Moveable bridges are frequently used over navigable waterways, where it is either impractical or too expensive to provide a bridge with sufficient vertical clearance over the water to allow the passage of shipping. Such bridges tend to be either lift or swing bridges, although there are other types.
Minimum self-weight is important as it reduces the size of the counterweights and the capacity of the mechanical plant required to open and close the structure. Hence, steel is ideal for the decks of moveable bridges.
The Ennerdale bridges (shown left) comprise a pair of single leaf bascule spans, one for each carriageway. Each deck weighs 400T and has a 300T counterweight.
The Novi Sad cable-stayed footbridge in Norwich rotates in plan about the central pier to allow river traffic through. The mechanical plant is hidden in the central support, and the shorter back span contains steel ingot counter-weights.
This is an example of a double leaf bascule bridge, in the port of Barcelona. Each leaf cantilevers 55m out from the trunnion, and is opened by twin hydraulic pistons located in the abutments (shown right).
This example is from Manchester, and is a vertical lift bridge over the ship canal at Trafford Park. The 600T deck rises 15m to allow the passage of shipping. The reason for specifying a lift bridge in this urban setting was the lack of space for approach embankments to get the road up and over the canal with a 15m vertical clearance.
|The Gateshead Millenium Bridge is perhaps unique in the way it opens. Being neither a lift nor a swing bridge, it opens by rotating and the movement has been likened to the blinking of an eye. This 125m long footbridge over the River Tyne links Gateshead with Newcastle comprises a pair of arches (one forming the deck and one supporting it) which pivot around their common springings. The bridge is opened by hydraulic rams housed in the supports.|
|The Yumemai Bridge in Japan is a 410m long floating arch bridge supported on two massive pontoons (58m x 58m x 8m). This structure opens by rotating through 90 degrees in plan, pivoting about one end. The reason for using a floating low level bridge was that the ground conditions were poor (reclaimed land) and precluded a fixed high level bridge or tunnel (shown right).|