A moon bridge, which is sometimes called a drum bridge, is a decorative pedestrian structure that is often placed over water, giving it the basic form of a bridge. This type of bridge can be made from any number of materials and is defined, not by the way it bears weight, but by the tall, rounded shape under the bridge. The half-circle shape is designed in such a way that when water is under the bridge it is reflected into a full circle, giving the appearance of a full moon or a drum. In many areas, this type of bridge is purely decorative, but at one time constructing a moon bridge was an excellent way to allow boat traffic beneath the bridge and foot traffic above without requiring a gradual approach for the bridge.
Most people associate this type of bridge with Japan, where the design originated. It is a bridge design well suited to narrow flows of water that might be found in canals or decorative gardens. In most cases, the ascent and descent used to cross the bridge are quite steep, and therefore it is not practical to make this bridge too large. Sometimes, the bridge is so steep that it must be climbed like a ladder using rungs affixed to the bridge itself. This type of bridge is therefore not often wheelchair accessible, and alternative accommodations must be made.
There are two basic types of moon bridges relating to how the bridge is designed. A beam bridge design does not require a long support on either side and can be created in the appropriate shape of an arch. Arch bridge versions of the moon bridge, on the other hand, do require longer approaches in order to remain stable. Depending on the situation, either design may be appropriate.
In certain cases, a bridge might bear the distinctive half-circle design of a moon bridge but lack the climbing aspect. For many people, these are still considered moon bridges because of their appearance. Others believe that this design should be restricted to those bridges that are highly steep.
The visual effect of this bridge makes it a popular design for gardens, particularly those with water or small paths. Even though most people associate this type of bridge with Japan, it is also frequently seen in other styles of Asian gardens. Depending on the decorations and embellishments used on the bridge as well as the materials from which the bridge is made, a moon bridge can look quite different from the traditional steep Japanese design.