Keywords: aluminium bridges, fatigue life, deflection reduction, numerical analysis, bridge strengthening, Eurocode 9.

This research reviews a number of existing bridge strengthening methods and recommends a generic flexural strengthening retrofit solution for in-situ aluminium bridges [1]. This research also determines the structural requirements for reducing the deflections and increasing the fatigue life of an existing 105 meter span aluminium bridge. As fatigue life is dependent upon the stress range and not the maximum stress, the maximum and minimum stresses within each member of the originally designed bridge, subject to a range of loading conditions are be determined. From this analysis the high stress range fatigue-critical sections are identified and an appropriate range of alternative members developed to improve fatigue life [2].

The results of the first part of the research showed that external post tensioning with CFRP considerably reduced the mid-span deflection and significantly increased the fatigue life. The IS panelling exhibited strong potential for the retrofitting of aluminium bridges the panelling requiring changes to the design of primary and secondary members. CFRP plate bonding is currently the most prolific form of retrofit bridge strengthening and is conducive to aluminium bridge strengthening. The option of pre-stressing the CFRP plates has considerable possibilities in reducing deflection and increasing the fatigue life of structural members. Aluminium plate bonding exhibits the advantages of standard CFRP plate bonding but with minimal reaction or corrosion problems. Thus the conclusion of the first part of the research is that was that aluminium plate bonding was the priority method for reducing vertical mid-span deflection and increasing the fatigue life of aluminium bridge structures [1].

In the second part of the research the initiation point was a portfolio of original design calculations and layout drawings for a 105 metre span aluminium bridge. As the fatigue life of an aluminium structure is dependent upon the stress range and not the maximum stress, the maximum and minimum stresses within each member of the originally designed bridge, subject to a range of loading conditions, was determined. This required a complete three dimensional analysis of the bridge from which the fatigue-critical sections with high stress ranges were identified. An appropriate range of alternative members was developed to improve the bridge's fatigue life. These designs were analysed to determine the increases in fatigue life [2]. The most obvious modification to the design was to increase the member section sizes, but this resulted in increased stress ranges and a consequent reduction in the bridge's fatigue life. The most effective modification that leads to the increase in fatigue life was the addition of diagonal members in each panel of the bridge. However, to ascertain the precise number and location of the additional diagonal members required extensive computer analysis. The results showed that the addition of extra members in just one specific panel increased fatigue life by 17%. With all the panels having an additional member, fatigue life increased by 400%. By the incorporation of an optimisation algorithm in the analysis a specified increase in fatigue life could be related to the number of panels requiring additional members. For example to increase the fatigue life by 100%, only 4 or 5 panels would require additional members [2].

1

D. Hill, Reducing the deflection and increasing fatigue life of insitu aluminium bridges through retrofitting measures, MEng dissertation, University of Newcastle upon Tyne, 2005.

2

M. Colledge, Improving the fatigue life of an aluminium bridge subjected to high fatigue loading, MEng dissertation, University of Newcastle upon Tyne, 2005.

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