic reserve strength of CFS sections subjected to unsymmetrical centroidal axis bending

Abstract

The inelastic reserve strength of cold formed steel (CFS) sections in AISI-S100 & AS/NZ 460 standards are expressed in direct strength method (DSM) for local and distortional buckling modes, which are derived from the studies on lipped channel and lipped Zee sections subjected to bending about the symmetric(major) centroidal axis. This study aims to check the adequacy of existing provisions to LC and Hat sections subjected to bending about un-symmetric(minor) centroidal axis such that the lipped edge is under compression. The Non-linear finite element analysis was carried out with the variation of width/thickness ratios of flange and lip depths between zero and adequate depth for three different grades of steels. Further, the strength was estimated analytically by imposing the complete stress-strain profile including strain hardening component across the section. Though the above sections exhibit distortional mode from the analysis, the inelastic reserve strength estimations from existing DSM distortional equations were shown to be conservative compared to numerical and analytical results. To address the conservative strength estimations, the present work proposes modifications in two stages 1) A new equation for limiting strain ratio ‘C' as a function of non-dimensioned slenderness ratio ‘λ’ and applied strain/stress gradient ‘ψ’. 2) An additional strain hardening component derived to be used with existing DSM provisions. Further, the strength was also estimated through continuous strength method (CSM) by deriving a generalized shape factor ‘a_g’ in the form of design tables and charts. The proposed DSM and CSM strength equations exhibit a good correlation with the past experimental data and yields more reliable strength predictions.