Keywords: stiffened plate, longitudinal stiffener, ultimate behaviour, local buckling, overall buckling, finite element analysis, virtual experiment, parametric study, non-conventional fabrication, welding residual stresses, imperfections.

In the conventional design, metal plate girders come with frequently located vertical and one or two horizontal stiffeners in the web. Although multi-stiffened plates with longitudinal stiffeners may result in an optimal geometry with thin web plates even in case of plate girders under dominant bending and shear [1], it is hardly applied in practice. The explanation for this can be found in the conventional fabrication process (each stiffener is welded directly to the plate): the increased welding demands causes higher fabrication costs. However, in the case of aluminium, applying extruded profiles leads to a very efficient and economic structure. In such a case, the extruded profile includes at least one stiffener and the adjacent plating. The multi-stiffened plate is then built up by these profiles welded to each other. In addition to the great advantage of optimal cross-section geometry can be utilized in this way, the fabrication process becomes less time-consuming and significantly less expensive (especially for mass production). Additionally, in contrast to the conventional method, the residual stress distribution due to welding changes so that the tension stresses arise in the middle of each sub-panel, which may improve the structural load capacity that is highly influenced by local plate buckling. As a further benefit, eliminating material softening due to welding in the extreme fiber also increases the resistance against bending. Finally, new joining techniques, such as Friction Stir Welding (FSW, [2]), promises better performance as a result of to the smaller material softening and geometrical imperfections. Similar advantages may also apply to the case of steel, e.g. with rolled profiles.

Nevertheless, previous researches on the ultimate behaviour of stiffened web plates concentrate on the case that compressive residual stresses highly reduce the load capacity. This is why our research deals with the buckling and ultimate behaviour of multi-stiffened metal plates, assuming new fabrication processes, such as the application of extruded aluminium profiles joined by the above-mentioned FSW.

In the paper, the influence of non-conventional fabrication processes, residual stresses and imperfections on the ultimate behaviour of longitudinally stiffened web plates is studied.

Firstly, the theoretical background of the corresponding phenomena and analytical solutions are briefly discussed. Results of parametric studies on the above effects are discussed in detail. For this purpose, finite element simulation and virtual experiments using ANSYS [3] are invoked instead of the time-consuming and expensive real experiments. Finite element models on three modelling levels (unstiffened sub-panel, stiffened web plates and plate girders) are developed. Geometrical as well as material non-linearity is included in the models. Additionally, residual stresses due to welding are considered. The simulations are executed on the basis of displacement-controlled calculations and the force-controlled arc-length method for local and the global investigations, respectively.

The model is verified in the way that numerical results are compared with experiments of steel plate girders designed in accordance with the concept discussed in [1]. With the help of these models, a parametric study through the non-linear simulations is completed with respect to the residual stress distribution and imperfections. General conclusions are drawn for the influence of non-conventional fabrication processes, residual stresses and imperfections, and the practical applicability of multi-stiffened plates is summarized.

Additionally, results are compared to the design methods given in the European standards Eurocode 3 (EC3 [4]) and Eurocode 9 (EC9 [5]), and, based on the numerical outcome, proposal is made for the proper design methodology.

1

Kitamura, K., Okura, I., Vigh, L.G., Utaki, T., Mikawa, K., "Proposal for new girder system, utilizing special features of aluminum alloys", Proc. of JSCE Annual Meeting, Japan, 2004, (in Japanese, accepted for publications).

2

Kallee, S., Nicholas, E.D., "Friction Stir Welding, TWI homepage", http://www.twi.co.uk.

3

"ANSYS Structural Analysis Guide", Online Documentation, ANSYS Inc., 2001.

4

"prEN 1993-1-1, Eurocode 3: Design of steel structures - Part 1-1: General rules and rules for building", 2003.

5

"prENV 1999-1-1, Eurocode 9: Design of aluminium structures - Part 1-1: General rules - General rules and rules for buildings", 1997.

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