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

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.

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.

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.

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.

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-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.

This section is compiled, with permission, from information provided by several official and unofficial sources, as well as additional content by Paul Rands and other contributors.

Click or tap on a region below to explore an extensive gallery of bridges across Australia.