|The watch tower 角樓 (jiǎolóu) on the southeast corner of the Inner City walls of the Forbidden City is one of four. These watch towers were built during the reign of Yongle of the Ming Dynasty and were part of the original defenses. There is a watch tower at each of the four corners of the palace. They were renovated during the Qing Dynasty, but the original design was maintained.
Take a careful look at the complexity of the construction and imagine the tools and measurement devices that existed in 1420. There are 9 girders, 18 posts and, 72 ridges on each of the four watch towers. The construction is based on mortise and tenon and interlocking notched supports. It required a small army of master craftsmen to complete the roofs. The story is that the inspiration for the design came from an elaborate bird cage constructed by a traveling carpenter. To construct such a roof in soft bamboo to make a bird cage is one thing, to transfer that design to a weight carrying structure is another.
Below, there are two pictures of a children's puzzle that shows how the beams of the traditional Chinese structure are locked together. Perhaps such toys were commonly used in ancient times to transfer the concept of the construction from one generation to the next.
The basic joint requires six members. The first picture shows the six and the second show the finished joint. The joint can flex in any direction so it is particularly stable during an earthquake. If the wood is properly seasoned before construction, the joint should remain tight for hundreds of years. Loose joints can be strengthened by adding wedges, but the wedges have to be balanced across the joint.
Last update: January 2010
© Marilyn Shea, 2009