Buckling of T-section beam-columns in aluminium with or without transverse welds

• This thesis deals with buckling of T-section beam-columns inaluminium with or without transverse welds. Totally 26 unweldedand 39 transversely welded T-section beam-columns were tested.Five of the welded beams were clamped. All unwelded and therest of the welded beams were simply supported for bending. Thewelding affects the load-carrying capacity of the beam-columns,because it introduces a heat-affected zone with reducedstrength. All beam-columns had the same theoreticalcross-section dimensions. The thickness of the flange and theweb was 6 mm. The depth and the width were 60 mm. Thetheoretical beam lengths were 500, 1020 and 1540 mm,respectively. Tensile tests of both the parent and theheat-affected material were made in order to determine thematerial properties.Comparisons were made between the buckling tests and threecodes, the European aluminium code Eurocode 9, the Britishaluminium code BS 8118 and the Swedish steel code BSK. Someinterpretations of the codes had to be made, because the codesare not totally clear when applied on T-sections. Most problemsare related to the fact that the section modulus is not thesame for the two edges. In the interaction formulas, only theedge in compression was considered when the bending momentcapacity was calculated. The chosen interpretation of the codeswas often very conservative when compared with the bucklingtests.The general-purpose finite element program Abaqus was usedto develop numerical models of the tested beam-columns. Shellelements were used. The models were calibrated with the resultsfrom the buckling tests. The stress-strain curves used in thefinite element calculations were obtained from the tensiletests. The results of the finite element calculations weresatisfactory. The numerical models could predict the loadcarrying capacity accurate enough. A similar deformed shape ofthe tested and calculated beam-columns was also obtained.Different modifications of Eurocode 9 were analysed in orderto improve the results. One modification was that the ultimatestrength of the heat-affected zone was used instead of theyield strength of the parent material whenthe bucklingreduction factors of a welded section were calculated. Thecalculation of the bending moment capacity in the interactionformulas was also modified. The plastic section modulus wasused to calculate the bending moment capacity when the tip ofthe web was in tension. When the bending moment acted in theopposite direction, the calculation of the bending momentcapacity was based on a modified classification of the webelement. The investigation in this thesis indicates thatEurocode 9 is too severe in the classification of thecross-section. The way the bending moment capacity iscalculated for unsymmetric cross-sections in the interactionformulas needs to be further analysed. Tensile failure at thetip of the web was also discussed. This thesis shows that thecodes need to be improved when it concerns unsymmetriccross-sections. Some information how Eurocode 9 can be improvedis given.Keywords: Buckling, buckling tests, Eurocode 9, aluminiumbeam-columns, beam-columns, T-sections, finite elementanalysis, transverse welds, codes, unsymmetriccross-sections.

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