Experimental investigation of built-up double-Z members in bending and compression

Using the conventional cold-formed shapes in built-up assemblies (composed members, trusses, etc.) has allowed designers of light-steel framing to widen the applications of cold-formed steel (CFS) to structures of larger scale. Built-up elements, fastened by bolts, screws or welds are usually symmetrical, more stable and of higher capacity. However, they are not addressed in the various design codes for CFS structures. Additional flexibility is caused by slippage between the constituents of a composed member, especially if bolts are used as interconnectors and hole clearances are provided to ensure the ease of assembly. The paper presents experimental studies on the behaviour of double-Z built-up members (bolted). First, twelve members are tested in weak-axis bending, to evaluate the stiffness of the member for the axis of symmetry. Second, the overall behaviour is analysed in eight compression tests on slender members. The goal is to determine the actual failure mode and overall buckling capacity of these members.

► The study focuses on built-up members from cold-formed steel profiles.
► The double-Z built-up members is a relatively new solution for portal frames.
► The results of two measurement campaigns on full-scale members are presented.
► The quality of the flexible interconnection between the Z-profiles is evaluated.
► Strain gauges and an optical measurement device are used for the tests.