An initiative was taken in 1975, to harmonise the technical rules, across the European Union, for the design of construction works. These rules, called Eurocodes, were designed to replace national rules and in the UK will replace British Standards [1]. The specific Eurocodes, relevant to civil and structural engineering, are the Structural Eurocodes and Eurocode 9: Design of aluminium structures, is the initiation point for this research [1].

In the design office, steel and concrete are the predominant construction materials. From this starting point, this research considered that design engineers, familiar with the design of steel structures, would find it considerably easier to design in aluminium if they were shown how an existing steel design could be changed to an aluminium design. As the time to assimilate Eurocodes into design office practice will be short and there could be a common misconception that aluminium can be designed as if it were ultra lightweight steel, this research considered how the introduction of the aluminium Eurocode into the design office could be facilitated.

A cable stayed steel bridge for a proposed monorail crossing over the river Tyne at Scotswood was designed for an integrated transport system in Tyne and Wear [2]. The bridge design consisted of a series of 5m long sections of deck each supported by a pair of steel cables at the ends of the top chord. These cables were connected to a central truss which supported each section. The loads from the deck section were eventually carried by the cables to two towers on opposite banks of the river. This research considered only the design of the truss. The distance between the top and the bottom chord of the truss was 3m with each bay along the top and bottom chords being 3.7m in length. The loading conditions adopted for the aluminium structure were identical to those used for the steel structure, other than the self weight and did not consider transient or accidental load cases. The dead loads considered were the self weight of the truss, the monorail track and the pedestrian walkways. The live loads were the monorail train and the pedestrian loads. Once the design had been analysed in aluminium, methods for improving the design to make the structure more suitable for aluminium were investigated. Modifications to the structural design were outlined to illustrate how best to utilize structural aluminium.

The first consideration was to change the existing steel CHS members to aluminium. This resulted in an increased deflection of 185%. However the weight of the truss was reduced by 65% making for easier on-site erection and reduced transport cost. The next step was to increase the section sizes of the CHS member's at the most critically loaded positions. The increase in deflection was 98%. A further step was to increase the second moment of area of the CHS by adding stiffening plate outstands. The deflections were increased by 113%.

Consequently various structural changes are made to the design to increase the capacity of the members to resist the loading. Structural changes such as reducing the length of the web members and those members subjected to the worst axial forces were considered to determine an optimal solution.

Having determined that simple changes in member properties and section sizes could increase the capacity of the truss, additional work was undertaken to use structural aluminium more efficiently. Two key modifications were introduced. The first was to reduce the length of the web members so that the tensile forces in the bottom chord would be reduced and the buckling resistance of the web members increased by the inclusion of an additional chord midway between the upper and lower chords of the truss. This design increased the number of connections and showed that the addition of the midway chord increased the buckling resistance of the web members and reduced deflections. The second was the incorporation of two additional web members in the central web panel of the truss. The two additional members increased the tensile resistance and reduced the deflections. In comparison to the truss with the midway chord, the addition of the two web members was considered to be more efficient due to fewer 'extra' connections.

The research showed the extent to which an existing steel design could be reordered to provide a satisfactory aluminium structure and a means of educating design engineers in the design of aluminium structures.

1

EN 1999, Eurocode 9: Design of aluminium structures, Part 1-1: General structural rules, CEN, Brussels, 2005.

2

Jackson, R.I., Aluminium conversion of a steel monorail transport bridge, MEng dissertation, University of Newcastle upon Tyne, 2005.

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