How do suspension bridges hold up their weight?

A suspension bridge hangs its roadway from large steel cables rather than propping it up from below. The cables are in tension, meaning they are pulled tight. They rest on tall towers and are fixed at each end in anchorages. Weight travels from the deck to the cables, then to the towers and anchorages, and finally into the ground. Britannica describes the structure as an upside-down arch in tension, with only the towers in compression. [1] [2]

The Golden Gate Bridge's own explanation calls this route the load path. Traffic and the roadway weigh on the deck trusses. These are not strong enough to span the whole strait, so vertical suspender ropes, spaced every 50 feet, hold them up. The ropes pull the weight up to the curving main cables. The cables place all of it on the tops of the two towers, and the towers carry it down to concrete foundations that extend into bedrock. [3]

The Golden Gate guide compares the towers to a backpack wearer's body. The straps lift the weight to the shoulders, and the load then has to travel through the legs to the ground. [3]

Two kinds of force are at work. The towers are squeezed in compression and support most of the bridge's weight. The cables are stretched in tension, and that pull is passed to the anchorages. Anchorages are massive concrete blocks or solid rock. Inside them the cables spread out into separate strands, which distributes the load evenly and keeps the cables from pulling free. The anchorages resist the inward pull of the cables. [4] [5] [6] [1]

Steel suits the cables because it is very strong under tension. NOVA notes that a single steel wire only 0.1 inch thick can support over half a ton. Modern cables are made of thousands of such wires bound tightly together. Earlier bridges used iron chains, and some of the earliest cables were twisted grass. [2]

Because the deck hangs in the air, it can move under load. Britannica says the deck must therefore be heavy, stiff, or both. Most suspension bridges add a deck truss under the roadway, which stiffens it and reduces swaying and rippling. Deflection theory, first published in 1888 by Josef Melan, has been used in design since the early 20th century to calculate how the deck and cables deflect together under load. [1] [5]

Wind is the main challenge, since these bridges are light and flexible and move both up and down and sideways. Engineers now take special measures to ensure aerodynamic stability. The 1940 Tacoma Narrows Bridge is the most famous example of aerodynamic instability in a suspension bridge. [6]