BingBumpier
BingBumpier
Every bridge solves the same problem in a different way — how to carry a load across a gap without sagging into it. The five or six major structural types are essentially five or six different answers to that question, each with strengths that make it cheaper or more practical for a specific span length.
The simplest possible bridge: a horizontal beam supported at both ends. Logs across a creek. Most highway overpasses you drive under. They are cheap and quick to build but cannot span very far without supports underneath, because the beam has to bend less than its strength allows. Practical limit is about 250 feet for steel girders, less for concrete.
An arch redirects the downward force of the load into outward compression along the curve, which is taken up by the abutments at each end. Stone and brick arches built by Romans and medieval engineers still stand because compression is what stone is good at. Modern concrete and steel arches can span several thousand feet — the longest is over a third of a mile.
A network of triangles, usually of steel, that distributes load across many small members. Triangles are rigid because their shape cannot deform without changing the length of a side, which is why trusses are so efficient at handling variable loads. Most older railroad bridges are trusses. Practical span up to about a thousand feet.
The deck hangs from vertical cables, which hang from main cables that drape between two tall towers and anchor into the ground at each end. The towers carry compression, the cables carry tension, and the deck just floats. Suspension bridges hold the records for the longest spans — the Akashi Kaikyō Bridge in Japan spans 6,532 feet between towers.
A close cousin of suspension bridges but with the cables running directly from the deck to the towers rather than draping. They are more rigid, faster to build, and have become the dominant choice for new long-span bridges in the last forty years. The Russky Bridge in Vladivostok holds the cable-stayed span record at about 3,622 feet.