Strength and Stiffness Characteristics of Built-up Members of Cold-Formed steel profiles – experiments and numerical investigation

New, optimised section shapes are being developed and implemented by industry for portal frames of cold-formed steel profiles. These should meet the perpetual need for larger column heights and spans. For such structures, the most practical solution for longer column/girder members under larger loads remains the double section. Fabricated from two standard Z, C or Σ shapes, it has bolted interconnections discretely along the length. The actual strength and stiffness characteristics of such sections are not predicted by design codes and, in practice, conservative assumptions are made. The European standard EN 1993-1-3 does not suggest how profile interaction could be implemented in design. A built-up member, made from two non-symmetrical Z-sections, was tested so that the actual strength and stiffness characteristics can be obtained. 12 tests were performed in weak axis bending. The spacing between interconnections was varied, so that its effect can also be studied. Both standard displacement transducers and a new generation optical measuring device were used to measure displacements. The results showed that ignoring the interaction between the section parts significantly reduces the calculated member resistance. A considerable gain in strength and stiffness was observed compared to the calculations. This implies that the assumed stress distributions are not correct if the interaction is ignored in design. Non-linear FEA was carried out using ANSYS to model the behaviour of the members. Results showed good agreement with the experiments for stiffness, failure modes and ultimate loads. An upcoming test campaign on members in compression will test the conclusions of the research made so far.status: publishe