Pitched roof portal frames are often made of welded plate elements with tapered web classified as class 3 (semi-compact) and 4 (elastic) according to EN 1993-1-1. Moreover the class of flange, with constant variation along the element, can range from class 2 (compact) to class 3(semi-compact), function of their width to thickness ratio. Under gravitational load, that is the dominant case in design practice, these elements are mainly subjected to combined action of bending moment and axial compression force. Hence the tapering of the elements is made in accordance with internal force distribution under gravitational loads. Due to their cross section class type, the failure in buckling can be of different types. When class 2 and 3 sections, are restrained against lateral and/or torsional buckling, the coupling between sectional plastic buckling and overall in plane buckling of the members in compression and/or in bending is possible. When class 4 (slender) sections are used, which generally is the case of the rafter at the maximum height of the tapered web, the sectional buckling (e.g. local buckling of walls or distortion) may occur in the elastic domain. If no lateral restrains, or when they are not effective enough, the lateral torsional mode could characterize the global behaviour of the frame members, or interaction with sectional buckling modes may occur as well. The paper presents the results obtained experimentally, along with FEM analyses and a parametric study results. The failure mechanism, and hence the ultimate capacity is mainly influenced by the cross sectional and/or overall slenderness as well as the applied lateral restraints.
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