Keywords: steel structures, columns, cold-formed, finite element analysis, geometric imperfections, sensitivity analysis.

The representation of geometric imperfections through elastic buckling modes has been widely used in numerical analysis. However, there is no considerable consensus about which single buckling mode, or set of modes, that can provide useful representations of imperfection distributions. Yang and Hancock [1] identified, from the elastic eigenvalue buckling analysis, that initial imperfections based on a single buckling mode can lead to non-conservative results due to the possibility of interaction among some modes and suggests a number of analyses should be conducted to investigate the sensitivity of a structure to imperfections.

In the context of the present work, a series of pinned-end column tests with plain and lipped channel sections are simulated. The numerical analyses were carried out to investigate the effects of local, global and distortional imperfections on the column's ultimate load. Two ways to drive the modelling of imperfection were appreciated. Firstly, each type of the imperfection investigated was evaluated in an isolated way and, after that, in the sense of an associated way. A sensitivity analysis on the geometric imperfections was also performed, focusing on the form, magnitude and signal of the imperfections.

The consideration of buckling modes to represent the initial geometric imperfections was performed by the construction of numerical models using the computational code ANSYS v9.0, the results of which are generally satisfactory. Based on an analysis of ultimate load, it was shown that the choice of the proposed strategy can lead to satisfactory numerical results.

It is important to mention that in the context of isolated mode analysis (local, global or distortional imperfections on the columns ultimate load), in which the imperfections are based on a single buckling mode, the consideration of only a single buckling mode to represent the geometric imperfection can lead to satisfactory numerical results in terms of ultimate load. Moreover, plain channels are suggested more susceptibility to local imperfection than lipped channels and global imperfection sensitivity was similar for both sections. The effect of local imperfection on plain and lipped channels collapse deformed configuration results are different for each one. The minor axis displacement developed for the local imperfection provides an opposite axis displacements for plain channels in relation to the lipped ones. Plain and lipped channels whose ley/ry ratios were approximately 90 and 120 were the most susceptible to local and global imperfection. On the other hand, in the context of couple analysis, in which the imperfections are based on a combination of buckling modes, resulting in an important tool to investigate the influence of one type of buckling mode on each other, the predominance of one type of imperfection on the other depends on the sign combination, the amplitude combination and, the local and global slenderness of the section and columns respectively.

In case of plain and lipped channels, the global imperfection sign is the main factor to define conservative or non-conservative coupled imperfections. The global imperfection signal has a different influence on the plain and lipped channels deformed geometric configuration and the ultimate load.

1

D. Yang, G.J. Hancock, "Numerical simulation of high strength steel lipped-channel columns", Department of Civil Engineering, University of Sydney, Australia, Research Report R843, 2004.

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