Keywords: manufacturing imperfections, assembling imperfections, pitched-roof portal frames, tapered members.

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 of the structure. As a result of the nonrectangular shape of the element, thin web sections may be obtained at the maximum cross section height. Usually class 3 to class 4 webs may be used for the case of double T welded cross sections. The buckling strength of these structures is directly influenced by the lateral restraining, end support and initial imperfections [1].

The main objective of this paper is to analyse the sensitivity of single storey steel structures made of variable cross sections to different type of lateral restraints and also to manufacturing and erection imperfections. The previous studies [2,3] made by several authors all around the world, highlight the importance of taking into account different initial imperfections, both in the case of gravitational loads and horizontal loads. The considered imperfections might be described as: column vertical deviation (in or out-of-plan), initial bow imperfections, cross sectional imperfections, coupling between previously defined imperfections.

The paper summarises the outcome of a numerical study on a considerable number of pitched roof portal frames designed with tapered elements. Different types of lateral restraints (e.g. rigid and elastic) were considered. The magnitude of the imperfections was set equal with those prescribed by EN1993-1-1 [1].

A number of pitched roof portal frames commonly used for the execution of single storey buildings were analysed (12, 18, 24, 30 m spans and 4, 5, 6 m heights), to cover a wide range of structures. They have pinned column bases, tapered columns, tapered rafters and a pitch roof angle of 80. The tapering ratio is approximately equal to 2 for all cases. The rafter is composed of uniform and non-uniform regions.

For all the cases out-of-plane buckling of the frame elements was noticed to be the main failure mode regardless of the applied lateral restraints. There were cases for which the global lateral- torsional buckling of the frames was coupled with local buckling of the web. This was mainly observed when the restraints applied to the frame element are more effective against overall buckling (e.g. type 3 restraints). It was noticed that the imperfections considered have a low to significant influence on the final capacity of the frame, function than the applied lateral restraints. The difference between the imperfections considered is significant mainly for the combined cases.

The difference between elastic (actual) and rigid lateral restraints increases as the span increases. A maximum of 10% difference was recorded.

1

EN 1993-1-1, "Eurocode 3, Design of steel structures Part 1.1: General rules and rules for buildings", CEN, Brussels, Belgium, 2005.

2

Z. Kala, "Sensitivity Analysis of Steel Plane Frames with Initial Imperfections", Engineering Structures, 33(8), 2342-2349, 2011. doi:10.1016/j.engstruct.2011.04.007

3

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

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