Keywords: welded plates element, tapered members, pitched roof frames, out of plane buckling.

Single storey buildings possesses a steel main frame usually made of welded plate elements with a tapered web classified as class 3 (elastic) and class 4 (slender) according to EN 1993-1-1. The constituent elements are mainly subjected to compression, bending moments and interaction between these two. In the case of class 3 sections, when they are restrained against lateral and, or torsional buckling, the interaction between sectional plastic buckling and overall elastic buckling of the members in compression and, or in bending is possible. When class 4 (slender) sections are used, which is generally the case for a 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.

Even though many researchers have performed investigations on the uniform elements subjected to bending moment and axial compression force [1,2,3] information regarding the out-of-plane behaviour of tapered elements is still limited. The ultimate capacity for tapered steel plate girders subjected to shear was determined by Shanmugam [4] and the one for axially loaded thin-walled tapered columns with doubly symmetric sections by Salem [5].

The work presented in this paper considers the analyses both numerical and analytical of the tapered beam-columns, the parts of the portal frame, subjected to major axis bending moment and a compression axial force. The main failure mode of such members is likely to occur by in-plane bending, local buckling and yielding for members of short length, and by flexural-torsional buckling, local buckling and yielding for members of intermediate and long length.

Based on the results obtained, it can be concluded that the finite element modelling is reliable in predicting the ultimate capacity of the elements with tapered webs subject to both compression and bending with sufficient accuracy. The failure mechanism for the selected specimen series varies as a function of the web thickness and lateral restraints (i.e. global out of plane buckling governs in the case of thicker web specimens, whilst distortion of the compressed flange and local web buckling for the case of slender web). The ultimate capacity is significantly influenced by the width-thickness ratio of the web. The failure mode varies between local and global buckling as a function of the applied lateral restraints.

1

A. Taras, R. Greiner, "Torsional and flexural torsional buckling - A study on laterally restrained I-sections", Journal of constructional steel research, 64(7-8), 725-731, 2008. doi:10.1016/j.jcsr.2008.01.019

2

J. Szalai, F. Papp, "On the probabilistic evaluation of the stability resistance of steel columns and beams", Journal of Constructional Steel Research, 65, 569-577, 2009. doi:10.1016/j.jcsr.2008.08.006

3

B. Farshi, F. Kooshesh, "Buckling Analysis of structural steel frames with inelastic effects according to codes", Journal of constructional steel research, 65, 2078-2085, 2009. doi:10.1016/j.jcsr.2009.04.018

4

N.E. Shanmugam, H. Min, "Ultimate load behaviour of tapered steel plate girders", Steel and Composite Structures, 7(6), 469-486, 2007.

5

A.H. Salem, M. El Aghoury, M.N. Fayed, I.M. El Aghoury, "Ultimate capacity of axially loaded thin-walled tapered column with doubly symmetric sections", Thin Walled Structures, 47, 931-941, 2009. doi:10.1016/j.tws.2009.02.011

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