drawbridge

This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (April 2007)

Simple animation of a drawbridge in operation

A drawbridge is a type of movable bridge typically associated with the entrance of a castle surrounded by a moat. The term is often used to describe all different types of movable bridges, like bascule bridges and lift bridges.

Castle drawbridges

Drawbridge at the fort of Ponta da Bandeira; Lagos, Portugal

Medieval castles were usually defended by a ditch or moat, crossed by wooden bridge. In early castles the bridge might be designed to be destroyed or removed in the event of an attack, but drawbridges became very common. a typical arrangement would have the drawbridge immediately outside a gatehouse, consisting of a wooden deck with one edge hinged or pivoting at the gatehouse threshold, so that in the raised position the bridge would be flush against the gate,forming an additional barrier to entry. It would be backed by one or more portcullises and gates. Access to the bridge could be resisted with missiles from machicolations above or arrow slits in flanking towers.

The bridge would be raised or lowered using ropes or chains attached to a windlass in a chamber in the gatehouse above the gate-passage. Only a very light bridge could be raised in this way without any form of counterweight, so some form of bascule arrangement is normally found. The bridge may extend into the gate-passage beyond the pivot point, either over a pit into which the internal portion can swing (providing a further obstacle to attack), or in the form of counterweighted beams that drop into slots in the floor.

A bridge pivoted on central trunnions is called a turning bridge, and may or may not have the raising chains characteristic of a drawbridge. The inner end carried counterweights enabling it to sink into a pit in the gate-passage, and when horizontal the bridge would often be supported by stout pegs inserted through the side walls. This was clumsy arrangement, and many turning bridges were replaced with more advanced drawbridges.^[1]

The raising chains could themselves be attached to counterweights; in some cases a portcullis provides the weight, as at Alnwick. By the 14th Century a bascule arrangement was provided by lifting arms (called "gaffs") above and parallel to the bridge deck whose ends were linked by chains to the lifting end of the bridge; in the raised position the gaffs would fit into slots in the gatehouse wall ("rainures") which can often still be seen, as at Herstmonceux Castle. Inside the castle the gaffs were extended to bear counterweights, or might form the side-timbers of a stout gate which would be against the roof of the gate-passage when the drawbridge was down, but would close against the gate-arch as the bridge was raised.^[2]

Road, rail and canal

Drawbridge carrying a footpath over the Northampton Arm of the Grand Union Canal, England. The bridge can easily be moved by a light pull on the counterweighted chains.

With cars and trains in need of crossing small waterways without blocking boats from passing, drawbridges have a more peaceful reason for their existence. They also act as a road for cars which is convenient for both the cars and boats.

In Queensland, diamond crossings between narrow gauge cane tramways and main lines are being replaced by drawbridges, so that the rails of the main line are completely unbroken by gaps or weak spots. This allows the main line speeds to be raised.

Another rail use of drawbridges was on the Listowel and Ballybunion railway in Ireland. This was a sort of monorail, where trains hung on either side of a fence-like track. Road crossings were either a gate-like Stuart section of track or twin drawbridges, that allowed crossing at rail level.

Simple drawbridges may be found in short canal crossings, where the lifting mechanism is a pair of overhead beams with counterweights that are not a part of the bridge's load bearing structure.

Accidents

• April 23, 1853 – Rancocas Creek, New Jersey: Engineer of Camden & Amboy's 2 p.m. train out of Philadelphia, Pennsylvania misses stop signals (SPAD) and runs his train off of an open drawspan at Rancocas Creek. Fortunately, there are no fatalities.

See also

• List of rail accidents (pre-1950)

Gallery

Drawbridge crossing fortification ditches at Fort Ticonderoga	The famous Magere Brug (a double drawbridge) in Amsterdam	Another drawbridge in Amsterdam	USS Lake Erie, a Ticonderoga-class guided missile cruiser, passes under the Broadway Bridge on the Willamette River
Drawbridge, painted by Vincent van Gogh	Drawbridge at Doornenburg Castle	Wooden drawbridge in Amsterdam	Drawbridge connecting James Island (South Carolina) to the mainland
Drawbridge at Gloucester docks

References