A drawbridge is commonly used for all types of moveable bridges, including turning bridges, swing bridges, vertical-lift bridges, or bascule bridges.However, in the earlier days, drawbridges were used as a defense around the medieval period wherein a ditch or moat surrounded early castles, and a moving wooden bridge was used to walk across. This drawbridge was designed to be removed or destroyed during any eventuality.Drawbridges can also be called lift-bridge, turning bridge, swing-bridge, folding bridge, rolling bridge battlement, or machicolation. There are more than 615,000 bridges across the United States. More than 7.5% of the nation’s bridges are structurally in poor condition, and 42% of the bridges are over 50 years old.On average, more than 188 million trips are happening across these bridges every day. Chicago has the most of any drawbridges in North America and has one of the best design varieties on Earth. Chicago is surpassed only by Amsterdam as the city with the most drawbridges globally. The longest drawbridge is the Rethe Bascule Bridge in Hamburg, Germany, and this combined road and rail bridge spans about 308 ft (94 m) wide.Meaning Of Drawbridges A drawbridge is actually a bridge that can be raised so that people cannot cross it, or it allows ship or land vehicles to pass under it. In simple terms, it is a bridge that moves up and down to allow boats or vehicles to pass. In the earlier days, drawbridges were used to protect medieval castles from possible attacks or to let ships go underneath them. Invention Of DrawbridgesThe drawbridges are believed to have been built around 4,000 years ago, and one such is the fortress of Quban in ancient Egypt. Another drawbridge was found about 2,600 years ago in the Chaldean Kingdom of the Middle East. However, these bridges were not commonly used until the European Middle Ages.The first bridge was constructed in 1834 and was torn down in 1839. The primary material used in making drawbridges has been wood, stone, iron, and concrete. The other material included is aluminum, rubber, and other joint material. On modern drawbridges, iron had the most significant effect as steel was made from iron and used in reinforced concrete.Purpose Of DrawbridgesLike all other bridges, drawbridges allow people to cross large water bodies. However, the drawbridges move, and river traffic is not disrupted and can flow easily.The medieval drawbridge designs were simple and were handled manually. These bridges were made to enable inhabitants of medieval castles to enter and exit the castle easily. The drawbridges in those days were used to protect the castle from the attacks of the enemies and to avert the weapons and prevent them from reaching the castle walls and the gate passage.The castle drawbridges were typically designed outside a gatehouse wall of a medieval castle. It consisted of a wooden bridge deck with an edge hinged or pivoting at the gatehouse threshold. The raised bridge will act as an additional barrier to entry and be backed with more gates. This arrangement made it more difficult for the invaders to enter the castle drawbridge and reach the gate passage. The only option left for the invaders was to stop the food supply inside the castle, starve the occupants and force them into submission.Features And Uses Of DrawbridgesThe two general types of bridges are fixed and movable bridges.The fixed bridge can be either a light bridge or a heavy bridge, further classified into its basic structure: beams, trusses, arches, grinder, suspension, and cable-stayed. Steel is obviously one of the primary materials used in the construction of bridges.Concrete beams are used in the construction of small bridges. When there is difficulty in placing piers, steel suspension bridges are built wherein the decks are suspended from the cables and towers. The St. John’s Bridge is an ideal example of a steel suspension bridge. A movable drawbridge works differently; it’s slightly complex in structure and requires machines to open a part of the bridge to enable the ships and traffic to move through the bridge. Whenever there is less clearance for construction and insufficient space to build a large structure, movable bridges are preferred, including the vertical lift, swing, and bascule bridge.Much like an elevator, cables, pulleys, motors, and counterweights are used in a vertical lift bridge to raise a bridge section. One of the oldest of such a bridge is the Hawthorne Bridge in Portland, Oregon. It allows vessels to pass underneath, and the main span can be lifted to a raised position level to 110 ft (33.5 m).With the help of motors, gears, and counterweight, the bascule bridge’s span moves upward. There is no vertical obstacle to river traffic when opened, like the vertical lift bridges. The Broadway Bridge is an example of the Rall-type, double-leaf bascule bridge, wherein the bridge’s machinery is placed above the structure. An example of a Strauss-type, double-leaf bascule bridge is the Burnside Bridge; the machinery is well protected from weather and found in the large pier structures.The swing bridges are rare nowadays and were once commonly used to allow the river traffic to pass at crossings too wide for a drawbridge. Vessels move past from either side of the bridge. The only swing bridge operating in Oregon’s highway system is the Umpqua River Bridge in Reedsport. In the Castle Drawbridge, the moving deck is affixed to the counterweights and steel cables on towers at the end of the bridge. As soon as the drawbridge mechanism is activated, the middle section is pulled upward, allowing the ship to pass underneath.Sound signals are used to alert the opening and closing of the drawbridges. A prolonged blast using a horn for four to six seconds in duration, followed by one short blast for a second.The bridge tender will acknowledge the same sound signal, confirming that the bridge can be opened immediately. Whenever a drawbridge signal is red, you have to proceed with caution until you see a boat and stop completely at the marked stop line.However, the bridge is in operation for motor vehicle traffic and pedestrian, and the roadway is closed. The operator of the moving bridge balances the leaves in a vertical position. When a ship has to pass below the weights shift allowing the leaves to rise as soon as the operator activates the system.
A drawbridge is commonly used for all types of moveable bridges, including turning bridges, swing bridges, vertical-lift bridges, or bascule bridges.
