Keywords: aluminium structures, Eurocodes, finite elements, structural design, shell buckling.

This paper considers the complexity inherent in changing from national codes, such as British Standards, which are based on hand calculations, for the design and analysis of aluminium structures to the use of Eurocodes which require the use of finite element analysis.

It is a common misconception that aluminium and its alloys can be designed as ultra lightweight steel. Aluminium has a Young's modulus and a mass that is approximately one third that of steel. Consequently aluminium deflects three times more than steel, is more susceptible to fatigue and to buckling.

For shells, the buckling phenomenon is highly complex, being described by nonlinear partial differential equations. Further, the buckling of aluminium shells is most sensitive to the small geometric imperfections induced during the fabrication process, the non uniformity of loading and the possible reduction in strength, due to the heat affected zone at welds.

The use of the finite element method has meant that complex aluminium structures can be analysed and numerical predictions made. However the controlling parameters of aluminium shells must be considered carefully to ensure that the numerical results obtained produce reliable design strength predictions [1].

The majority of analysis and design calculations undertaken by design engineers are carried out using computers. Virtually all current design software will have to be rewritten to accommodate the Eurocodes. A major consideration in rewriting the computer software is that existing British Standard's design methods are based on hand calculations coupled with safety margins.

Presently, the majority of analysis and design calculations undertaken by design engineers, using British Standards, are carried out using computers. The Eurocodes assume that advanced computer techniques of analysis and design are ubiquitous or will become so exceptionally rapidly. Consequently it has become necessary to identify which parts of the Eurocode design process can be replaced by computer software and which parts can not. The answer depends upon the type of analysis being used. For example, for shells, the buckling phenomenon is highly complex, being described by nonlinear partial differential equations.

The introduction of the finite element method to the design process has meant that complex aluminium structures can be analysed and numerical predictions made. However the controlling parameters of aluminium shells must be considered carefully to ensure that the numerical results obtained produce reliable design strength predictions.

Eurocode 9 is divided into five parts, namely: general structural rules, structural fire design, additional rules for structures susceptible to fatigue, supplementary rules for cold-formed sheeting and Part 1-5 supplementary rules for shell structures [2,3,4,5,6]. Part 1-5, the most complex part of Eurocode 9 is considered in the light that it describes eight different types of analysis that can be used and assesses those areas of design where the use of finite element analysis radically affects the design of aluminium structures [6].

A major innovation in Eurocode 9 is the regulation of the use of finite element computer calculations in the design of aluminium structures. However care must be taken in the choice of finite element analysis. A linear elastic bifurcation (eigenvalue) analysis will not accurately model the sensitivity to geometric imperfections and a geometrically linear analysis cannot detect snap through buckling. The highest quality calculations are found by using geometrically and materially non-linear analysis with imperfections included, but there is difficulty in identifying the form and the amplitude of the imperfections to be used.

Time to assimilate the Eurocodes into design office practice is potentially very short. Consequently, as the Eurocodes are published they will coexist with the existing relevant British Standards for a maximum of three years, after which the British Standards will be withdrawn. Designs will then be to Eurocodes with computer software taking over from the more traditional methods of design.

This paper considers some of the situations that must be understood by engineering designers when changing from the use of national codes such as British Standards to Eurocodes.

1

Teng, J.G. and Rotter, J.M., "Buckling of thin metal shells", Spon Press, London, 2004.

2

prEN 1999-1-1 "Eurocode 9:- Design of aluminium structures, Part 1-1: General structural rules", CEN, Brussels, 2004.

3

prEN 1999-1-2, "Eurocode 9:- Design of aluminium structures, Part 1-2: Structural fire design", CEN, Brussels, 2004.

4

prEN 1999-1-3, "Eurocode 9:- Design of aluminium structures, Part 1-3: Additional rules for structures susceptible to fatigue", CEN, Brussels, 2005.

5

prEN 1999-1-4, "Eurocode 9:- Design of aluminium structures, Part 1-4: Supplementary rules for cold-formed sheeting", CEN, Brussels, 2004.

6

prEN 1999-1-5, "Eurocode 9:- Design of aluminium structures, Part 1-5: Supplementary rules for shell structures", CEN, Brussels, 2005

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