Types Of Drawbridges
There are many types of drawbridges that are either in use today or have been in the past. These range from floating pontoon bridges, bridges that lower themselves down into the water to allow boats to pass above and even a bridge with a single, narrow, hand removable plank that allows for only the masts of sailboats to pass through.
But the most common types found today can be divided into three main categories. These are the Swing Bridges, the Bascule Bridges and the Vertical Lift Bridges.
Swing Bridges
Swing Bridges pivot or turn on a center pivot bearing. These bridges are simple in design and generally operate with very few problems. They can be either symmetrical (balanced exactly in the center of the span) or asymmetrical (balanced off-center) and offer the advantage of allowing for two separate navigation channels if desired. Once open, they offer unlimited vertical clearance to vessels. They are susceptible to higher winds and are most often installed across relatively narrow channels.
Bascule Bridges
Bascule Bridges operate on the principle of the lever. The word “bascule” originates from the French word for a lever device which raises one end while lowering the other. Bascule Bridges can have either one or two spans depending on their design. Bascule Bridges are fairly straight forward in design and once open offer unlimited vertical clearance. They can also be susceptible to wind forces and are most often installed across narrow to medium sized channels.
Vertical Lift Bridges
Vertical Lift Bridges are constructed with two or more towers on either side of a long, movable span. Wire ropes are connected to the span and pass over huge sheaves or pulleys attached to or inside the towers. The other end of these wire ropes support a counterweight which lowers as the bridge span lifts and raises as the bridge span lowers.
These bridges are more complex in design but are less susceptible to wind forces and can span a fairly wide navigation channel. However, they also restrict vertical clearance to some degree even when fully open.
Somerset_Drawbridge_center.jpg used with permission by Pi3.124