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In: Engineering
Short answer: very carefully.
Long answer: it really depends on what kind of bridge. The big truss bridges you see are either designed to be structurally-sound while being built, meaning they are self-supporting structures all during construction and then when finished. Other truss bridges need temporary shoring/scaffolding. Other truss bridges are built on land and then rolled into place, or built on land, floated into place, and lifted into place by a crane.
Suspension bridges have the towers built first. Then they slowly string the main cables over the towers one wire at a time until the main cables are done. Then they slowly hang the deck structure one beam at a time.
Cable stay bridges are similar to suspension bridges. The tower is built first, then section of deck are installed and cabled into place.
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