Photo of the Sydney Harbour Bridge at dusk

Sydney Harbour Bridge at dusk, from Lavender Bay, June 2023. Image © Paul Rands

General Information:

Bridges are vital structures that span horizontally between supports, designed to carry vertical loads and connect communities. While the basic concept, a beam supported by two pillars, may seem straightforward, the engineering challenges involved are significant and apply to every bridge. The supports must be robust enough to uphold the structure, while the span must be strong enough to bear the loads it carries. In most cases, spans are kept as short as possible for efficiency, but longer spans are essential in locations where strong foundations are scarce, such as across deep estuaries or wide rivers.

Four primary materials are used for bridge construction, these are wood, stone, iron, and concrete. There are six basic bridge forms, which are: 1

Beam:

The beam bridge is the most common form of bridge you'll find. It works by bearing vertical loads through bending, when the beam bends, the top experiences horizontal compression, while the underside is put under horizontal tension. The supports transfer the weight of the beam downwards through vertical compression into the foundations, ensuring a sturdy and reliable crossing.

Truss:

A single-span truss bridge works like a simple beam, holding up weight by bending. This bending puts compression on the top parts, tension on the bottom parts, and either pushes or pulls on the vertical and diagonal pieces, depending on how they're placed. Truss bridges usually need less material to handle heavy loads.

Arch:

An arch bridge holds weight mainly through compression, which pushes both down and sideways on its foundations. The foundations need to stop the arch from sinking or sliding. Even though the foundation design is more complex, the bridge itself usually needs less material than a beam bridge of the same size.

Suspension:

A suspension bridge holds weight using curved cables under tension. The weight is passed to the towers, which push it down into the ground, and to the anchorages, which hold the cables in place and resist their pull. A suspension bridge is basically an upside-down arch, where the cables are stretched instead of compressed. Since the roadway is suspended in the air, it must be made strong and stable to avoid too much movement when used. This means the roadway needs to be heavy, stiff, or both.

Cantilever:

A beam is called cantilevered when it sticks out with support only at one end. A cantilever bridge usually has three sections. The outer sections are anchored on the shore and extend out over the gap. The middle section rests on the ends of the outer sections, acting like a regular beam or truss. It handles weight through tension in the lower parts and compression in the upper parts. The cantilever sections handle weight the opposite way, with compression in the lower parts and tension in the upper parts. Towers near the middle transfer weight down to the foundation, while towers near the edges pull the weight back toward solid ground.

Cable-stay:

Cable-stayed bridges support the main span with diagonal cables that are under tension. The towers direct the cable forces downward to the foundations through vertical compression. The tension in the cables also applies horizontal compression to the bridge deck.

Burnett Bridge (Bundaberg, QLD): 2

The route features the Burnett Bridge (also known as Burnett River Traffic Bridge) joins Quay Street, Bundaberg with Perry Street, North Bundaberg, across the Burnett River. It is 412 metres in length and has eight 51.9 long spans with cylindrical piers and concrete abutments. The bridge carries a 7.3 metres roadway and a footpath. The road is on transverse metal troughing spanning between the lower chords of hog-back lattice trusses.

The bridge is a metal truss bridge and a significant example of the work of Alfred Barton Brady who, during 37 years of work for several departments of the Queensland government, made a major contribution to the development of the state in both architecture and engineering design.

When it was opened to traffic on 24 August 1900, the Burnett Bridge on was the fifth longest metal truss bridge in Australia. It was completed at a time when funds for local authorities were scarce because of priority given by the government to developmental railways. Half the cost was met by endowment; the other half was advanced to the local authorities as a loan repayable in forty years.

Originally the two sides of the river was linked via ferry, but it was replaced with a timber bridge in the 1870s before the current bridge was built.

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Burnett Bridge:

Northbound on the Burnett Bridge over Burnett River, Bundaberg, December 2009.

Image © Rob Tilley

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Burnett Bridge:

Looking southbound on Burnett Bridge, North Bundaberg, March 2006.

Image © Paul Rands

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Burnett Bridge:

Looking southbound on Burnett Bridge, North Bundaberg, March 2006.

Image © Paul Rands

Kings Bridge (Launceston, TAS):

Carrying Trevallyn Road over the confluence of South Esk River and River Tamar in Launceston, the fabrication of Kings Bridge commenced in Manchester, England, when completed, the span was shipped to Australia. The designer of the first span was William Thomas Doyne, (1823-1877), a civil engineer born in Ireland. At the request of the West Tamar Road Trust, Doyne designed the elegant bridge to cross the South Esk River at Launceston. It was described as a wrought-iron arch spanning 190 feet. 3

In 1864, Kings Bridge arrived in Launceston and was assembled on a pontoon, floated into position and then lowered on to its abutments on the receding tide. Kings Bridge was originally a single lane metal arch that connected Launceston with the West Tamar region. 3 On the 4th of February 1864, Kings Bridge was officially opened at a cost of £12 000. 3

A duplicate adjacent span of Kings Bridge was fabricated by Salisbury's foundry in Launceston in 1904, and was similarly erected and floated into position like the original span.

By the late 1960s, the traffic volumes over Kings Bridge had reached levels that required a wider crossing to be built. On the 2nd of November 1973, Paterson Bridge over South Esk River opened to traffic, adjacent to and bypassing Kings Bridge. The Paterson Bridge was named in honour of the first Governor of the colony of Tasmania. 4 Kings Bridge remained in use for local traffic for some years after, before it was refurbished, and then re-opened to traffic on the 22nd of June 1988.

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Kings Bridge:

View of Kings Bridge over the confluence of South Esk River and River Tamar in Launceston, looking towards Cataract Gorge, July 2018.

Image © Paul Rands

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Kings Bridge:

View of Kings Bridge over the confluence of South Esk River and River Tamar in Launceston, looking towards Cataract Gorge, July 2018.

Image © Paul Rands

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Plaque:

Foundry plaque on the duplicate span of Kings Bridge, Launceston, July 2018.

Image © Paul Rands

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Plaque:

History plaque on Kings Bridge at Launceston, July 2018.

Image © Paul Rands

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Plaque:

Refurbishment plaque, Kings Bridge, Launceston, July 2018.

Image © Paul Rands

Spiky Bridge (Swansea, TAS):

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Spiky Bridge:

Spiky Bridge at Swansea, July 2018. A sign at the location reports the bridge was built by convicts as part of the coach road that connected Swansea with Little Swanport and the east coast road to Hobart. Work commenced on its construction, along with Rocky Hills Convict Station in 1841.

Image © Paul Rands

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Spiky Bridge:

Spiky Bridge at Swansea, July 2018. A sign at the location reports the bridge was built by convicts as part of the coach road that connected Swansea with Little Swanport and the east coast road to Hobart. Work commenced on its construction, along with Rocky Hills Convict Station in 1841.

Image © Paul Rands

1 Britannica, Technology, Engineering, Civil Engineering, Bridge.
2 QLD Government, Environment and Resource Management, Cultural Heritage, Burnett Bridge.
3 ABC Northern Tasmania, Kings Bridge.
4 TAS Government, Libraries Tasmania, Paterson Bridge Official opening 2nd November 1973.