Movable Bridge

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.[1] 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)[2] – 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;[3] temporary deviations are published in the Coast Guard's Local Notice to Mariners.[4]

Types of moving 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
    • 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 multiple sections that curl vertically
  • Vertical-lift bridge – the bridge deck is lifted by counterweighted cables mounted on towers
  • 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
  • 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

Visual index of moving bridges

Accidents

  • September 15, 1958 – Newark Bay, New Jersey rail accident, Elizabethport, New Jersey: Central Railroad of New Jersey (CNJ) commuter train #3314 from Bay Head Junction to Jersey City ran a stop signal and an open derail protecting the Newark Bay 4-span lift bridge, and the train's two diesel locomotives and two of five passenger cars went into Newark Bay through one of the open spans. Four crewmen, including the engineer and fireman, and 44 passengers died by drowning.
  • September 22, 1993 – Mobile, Alabama, USA: In heavy fog and low visibility, a disoriented towboat pilot made a wrong turn and entered an un-navigable waterway. Due to inexperience and improper radar training, the pilot did not realize he was off-course and struck an unfinished swing bridge over the Big Bayou Canot around 2:45AM, knocking it out of alignment by approximately three feet (one meter), although his initial supposition in the low visibility was that one of the barges had run aground. The rails kinked but did not break, so no fault was indicated for approaching trains. Approximately 8 minutes later, an Amtrak train carrying 220 passengers derailed at the kinked portion of the rails, killing 47 and injuring 103 more.
  • November 23, 1996 – Kearny, New Jersey, USA: An Amtrak passenger and mail train derailed while crossing the Portal Bridge over the Hackensack River, sideswiping another passenger train in the process. Thirty-four people were injured. A broken rail joint on one pair of the bridge's movable rails at each end of the span caused a track misalignment, while still making electrical contact with the landward rails; thus signals were clear, derails closed, and a fault indication was not displayed.

See also

References

External links

  • Moveable Bridges in the British Isles
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