One of the advantages of a draw bridge is lower cost to build. A draw bridge doesn't have the high piers and long approaches of conventional bridges. A disadvantage of a draw bridge is the long wait times as traffic is halted when the bridge is raised for passage.
Drawbridges helps big boats pass through low bridges. Though this does cause lots of traffic, it helps the boats.
A drawbridge allows pedestrians and vehicles to cross on the roadway over the bridge most of the time, but can be pulled up out of the way to allow a ship that is too tall to fit under the bridge to pass on the river underneath the bridge.
for ships to go under
For over 8,000 years ago they have been in use in some form.
Drawbridges are usually found on old castles over the moat.
The Aztecs used a combination of wood, stone, and ropes to build drawbridges. They would create a framework using wooden beams and stone supports, then use ropes to attach the bridge to a mechanism for raising and lowering it. These drawbridges were commonly used in their cities to span canals and waterways.
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Castles had drawbridges to allow some people inside, usually by providing a bridge to get over a moat or ditch. Drawbridges could be brought up to keep other people out, such as enemies, since they could not get over the moat or ditch without it.
Castles over a moat.
Hydraulic systems may be used to open and close drawbridges.
A drawbridge (bascule bridge) is utilized where the low clearance of a bridge over a waterway would not allow some ships and boats to pass beneath it. This is most commonly seen on active waterways where a bridge is desirable for traffic, but taller/larger ships and boats MUST pass through. Alternatives to drawbridges include vertical lift bridges, where the middle section can be raised high enough to allow ships beneath it, and swing bridges that can pivot in the center. Counter-weighted drawbridges, however, are more common, because they were usually cheaper and easier to construct. In many cities with high vehicular traffic, or for interstate highways, drawbridges have been replaced by modern high-rise bridges.
Drawbridges.
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