Bridge Modifications in Darling Harbour

In 2002 I joined Aurecon (formally Connell Wagner) and was hired for a tunnel project under the Sydney CBD running east-west. As part of the project, the Western Distributor viaduct in Darling Harbour needed major modifications to accommodate the new tunnel exit and entry portals. 

Some incredible #engineering and #innovation happened on the project. I will give an overview of three major modifications that I was lucky enough to work on.

Market Street Viaduct Widening

The Market Street viaduct comprised two separate superstructures. On the eastern and, a multi-cell box girder and to the west a composite steel I-girder and concrete deck. The vertical and horizontal road alignment was complex. 

Two different approaches were used to widen the viaduct. The multi-cell box girder was widened using precast panels with an inclined strut that hinged from the existing deck and webs respectively to allow cast in-situ concrete to be placed, making the new and existing deck as-one. 

The steel composite spans were widened using steel beams and a concrete deck, a similar construction method to the existing viaduct. However, to do so required the existing substructure to be widened. The reference design called for a strengthened pilecap. This would have caused major disruption to the busy tourist area of Darling Harbour.

An advanced analysis was undertaken for the existing piers which demonstrated the substructure below ground did not require strengthening. Large “lego” type steel headstock extensions were placed with external post-tensioning to both strengthen the headstock and carry the widened bridge.

 Market Street Pier Relocation

The new tunnel entry, as Murphy’s Law would have it, ran straight through a pier that supported the multi-cell box girder spans of the Market Street Viaduct. Due to space and other utility constraints, the tunnel entry could go nowhere else.

The solution was to vertically extend the walls of the tunnel dive structure to support two new steel transfer beams that in turn supported the permanent bearing. The transfer beams were offset sufficiently to enable the existing bearing to be removed and replaced once the temporary jacks lifted the deck.

 Druitt Street Pier Modification

Similar to the Market Street Pier Relocation, the tunnel road alignment dictated the existing blade pier carrying a multi-cell box girder of the Druitt Street Viaduct needed to cut in half.

 A steel frame was designed to carry the new bearing with the legs of the frame acting in a push/pull arrangement for stability. I learned a lot about steel detailing from John Steele on this one.

Both the pier modifications, although difficult to build (as testified by my friend David Hicks who constructed the Druitt Street Pier Modification works) worked well and existing infrastructure was effectively re-used.

The lesson that I took from these modification works was to allow even greater factors of safety for jacking loads when modifying existing wide deck structures. The predicted temporary jacking loads were actually lower than the actual jacking loads most likely due to load re-distribution in the structure due to stiffness or settlement effects. The factors of safety covered the difference in both cases, however it is important to over-size jacks when dealing with large concrete or steel decks to sufficiently cover all loading events. This type of modification works normally occur during scheduled closures of major highways or railways so if something does not go as planned, this can cause major disruption to the public.