The folding bridges can be raised to allow ships to pass under them.
A collapsible bridge is a type of movable bridge designed to collapse to accommodate ship traffic that needs to move across the waterway the bridge spans. 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. The design of this bridge requires complicated and careful engineering, as the bridge must be designed to handle the stress of common use, as well as the stresses involved in bending.
Folding bridges can be used to accommodate pedestrian or vehicle traffic. Metal is a common construction material for these bridges because it is durable and very strong, and the movement of the bridge is normally controlled by hydraulic systems. In some designs the bridge is basically left down except when it needs to be raised, and in others the bridge is kept retracted until needed. The decision on which position to keep the bridge in is determined by the needs of the traffic in the area.
In a basic folding bridge, the hinged segments simply fold down, pulling to one side to allow traffic to pass. Other folding bridges actually roll up, in an intriguing variation on the traditional design. Curved bridges are made of several hinged segments that can be threaded together in a roughly circular shape, starting on one side of the bridge until the curved bridge reaches the shore. This visually interesting design is popular for folding pedestrian bridges.
An advantage of the folding bridge is that it can potentially be portable. “Portable” is a relative term, as vehicles would be needed to transport the bridge into place. Portability can be valuable in military applications, where advancing armies may want to bring their own bridges to make sure they don’t stop at waterways. Portability can also be helpful when a temporary bridge repair or replacement is required, and the portable folding bridge is moved into place for people to use until the new or repaired bridge is approved for use.
Movable bridges in general tend to be attractive as most people are used to fixed bridges and may find it interesting to watch a bridge move. The folding bridge stands out because its mode of movement is very different from that of more traditional mobile bridges, such as drawbridges and swing bridges.