Concrete arch bridge

Constructing an arch bridge can be tricky, since the structure is completely unstable until the two spans meet in the middle. The method of construction of precast concrete arches is similar to the method of construction used during the Roman period. Wedge shaped precast concrete segments are used as voussoirs and these are built up side by side on temporary supports. There are two basic methods of providing the temporary support system: the provision of a supporting system below the bridge and the use of a tower and stay system. Gladesville Bridge in Sidney, Australia was constructed using hollow precast concrete segments. They were placed on falsework supported from the ground. The false work was designed to accommodate a 80-foot 200-foot navigation channel. A diagrammatic representation of the construction of an arch using a tower and a stay system is shown in the figure below.

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In-situ archers are generally cast using shuttering supported the from ground. A notable cast in-situ reinforced concrete bridge is the Plouggastel Bridge in France. Freyssinet used a shuttering system which was floated into position and supported off the river bed.

One of the most revolutionary arch bridges in recent years is the Natchez Trace Bridge in Franklin, Tennessee, which was opened to traffic in 1994. It's the first American arch bridge to be constructed from segments of pre-cast concrete, a highly economical material. Two graceful arches support the roadway above. Usually arch bridges employ vertical supports called "spandrels" to distribute the weight of the roadway to the arch below, but the Natchez Trace Bridge was designed without spandrels to create a more open and aesthetically pleasing appearance. As a result, most of the live load is resting on the crowns of the two arches, which have been slightly flattened to better carry it. Already the winner of many awards, the bridge is expected to influence bridge design for years to come.