Keywords: cold-formed steel columns, lipped channel columns, buckling mode interaction, local-distortional-global interaction, experimental investigation.

This paper deals with the structural behaviour and ultimate strength of fixed-ended cold-formed steel (CFS) lipped channel columns experiencing various levels of local-distortional-global buckling mode interaction. After briefly addressing the column specimen geometry selection (carefully chosen to ensure more or less closeness between the critical buckling stresses associated with these three buckling mode types), the paper reports the results of an experimental investigation involving a set of seventeen columns with several cross-section dimensions, lengths and yield stresses, which is aimed at (i) providing experimental evidence of the occurrence of the triple mode interaction under consideration and (ii) quantifying its effect on the column deformed configuration evolution and failure load erosion (drop). Various aspects concerning the test set-up and procedure are described in some detail before presenting and discussing the column experimental results obtained, which basically consist of (i) measured cross-section dimensions, lengths and initial displacements (geometrical imperfections), (ii) stress-strain curves and yield stresses determined from tensile coupon tests, (iii) recorded non-linear equilibrium paths (applied load vs. various relevant displacements) and ultimate strength values, and (iv) observed deformed configurations and collapse mechanisms. The test results are used, in a companion paper (Part II), (i) to calibrate and validate numerical simulations and (ii) to support the proposal and to assess the merits of design approaches based on the direct strength method and intended to handle L-D-G interactive failures.

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