Buckling of steel flanged cruciform columns under axial loading

Abstract

Doubly symmetric cruciform sections are susceptible to torsional buckling and are therefore not widely used in the construction industry; however, they are becoming more popular when two-way frame action is needed. There is very limited research on the structural behaviour of cruciform columns. In view of this, very little has been done to understand the buckling behaviour of flanged cruciform columns fabricated from beam sections. This dissertation aims to address this literature gap by explaining both the elastic and plastic buckling behaviour of these sections with regards to axial loading. Furthermore, by analysing existing design procedures presented by EN 1993-1-1 (2005), the validity of applying such procedures to the flanged cruciform section are to be questioned. This is done by first performing numerical analysis to a standard steel flanged cruciform section – consisting of a critical load analysis, as well as an inelastic analysis due to initial imperfections. Following this, by making use of LUSAS (2018), a Finite Element structural analysis software package, FEM analysis is performed – consisting of a critical eigenvalue analysis, as well as a non-linear elasto-plastic analysis. Hence, the results obtained from the FEM analysis are compared to the values obtained from the numerical analysis. An experimental test is carried out to allow for the comparison of the theoretical buckling capacity with that obtained from physical experimentation. By using numerical analysis, it was possible to obtain the theoretical critical elastic buckling load, and as expected torsional buckling failure was the predominant mode of buckling for lower values of slenderness, whilst flexural buckling failure would take place for members of higher slenderness. This showed that by making use of Eurocode (‘EN1993-1-1’, 2005) to design flanged cruciform columns there are certain risks involved in the assumptions taken in the design code. Computer analysis with LUSAS (2018) showed that the results confirmed the theoretical findings. Experimental analysis showed different results from what was expected from theoretical and computer analysis; however, due to the limited amount of experimentation further analysis could not be performed. Thus, the dissertation provides a good guideline for structural engineers when designing for steel flanged cruciform columns under axial loading.