Experimental study of cold-formed steel built-up columns

A comprehensive experimental programme was contrived with the aim of investigating the behaviour and the capacity of cold-formed steel built-up columns with particular emphasis on the effects of connector spacing and contact between individual components. A total of 24 built-up columns, including four different cross-sectional geometries, were tested between pin-ended boundary conditions, while applying the load with nominal eccentricities of L/1000 or L/1500. The columns were designed to fail by interaction of cross-sectional buckling of the components, possible global-type buckling of the components between connectors and global buckling of the whole column, and all these failure modes were successfully achieved. The built-up sections were fabricated from flat plates, plain channels and lipped channels and were assembled with either bolts or self-drilling screws. The connector spacing was varied between specimens of the same cross-sectional geometry. Tensile coupons were taken from the flat portions and the corners of the sections in order to determine their material properties, while detailed measurements of the geometric imperfections of each specimen were also carried out using a specially designed measuring rig. In addition, the isolated behaviour of both the bolts and the screws used in the specimens was investigated through single lap shear tests. It was observed that the buckling patterns in the built-up specimens were affected by contact between the various components and by the spacing between the connectors. However, in the cases where global buckling of the components in between connector points was not critical, the connector spacing had a minor influence on the ultimate capacity of the columns.