Explained
How a suspension bridge works.
Hanging a road from two ropes
A suspension bridge carries its deck by hanging it from two huge cables. The cables drape over a pair of tall towers and are tied down at each end into massive anchorages. Vertical hangers connect the deck to the cables every few metres. It's the form used for the world's longest spans, because steel wire is extraordinarily strong when you pull on it, and a hanging cable uses that strength very efficiently.
Follow the load and the design makes sense. A truck on the deck pulls down on the nearest hangers. The hangers pull down on the main cables. The cables, in tension, carry that force along their curve to the tops of the towers and on to the anchorages. At the towers, the cables press straight down, so the towers are squeezed in compression and pass the load into their foundations. At the ends, the anchorages resist the cables' pull with their sheer weight and their grip on the rock or soil.
Cables made of wire
The main cables aren't single ropes. They're spun on site from thousands of parallel high-strength steel wires, pulled across the gap one loop at a time, then squeezed into a round bundle and wrapped for protection. Each of the Golden Gate Bridge's two main cables contains 27,572 wires. Because the wires are parallel, any one breaking takes only a tiny share of the strength away.
Stiff against the wind
A long, light deck can behave like a sail or a wing. In 1940 the first Tacoma Narrows Bridge in Washington State twisted itself apart in a moderate wind, through an effect called aeroelastic flutter. Since then, decks have been made stiffer against twisting, using deep trusses or streamlined steel boxes, and designs are tested in wind tunnels before they're built. Today the longest suspension span in the world belongs to the 1915 Çanakkale Bridge in Türkiye, at 2,023 metres between towers, opened in 2022. Drag the explode slider to separate the towers, cables, hangers, deck and anchorages and see how each one fits into the load path.
Parts glossary
- Towers
- Two tall towers hold the main cables up high. The cables press down on the tower tops, so the towers work mainly in compression.
- Main cables
- Thousands of parallel steel wires are bundled and squeezed into each main cable. The cables drape between the towers and carry almost the whole weight of the deck in tension. Fun fact: Each of the Golden Gate Bridge's two main cables contains 27,572 individual steel wires.
- Hangers
- Vertical steel ropes, called hangers or suspenders, connect the deck to the main cables at regular intervals and pass the deck's weight up into them.
- Deck
- The roadway is built as a stiff truss or box girder so it spreads loads between the hangers and resists twisting in the wind. Fun fact: The first Tacoma Narrows Bridge collapsed in 1940 after wind set its slender deck twisting; bridge decks are now tested for aerodynamic stability.
- Anchorages
- Massive concrete blocks at each end grip the ends of the main cables. They resist the cables' enormous pull with their weight and their grip on the ground.
