BUCKLING BEHAVIOUR OF POLYURETHANE FOAM-FILLED COLD-FORMED STEEL C-SECTIONS

Thin-walled cold-formed steel (CFS) structural members tend towards failing before reaching their yield strength due to local and distortional instabilities. Global buckling such as Euler or lateral torsional buckling can also occur, however, global buckling is not a specific failure for CFS members. Local buckling means plate buckling of a slender part of the cross-section. Distortional buckling occurs in open cross-sections such as C-sections, at which the flanges under compression buckle inward and outward. In order to improve the local buckling behaviour CFS members are so far provided with stiffeners, seams or corrugations. Though, these provisions are material consuming, complicate the production process and have only a small impact on distortional buckling prevention. Filling the cross-section with polyurethane foam has a positive effect on the local and distortional buckling behaviour. The polyurethane liquid components are filled into the cross-section and expand inside the profile. The foam supports the cross-section parts normal to their plane and keeps the cross-section in shape. Finite element (FE) elastic buckling analyses have shown, that the critical elastic local and distortional buckling loads are two to three times higher for foam filled section than for regular sections. This leads to an increase of the members load capacity of up to 50 %, depending on the slenderness. The improvement of the foam is also shown in tests on foam-filled C-section columns. Depending on the cross-section shape, the foam can lead to much higher load capacities of CFS members and hence to a more economical design.