The Behaviour of Beam-Column Elements with Variable I Cross-Sections considering Lateral Restraints

Abstract

Steel structural elements with variable cross section, made of welded plates, are largely used in the construction industry for both beams and columns in accordance with the stress and stiffness demand in the structure. These types of elements are mainly used for the design of singles storey frames with pitched roof rafters and pinned column base. Rafters and columns can be designed as tapered members made of steel welded plates, respecting the bending moment diagrams for gravitational load combination. Although they are partially restrained against out of plane buckling by side-rails and roof purlins cooperating with sheeting, in current design practice it is quite difficult to evaluate their influence. This paper deals with nonlinear elastic-plastic analysis and experimental tests performed on tapered beam-columns, parts of portal frames, subjected to both bending moment and compressive axial force. Failure of such members may occur by in-plane bending, local buckling and combination between these two, for members with low non-dimensional slenderness. While for members with intermediate and high non-dimensional slenderness, flexural-torsional buckling, local buckling and combination of these two may occur in function of the web slenderness. Numerical results will be compared with experimental tests, obtained for a relevant number of specimens, in order to check the reliability of the numerically determined ultimate capacity of the elements. The influence of both lateral and torsional restraints applied on the beam-column will also be discussed.