Most suspension bridge towers are composed of steel, however a handful are made of steel-reinforced concrete. The primary structural material for the deck is usually steel, although some longer-span bridges have been built with heavier materials such as cast iron or even wood. A suspension bridge's cables are its most vital component; without them, the structure cannot support itself. Cable tensioners used on modern bridges attach to the cable and pull it evenly across each tower. On older bridges, a person would walk along the roadbed near one tower and adjust a rope attached to the other tower to keep it equal in length so that there was no slack in the cable when it crossed over small obstructions like ditches or fence lines.
The term "bridge" comes from the French word "ceinture", which means "belt" or "girdle". Bridges were originally designed to protect roads against river floods and later used as military defenses against invaders. Today they are used for many other purposes including vehicles, pedestrians, and rail traffic. There are several different types of bridges, depending on their use and design philosophy. Within each type of bridge there can be subtypes depending on how the bridge is constructed and supported.
The steel wires that compose the cables on certain suspension bridges have been galvanized (coated with zinc). Galvanized wire has several advantages over plain wire: It's more corrosion resistant, it's easier to work with when constructing the frame or rigging of the bridge, and it's less likely to kink or break.
Only one major bridge in Canada is made up exclusively of concrete: The Jacques-Cartier Bridge connects Montreal and Pointe-Claire, Quebec. It opened to traffic in 2007 after being completed two years earlier.
You may have seen photos or videos of the Golden Gate Bridge online. It's a suspension bridge that crosses the San Francisco Bay between California and is one of the most photographed landmarks in the United States. It was built from 1936 to 1990 and holds the title of world's longest wooden suspension bridge structure. The main part of the bridge consists of three separate sections each weighing approximately 14 million pounds, connected by two large vertical suspenders attached to both sides of the bay. Each section is made up of four longitudial boards laid side by side with the grain running vertically, tied at the top and bottom with horizontal members and connected by vertical posts.
When a bridge over a body of water requires piers, foundations are formed by lowering caissons into the riverbed and filling them with concrete. Towers are erected atop caissons in the case of suspension bridges. The initial suspension bridge towers were made of stone, but they are now made of steel or concrete.
Bridges are built according to plans provided by civil engineers. The process begins with determining the required load-bearing capacity of the structure. This information is used to select the proper material for the construction (steel or concrete for example). Next, the engineer will design the layout of the reinforcement within the material (roof loads, vertical forces etc.). Then, actual samples of the chosen material are constructed in the laboratory and tested to determine their strength. Last, when all the details are agreed upon, a contract is signed with a builder who has access to the necessary equipment and labor force to complete the project.
In conclusion, bridges are built by contractors who have access to heavy equipment such as cranes and bulldozers.
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