Bridgewater Bridge, Hobart
Background
Construction commenced in 1938 but was halted during the Second World War, with the bridge opening to limited road traffic in 1942.
Designer and Chief Engineer for the Public Works Department, Allan Knight, in 1938, was determined to consider weld fatigue in the design of the all-welded Bridgewater Bridge structure. He engaged David Issacs, an engineering consultant from Victoria, to assist with the design. Isaacs opted for the use of butt welds, especially where the welds were to be flush-ground and located in thick metal where accurate alignment of the members was possible on opposite sides of the weld.
The initial crossing consisted of a 730m long rockfill causeway leaving a 340m gap for a shipping channel. During construction the rockfill steadily subsided into the mud below and progress was consequently protracted. The gap was successively crossed by a punt 1835, a rolling span 1849, a swing span rail bridge 1874, a swing span road bridge 1893. The full deck was completed in 1944, the lift span in August 1946 and the rail crossing in October 1946. River navigation through the misaligned spans of the three bridges during construction was difficult, so two of the redundant bridges were demolished to aid navigation.
Equipment and Subsequent History
The bridge is supported by reinforced concrete piers on concrete encased timber piles with the piers supporting the lift span towers constructed on reinforced concrete caissons. There are eleven all-welded plate girder spans with a concrete deck, two on the northern approach of the three steel truss spans and nine on the southern side.
The lift span and flanking spans at either end are of strong, lightweight truss construction. All three are through trusses that these days constrain the dimensions of road and rail vehicles travelling over the bridge. The 42.9m lift span has had a number of decking changes, initially a light weight Oregon timber deck, with the current system comprising vertically oriented planks compressed together into a monolithic slab using transverse post tensioning. The waterway opening has a clear distance of 36.5m between piers.
The heavy lift span is counterbalanced with two large concrete counterweights, with each counterweight supported by six 44 mm wire ropes at each of its two ends and each group passing over a large diameter cast sheaves with two grooved sheaves mounted on top of each tower. The sheaves support the combined mass of both the lift span and its counterweights. The necessary drive was originally provided from a side valve V8 petrol engine, with lever operated brakes and manually operated rail locks. This system was later changed to twin wound rotor electric motors with a diesel engine backup.
The bridge now carries some 20,000 vehicles per day, with 10 per cent commercial vehicles. A regular and rigorous maintenance regime is followed with daily inspections of the critical components such as counterweight and operating cables. The bridge has also seen significant strengthening and rectification works over the last 40 years, including structural modifications to increase the load capacity, the strengthening of piers with transverse steel beams encased in concrete and improved reliability of the lift span.
A major scoping refurbishment project was commenced in 2007 with the aim of maintaining the current bridge for a maximum of 15 years. This work was undertaken in 2010.
This current bridge has now been operating for over 80 years. Construction of a replacement bridge has commenced downstream of the current bridge and causeway.
Engineering Heritage Recognition Program
| Marker Type | National Engineering Marker (NEM) |
| Award Date | September 2018 |
| Heritage Significance | Original causeway built by convicts in 1830-34 remains part of the crossing. The causeway is one of the largest convict-built engineering works in Australia. The Bridgewater bridges overcame a major obstacle for the transport of people and freight in early Tasmania. Steel approach spans, the lift-span towers and the lift span are early examples of electric welding. The measures taken to avoid fatigue cracking in the welds were pioneering at the time. The current Bridge is the largest surviving lift span bridge in Australia and the only one of its kind in Tasmania. It may well be the oldest all-welded railway truss bridge and oldest all-welded railway lift span existing in the world. |
| Nomination Document | Nomination document |
| Interpretation Panel | Interpretation panel |
