Observations on the design and modelling of some joined thin-walled structural sections

In a recent experimental and analytical study for the axial progressive collapse of some thin-walled structural sections, no consistent reliability could be obtained for the prediction of crush parameters for different geometries and materials with an analytical approach that relied on a yield hinge model. Square and spot-welded top-hat sections were compared. This situation occurs because the assumed equivalence between the top-hat and square sections according to the yield hinge model of the folding structures, does not appear valid for the actual structures. Some of the dependences between the actual structure and the model are discussed, particularly for the top-hat geometry. Observations on a beneficial positioning of the spot-welds in a top-hat section are reported, and common recommendations for an optimum weld pitch are critically examined. Modes 1 and 2 deformations are identified for the well-known quasi-inextensional, regular (asymmetric) progressive folding of thin-walled sections having a rectangular (core) cross-section, such as square and top-hat sections. The deformation modes are characterized by different proportions of inward and outward movements of the material during folding. Possible implications for the stability of collapse and for the analytical formulae are indicated.