Bridges

The structural assessment of two underbridges to Australian Standard AS 5100.5 indicated that the design fatigue life had been exceeded for steel shear stirrups forming part of the prestressed concrete girders comprising the superstructure of the bridge. A detailed inspection of the bridge showed no sign of cracking or distress and the strength load rating was satisfactory. In fact calculations indicated that the concrete was uncracked in shear under the design loading. The bridges were constructed around 30 years ago. Shear strengthening was impractical and superstructure replacement was costly, particularly as the two underbridges were representative of 21 similar structures. On this basis, Aurecon was engaged by Sydney Trains to undertake a detailed fatigue assessment including comparison of the provisions of AS 5100 with other international Standards (Eurocode, German DIN EN 1992-1-1/NA:2013-04, AREMA, AASHTO LRFD, Canadian CSA-S6-06 and the draft fib Model Code 2020). This work included contact with the AS 5100.5 Standards subcommittee.

The findings of the investigation noted that; it is valid to consider part of the concrete component in resisting shear (even when it is cracked) for fatigue loading, that the angle of inclination between the concrete compression strut and the longitudinal axis of the member should be modified due to the potential for working of the shear crack under repeated loading. This investigation informed on the amendment to AS 5100 and indicated that fatigue strength of the two bridges was satisfactory based on projected track usage, for at least the duration of its originally intended design life.