Narrows Bridge Duplication (2001) - 2012 Booklet
By 1998 the original Narrows Bridge was carrying 155,000 vehicles per day and the State Government announced plans to widen it. The initial solution was to build a smaller bridge immediately west of the existing structure and to join the decks of the two bridges to form a contiguous roadway. However this plan was superseded by a decision to build a new bridge separated from the original bridge by a gap of six metres. Main Roads WA called tenders for the design and construction of the new bridge in July 1998, with the proviso that the new bridge had to have the same profile and look the same as the original.
Leighton Contractors, in association with designers Connell Wagner, was, in March 1999, awarded a $49 million contract to design and construct the bridge. Their proposal was to use the incremental launching technique, whereby 28 metre concrete bridge beam segments were cast in a bed on the south abutment, post tensioned to the previous completed segment, and pushed by hydraulic jacks 28 metres northwards over permanent and temporary piers until the front end of the twelve stages reached the north abutment. The original Narrows bridge has eight rows of I beams connected by diaphragms at the piers and intermediate points. The new bridge has four rows of larger I beams, the east and west pairs connected with diaphragms before launching separately. Incremental launching is best suited to bridge beams of constant depth, and because of the specified curved soffit of the Narrows bridge beams, during launching the pairs of beams had to be supported on elevated temporary supports, these being removed when the beams were lowered onto permanent piers after the launching was completed. Then an insitu slab was cast along the bridge centre line joining the east and west pairs of beams.
The bridge was opened to traffic on 26 February , 2001.
Author:
Don Young
