Experimental Investigation on Cold-Formed Steel Stiffened Lipped Channel Columns Undergoing Local-Distortional Interaction

Although a significant number of experimental results concerning the behavior and strength of CFS columns affected by local-distortional (L-D) interaction are available in the literature, most of them dealing with lipped channels, the specimens providing clear experimental evidence of this coupling phenomenon and ensuing failure load erosion are relatively scarce - certainly, much less than those collected to propose, calibrate and validate the existing local, distortional, global and local-global Direct Strength Method (DSM) design curves. Naturally, reliable and illuminating experimental data are essential to establish efficient DSM-based design rules to handle column L-D interactive failures, which are still lacking in the current CFS specifications worldwide. Aiming to bridge this gap, Martins et al. (2017) recently proposed a well-founded preliminary version of the sought DSM-based design approach. It just needs to be ""fine-tuned"", a task that (i) requires carefully planned fresh experimental failure loads and (ii) should pave the way for the codification of the aforementioned DSM-based design approach - the ultimate goal of this research effort.

The objective of this work is to report a carefully experimental investigation, planned at the University of Lisbon and carried out at the University of Hong Kong, concerning the behavior and ultimate strength of CFS web-flange-stiffened (WFS) and web-stiffened (WS) lipped channel columns undergoing L-D interaction. It involves 31 specimens (16 WFS+ 15 WS), brake-pressed from high-strength zinc-coated grades G450, G500 and G550 structural steel sheets, exhibiting critical distortional-to-local buckling load ratios ranging between 0.8 and 2.0. The column geometries were carefully selected to enable the testing of fixed-ended columns undergoing true L-D interaction and secondary (local or distortional)-bifurcation L-D interaction (ensuring evidence of the latter required the selection of rather slender columns) - fortunately, all the tested specimens exhibited the expected L-D interactive failure nature. The specimen material properties were obtained from tensile coupon tests and their initial geometrical imperfections were measured prior to testing - they will be essential to perform the numerical simulations currently being planned. The experimental results presented and discussed consist of column (i) load-displacement equilibrium paths, (iii) deformed configuration evolution along those paths, including the failure mode and (iii) failure loads. Finally, the experimental failure load data obtained are compared with their estimates provided by the currently available DSM-based design approaches for columns affected by L-D interaction - the fresh light shed by this comparison will contribute to the timely codification of a DSM design procedure in the near future.

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