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How do suspension bridges work?

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Quick answer

A suspension bridge holds its deck from above using main cables that hang in a curve between towers and are anchored at both ends. The deck's weight pulls the cables into tension, and the towers pass that load down to the ground, allowing long spans with few supports.

A suspension bridge carries its deck on cables that hang in a curve between tall towers and are anchored at both ends. The deck's weight pulls the cables into tension, and that tension passes through the towers into the ground, letting the span cross wide gaps without many supports.

Step by step

  1. Build tall towers on firm foundations on each side of the gap, or in the water if needed.
  2. Stretch a pilot cable across the gap, then use it to pull thicker main cables made of many steel wires back and forth.
  3. Anchor the ends of the main cables into massive concrete or rock anchorages buried in the ground.
  4. Hang vertical suspender cables from the main cables down to the deck sections.
  5. Lift and attach the deck pieces to the suspenders, starting from the towers and working outward.
  6. Add stiffening trusses or girders to the deck so it resists wind and traffic loads without twisting.

Key things to understand

  1. Two or more towers rise from foundations in the water or on the banks, holding the main cables high above the deck.
  2. The main cables are made of thousands of thin steel wires spun together, giving great strength and flexibility.
  3. Each cable hangs in a smooth curve close to a parabola, and its ends are tied to massive anchorages buried in the ground or rock.
  4. Vertical suspender cables drop from the main cables to hold up the stiffening deck, which carries traffic and resists wind and movement.
  5. The deck's weight pulls the main cables, putting them in tension; the towers carry that load in compression down to their foundations.

Labelled diagram

  1. Anchorage

    A massive concrete or rock block that holds the end of the main cable firmly.

  2. Tower

    A tall vertical structure that supports the main cables and carries load to the ground.

  3. Main cable

    A thick steel cable that hangs in a curve between towers and is under tension.

  4. Suspender cable

    A vertical cable that hangs from the main cable and holds up the deck.

  5. Deck

    The roadway or walkway that carries traffic and is supported by the suspenders.

How a suspension bridge carries load

Real-world examples

  • The Golden Gate Bridge in San Francisco uses two main cables, each made of about 27,000 wires, to span the Golden Gate Strait.
  • The Akashi Kaikyo Bridge in Japan, with a central span of about 1,990 metres, crosses a busy shipping lane in an earthquake zone.
  • The Humber Bridge in England connects two banks of the Humber Estuary with a single span of over 1,400 metres.
  • The Brooklyn Bridge in New York combines suspension cables with a web of stays, an early and influential design.

Common mistake

✗ Myth: The towers hold up the deck directly.

✓ Fact: The towers support the main cables, and the main cables hold the deck via vertical suspenders. The deck's weight pulls the main cables, which pull on the anchorages; the towers mainly carry the vertical load down to the foundations.

People also ask

What is a suspension bridge made of?
The main cables are made of thousands of high-strength steel wires bundled together. The towers are usually steel or concrete. The deck may be a stiffening truss of steel or a box girder, and the anchorages are massive concrete or rock structures. Suspenders are thinner steel cables.
How does a suspension bridge hold up cars?
The weight of the cars pushes down on the deck. The deck pulls down on the vertical suspender cables, which pull down on the main cables. The main cables, anchored at the ends, resist by going into tension. Where the cables pass over the towers, that tension presses down, and the towers push down on their foundations.
Why do suspension bridges sway in the wind?
Wind pushes on the deck and can create swirling vortices that make it twist or bounce. If the deck is not stiff enough, the motion can build up. Engineers add stiffening trusses, streamline the deck shape, and sometimes leave gaps to let wind pass through, reducing sway.
What keeps a suspension bridge from collapsing?
The main cables are anchored at both ends into solid rock or huge concrete blocks. The deck's weight pulls the cables tight, and the cables pull back, transferring the load through the towers to the ground. The whole system is in balance: tension in the cables, compression in the towers.
Why are suspension bridges good for long spans?
They can span long distances because the deck is supported by cables from above rather than by piers from below. This reduces the number of supports needed in deep water or steep valleys. The cables carry the load in tension, which is an efficient way to use steel.
How do suspension bridges handle wind?
Wind can cause the deck to twist or sway, so engineers add stiffening trusses or box girders to the deck and sometimes open gaps in the deck to let air through. They also test models in wind tunnels. Modern designs avoid the collapse that happened to the Tacoma Narrows Bridge in 1940.
What is the difference between a suspension bridge and a cable-stayed bridge?
In a suspension bridge, the main cables hang in a curve between towers and are anchored at the ends, with vertical suspenders holding the deck. In a cable-stayed bridge, the cables run straight from the towers directly to the deck, and the deck's weight is carried by the towers alone, without large end anchorages.

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