A drawbridge is a type of bridge usually made from heavy timber or some other type of wood. Typically, it is constructed over a moat or a ditch. It can easily be raised or lowered in order to let people pass through. In years past, a drawbridge was raised in order to keep the enemy away. In this case, they were usually used to protect castle gates.
In the early years, there were three types to choose from. The traditional drawbridge consisted of a sliding platform that could pull back and forth. This also featured a raised bridge, which was operated by chains and attached to the corners. It also included a bridge with posts that pointed upwards and had hanging chains.
A bascule drawbridge and a lift drawbridge are two other options. The bascule type utilizes a counterweight and allows boats to pass through. It was officially invented in the 1850s after engineers tweaked the original drawbridge design. The first was constructed in Saint Petersburg, Neva.
The most famous drawbridge in the world is the Tower Bridge, which runs across the River Thames in London. At first, the Tower Bridge was operated by steam powered from coal-burning boilers. Today, oil and electricity power the Tower Bridge.
A lift drawbridge is commonly called a vertical lift drawbridge. It gets this name because it rises vertically, though still remaining in a parallel position with the deck. It rises with the help of counterweights, which must weigh more than what is to be lifted. This type is constructed from steel and is typically used to lift automobiles and trucks. It is also used for light and heavy railroad work.
The vertical lift is less expensive to construct then the bascule type. The only disadvantage of a lift drawbridge is height restriction. Since the height is restricted, the passageway hoist used with the bridge is generally far above the platform base.