Hobart Bridge

The eastern end has been detached from its shore connection.

The Hobart Bridge was a pontoon bridge that crossed the River Derwent, connecting the eastern and western Shores of the City of Hobart, Tasmania, Australia.

History

Plans for a bridge to link the Derwent River’s two shores near Hobart date back to 1832. It was not until 1943 that the first bridge was completed, the Hobart floating bridge and lift span. The bridge was opened to toll traffic on 22 December 1943 and the collection of tolls continued until midnight on 31 December 1948.

Soon after its opening a violent storm blew in and damaged a section of the bridge, and to prevent the same happening again, the bridge was anchored to the riverbed in the middle and strengthening cables were added to stiffen the structure. After these modifications were completed the lifespan of the bridge was estimated as 21 years. It was closed on the 17 August 1964.

The bridge provided much better connection between the Eastern and Western Shores, and consequently development on the Eastern Shore sped up and became so dense by the mid-1950s that the floating bridge could no longer handle the amount of traffic that was crossing it. Congestion became a severe problem, and in the late 1950s the decision was taken to construct a completely new bridge, the Tasman Bridge, which opened in early 1964.

Now separated in the middle, the eastern half is ready to be towed up-river to its final resting place in Gielston Bay. The new Tasman Bridge can be seen on the left. The steel towers at the far end of the old bridge are lift towers that raised a section of the bridge to allow ships to navigate up-river (to the Zinc Works, for example).

The floating bridge was closed to traffic on 17 August of that year, and the following day the locking pin was removed and the two concrete sections towed away. For several years they were moored, but one of them sank in November 1970, and the Council undertook to dispose of them. The two halves were cut up and sunk at various locations.

The lift span was left in situ for some years but in the end it too was demolished. Today the only reminders of the bridge are the eastern foot of the lifting section which is still in place, and the preserved locking pin. One of the pontoons was sunk at Alonnah, and remains in use as a public jetty. Another piece is sunk in Ralphs Bay, in about 40 feet of water.

Construction

The Hobart Bridge was of unique design and construction, and the first of its type anywhere in the world. It was a floating bridge with a lift span, constructed of hollow concrete pontoons, 24 in all, connected together forming a crescent shape curved upstream, and anchored in the middle.

The bridge was constructed in 12-pontoon sections which were then towed out into the river and connected to the banks and to each other in the middle. The total volume of concrete used in making these pontoons was 11,000 cubic yards (8,400 m³). The two halves of the bridge were made of ten 131 ft 6 in (40.08 m) pontoons, one 124 ft 6 in (37.95 m) section, and one 138 ft 6 in (42.21 m) section, joined in the middle by a 12 ³⁄₄ in (324 mm) vertical locking pin, which was saved when the bridge was demolished and is now on display outside the Royal Engineers Building in Hobart.

The total length of the roadway was 3,154 feet (961 m). The total width of the bridge was 40 ft 6 in (12.34 m). It had a two-lane roadway and a footpath on one side. At the western end a large lifting section was provided to allow ships to pass. It provided a vertical clearance of 145 ft 6 in (44.35 m) at low tide, and the opening section was 180 ft (55 m) wide. Four 600 horsepower (450 kW) electric motors were used to open the bridge, which took two minutes. The total weight of steel used in the construction of the bridge was 3,100 long tons (3,100 t).

References

• Parliament of Tasmania History site - Hobart to Tasman Bridge