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A moveable bridge, or movable bridge (common alternative spelling in American English), is a bridge that moves to allow passage (usually) for boats or barges.〔Schneider, C.C. (1907) (Movable Bridges ), Proceeding of the American Society of Civil Engineers, Volume 33, Part 1, Page 154.〕 In American English, moveable bridge and drawbridge are synonymous, and the latter is the common term, but drawbridge can be limited to the narrower, historical definition used in some other forms of English, in which ''drawbridge'' refers only to a specific type of moveable bridge. An advantage of making bridges moveable is the lower cost, due to the absence of high piers and long approaches. The principal disadvantage is that the traffic on the bridge must be halted when it is opened for passages. For seldom-used railroad bridges over busy channels, the bridge may be left open and then closed for train passages. For small bridges, bridge movement may be enabled without the need for an engine. Some bridges are operated by the users, especially those with a boat, others by a bridgeman (or bridge tender); a few remotely using video-cameras and loudspeakers. Generally, the bridges are powered by electric motors, whether operating winches, gearing, or hydraulic pistons. While moveable bridges in their entirety may be quite long, the length of the moveable portion is restricted by engineering and cost considerations to a few hundred feet. There are often traffic lights for the road and water traffic, and moving barriers for the road traffic. In the United States, regulations governing the operation of moveable bridges (referred to as ''drawbridges'') – for example, hours of operation and how much advance notice must be given by water traffic – are listed in Title 33 of the Code of Federal Regulations; temporary deviations are published in the Coast Guard's Local Notice to Mariners. ==Types of movable bridges== *Drawbridge (British English definition) – the bridge deck is hinged on one end *Bascule bridge – a drawbridge hinged on pins with a counterweight to facilitate raising ; road or rail * *Rolling bascule bridge – an unhinged drawbridge lifted by the rolling of a large gear segment along a horizontal rack *Folding bridge – a drawbridge with multiple sections that collapse together horizontally *Curling bridge – a drawbridge with transverse divisions between multiple sections that curl vertically *Fan Bridge - a drawbridge with longitudinal divisions between multiple bascule sections that rise to various angles of elevation, forming a fan arrangement. *Vertical-lift bridge – the bridge deck is lifted by counterweighted cables mounted on towers ; road or rail *Table bridge – a lift bridge with the lifting mechanism mounted underneath it *Retractable bridge (Thrust bridge) – the bridge deck is retracted to one side *Submersible bridge – also called a ducking bridge, the bridge deck is lowered down into the water *Tilt bridge – the bridge deck, which is curved and pivoted at each end, is lifted at an angle *Swing bridge – the bridge deck rotates around a fixed point, usually at the centre, but may resemble a gate in its operation ; road or rail *Transporter bridge – a structure high above carries a suspended, ferry-like structure *Jet bridge – a passenger bridge to an airplane. One end is mobile with height, yaw, and tilt adjustments on the outboard end 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「moveable bridge」の詳細全文を読む スポンサード リンク
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