Effect of thickness on the behaviour of axially loaded back-to-back coldformed steel built-up channel sections - Experimental and numerical investigation

Abstract

In cold-formed steel structures, such as trusses, wall frames and portal frames, the use of back-to-back built-up cold-formed steel channel sections are becoming increasingly popular. In such an arrangement, intermediate fasteners are required at discrete points along the length, preventing the channel-sections from buckling independently. Current guidance in the AISI &AS/NZS for such back-to-back built-up cold-formed steel channel sections requires the use of modified slenderness in order to take into account the spacing of the fasteners. Limited research has been done on back-to-back built-up cold-formed steel columns to understand the effect of column thickness and slenderness's on axial capacity. This issue is addressed herein. This paper presents the results of 60 experimental tests performed on back-to-back built-up cold-formed steel channel sections under compression. Detailed observations on different failure modes and column strengths were made through varying thickness, length and cross section of columns. A non-linear finite element model was developed which includes material non-linearity, geometric imperfections and explicit modelling of web fasteners. The finite element model was validated against experimental results. A comprehensive parametric study consisting of 204 models has been carried out covering a wide range of thickness and slenderness for the considered back-to-back built-up columns. Axial capacities obtained from the numerical study were used to assess the performance of the current AISI& AS/NZS standards when applied to cold-formed back-to-back built up columns; obtained comparisons showed that AISI& AS/NZS standards are un-conservative for stub and short columns which were failed by local buckling whereas standards were over-conservative for the strength of intermediate and slender columns which were failed mainly by overall member buckling. This paper has therefore proposed improved design rules and verified their accuracy using finite element analysis and test results of back-to-back built-up cold-formed channel sections, subjected to axial compression.