Moving Bridges
Often bridges need to be able to move to allow other things—often ships—to pass by or underneath them. There are three main types of moving bridges, the bascule, lift and swing bridges. A fourth special kind of bridge is used to move things across a barrier such as a river using moving platforms; this is known as a transport bridge.
The obvious advantage of these bridges is that they allow the bridge to be moved out of the way. Unfortunately this takes time which delays the people that had been using the bridge since they have to wait for it to be moved, the traffic to pass and for the bridge to be returned to its normal position.
These types of bridges work in much the same way as drawbridges work in ancient castles. One side of the bridge has very heavy weights, which force the lighter side up or let it down when motors shift the weights.
Two vertical towers work together like two people holding a table up to let someone pass below. They are usually quite ugly, but are efficient because they can be raised on lowered quite quickly to allow ships to pass. This means that people do not have to wait as long to use the bridge again.
Swing bridges rotate in place at a central area to twist the bridge out of the way. The problem is that the bridge and its turning machines take up space in the middle of the river.
This special kind of bridge are usually very long and turned out to not be very popular. Two large towers support a large beam between them which holds up a moveable platform. These "aerial ferries" were faster than a normal ferry that traveled on the water, and could still be used in bad weather that made the ships unsafe.
The only problem was is that they were built before cars and trucks became so popular, and could not handle the amount of vehicles trying to cross.
See also arch, beam, cable-stayed, suspension and truss bridges.