A folding bridge is a type of movable bridge which is designed to fold up to accommodate passing ship traffic which needs to move through the waterway spanned by the bridge. Folding bridges can be found in many regions of the world, with a number of variations on the folding design, some of which are quite creative. This bridge design requires some tricky and careful engineering, as the bridge must be designed to deal with the strain of ordinary use as well as the stresses involved in being folded.
Folding bridges can be used to accommodate pedestrian or vehicle traffic. Metal is a common construction material for such bridges because it is durable and very sturdy, and the movement of the bridge is typically controlled by hydraulic systems. In some designs, the bridge is primarily left down except when it needs to be raised, and in others, the bridge is kept retracted until it is needed. The decision about which position to keep the bridge in is determined by the needs of the traffic in the area.
In a basic folding bridge, hinged segments simply fold together, pulling to one side to allow traffic through. Other folding bridges actually curl up, in an intriguing variation on the traditional design. Curl-up bridges are made from a number of hinged segments which can roll themselves into a roughly circular shape, starting on one side of the bridge until the curled bridge reaches the shore. This visually interesting design is popular for pedestrian folding bridges.
One advantage to the folding bridge is that it can potentially be portable. “Portable” is a relative term, as vehicles would be needed to transport the bridge and move it into position. Portability can be valuable in military applications, where advancing armies might want to bring their own bridges to ensure that they are not stopped at waterways. Portability can also be useful when a temporary bridge repair or replacement is required, with the portable folding bridge being moved into place for people to use until the new or repaired bridge is approved for use.
Movable bridges in general tend to be eye-catching, as most people are accustomed to stationary bridges, and they may find the movement of a bridge interesting to watch. The folding bridge stands out, as their mode of movement is very different from that of more traditional movable bridges such as drawbridges and swinging bridges.