How Suspension Bridges Work: Cables, Towers, and Tension
Suspension bridges hang the deck from main cables held up by towers and anchorages. See how tension carries the load, using the Golden Gate Bridge as the example.
A suspension bridge carries its roadway from two enormous main cables that are strung over tall towers and anchored firmly into the ground at both ends. The deck hangs from the main cables on vertical suspender ropes. The cables are in tension, the towers are in compression, and the anchorages resist being pulled inward. Because steel is extremely strong in tension, this arrangement can span greater distances than beams, arches, or other bridge types: the bridges with the longest main spans in the world are suspension bridges.
The parts and how the load flows
Weight travels from the deck up the suspenders into the main cables. The cables carry it to the tops of the two towers, which push it down through their legs into foundations. The cables continue past the towers to the anchorages — massive concrete blocks, or in some bridges tunnels into rock — which hold the cable ends against the enormous pull. This chain turns a downward load on the roadway into tension in the cables and compression in the towers.
The shape of the cable is part of the design. A cable carrying only its own weight hangs in a curve called a catenary. A cable carrying a deck whose weight is spread evenly along its length is very close to a parabola. The cable's horizontal pull is set by the formula H = w × L² / (8 × d), where w is the deck weight per metre, L is the span, and d is the sag of the cable. A shallower sag means far greater tension, so designers usually choose a sag of roughly one-tenth of the span. This is the same family of problems as structural loads and beam bending, but here almost everything is stretched or squeezed rather than bent.
The Golden Gate Bridge as a worked example
The Golden Gate Bridge opened in San Francisco on 27 May 1937. Its main span is 1,280 metres, and for 27 years it was the longest suspension span in the world. Its towers rise about 227 metres above the water. Each of the two main cables is about 92 centimetres thick and is made of 27,572 individual steel wires, with a combined wire length of roughly 129,000 kilometres across both cables. Each cable is compacted from 61 strands, spun in place wire by wire — the method engineers use for most large suspension bridges.
Why does steel wire do the job?
Stress and strain govern the choice. Steel wire can be drawn to very high tensile strength, and a bundle of thousands of thin wires is tougher than a single thick rod because it avoids hidden flaws and allows inspection. The Brooklyn Bridge, completed in 1883 with a span of about 486 metres, was the first major suspension bridge to use steel wire for its cables, and it proved the concept at scale. Today the record belongs to the 1915 Çanakkale Bridge in Turkey, whose main span is 2,023 metres.
When bridges sway: aerodynamics and resonance
A long, light deck is flexible, so suspension bridges must be designed against wind and crowds. The Tacoma Narrows Bridge in Washington state collapsed on 7 November 1940 in a wind of only about 64 kilometres per hour. The cause was aeroelastic flutter: wind and deck motion fed each other, so each twist drew more energy from the airflow. Modern decks are stiffened with trusses or given streamlined box-shaped sections, and tested in wind tunnels before construction. Pedestrian effects also matter: the London Millennium Bridge was closed two days after opening in 2000 because walkers' footsteps synchronised with a small sway, a crowd dynamics problem fixed by installing dampers.
Suspension bridges grew out of the materials and ironworking of the industrial revolution, and they remain a clear example of engineering working with the strengths of its materials: put steel in tension, put concrete in compression, and let geometry do the rest.
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bridges civil engineering engineering structures tension