Stirling Bridge - 2012 Booklet
History of Stirling Bridge
A bridge over the river at Fremantle has always been a key focus of interest since the colony was settled in 1829. When completed Stirling Bridge was the latest construction to span these waters and to cope with increasing traffic loads in the area.
Stirling Bridge forms a link between Stirling Highway and Cockburn Road as part of the bypass to the City of Fremantle. It was designed to meet the traffic requirements generated by the continuing development of heavy industry in the Kwinana area a general urban expansion. The bridge has been planned in two stages. The first completed in 1974 and the second, to be built when required will duplicate the existing structure on the upstream side, adding another three lanes and giving a combined width of 35m.
Bridge Statistics
The Stirling Bridge is a continuous seven span twin post-tensioned segmental spine beam concrete bridge. The overall length is 415m. The individual spans are from the south 23.8m, 81.4m, 75.3m, 69.2m, 63.1m, 54.9m, and 47.2m respectively. The bridge is fixed on the south abutment and the movement taken in finger joints on the north abutment. The superstructure is supported on steel roller bearings.
The bridge deck is 16.4m wide comprised of a 14.6 m four lane roadway and a 1.8m wide footpath on the downstream side. The clearance in the navigation channel is 9m and over the roadway 6m. The bridge profile rises gently from the south abutment to a third of the way across then falls gradually to the north abutment. It has a cross fall grade of 1:40. The balustrade and guard rails are galvanized steel coated with high quality paint due to the corrosive environment. A 450 mm diameter water main is slung under the roadway between the boxes and provision was made for power and communication ducts through the inside of the boxes and under the footpath slab.
Quantities
- Pier piles: 74 Nos., 2,533m, 0.73m dia. by 13mm mild steel.
- Abutment piles: 21 Nos., 1,035m, 0.47m dia. by 9mm mild steel.
- Reinforcing steel: 1,110 tonnes.
- Concrete: 7,925 m³.
- Prestressing steel: 285 tonnes.
- Precast segments: 292.
Design and Construction
Design plans and specifications were prepared by Maunsell and Partners on behalf of the Client, the Main Roads Department of Western Australia. Tenders were called and the lowest tender at $2,560,000 was submitted by J.O.Clough & Son Pty Ltd. This was accepted on 19 June 1972 requiring a contract completion by 14 July 1974.
The contractor proposed to precast all the concrete superstructure segments at its Kewdale Precasting Yard and transport these to site on low loaders. They proposed to build temporary mid span piers and to use these and the permanent piers to support half span falsework trusses on which to erect the superstructure. Floating equipment would be used to construct the piers in the river. Purpose built lifting and moving equipment was specially designed to fit the purpose. The pier piles were top driven with a K35 diesel pile hammer using a flying leader. Piles were founded on a layer of dense silt known as the Swan River Silt which was about 40m below the water surface. A test pile was driven in Pier 2 and tested to 380 tonnes and the permanent piles were then driven to a set in excess of that to which the test pile was driven. A pile test was also carried out on a north abutment pile to ensure the required capacity was reached.
To support the individual segments and the false work trusses sand jacks were used to enable them to be released under load after stressing was completed. Once positioned and aligned on the falsework the prestressing cables were threaded through the ducts and the 75mm joints between the segments filled with a specially designed concrete mix.
The BBR post tensioning system was used. The main web cables were encased in ducts in the webs of the segments and comprised 81 Nos., 7mm dia. wires. There were six in each web and these were subsequently stressed to approximately 400 tonnes. Four 56Nos., 7mm dia. wire cables were placed in the bottom flanges of the spans and these were stressed to 275 tonnes. Cable ducts were grouted after stressing.
The bridge was constructed from the south end in ten stages; 1a,1b,2a,2b etc. as the two box girders advanced across the river. The respective stages for each box girder were joined at the quarter points of the spans. This was a rather delicate operation which required locking the built stage to the one supported on false work while the last joint was poured and the stressing begun. As the strength of the concrete increased so did the tensioning of the primary cables until a contiguous structure had been achieved.
The top edges of the boxes were clad with a small finishing panel to neatly define the line of the superstructure. These were cast with an ‘off white’ concrete and had a bush hammered finish. The roadway was sealed with a sprayed fibre glass reinforced bituminous coating then paved with two layers of asphalt.
The bridge was completed three months ahead of schedule and opened to traffic by the Premier of Western Australia Hon. Sir Charles Court, O.B.E., M.L.A. on 17 May 1974.
The bridge gracefully sits in its environment displaying the elegant lines of its thoughtful design. The reduction in depth of the bridge beams from a maximum of 3.4 metres at the south abutment to a minimum of 1.8 metres at the north abutment complements the reduction in span lengths and soffit clearance height from south to north and produces a pleasing appearance, particularly when viewed in elevation. At the time of its construction it was the longest bridge in Western Australia.
Author:
Peter Knight
