The circular form has several advantages:
1. Technically a circular form is easier to build because of the identical cross-section throughout and without the need for complex roof and wall corner construction. See also the section "Construction"
2. The circular form allows more economic use of material. Wood is more expensive to obtain, transport and work than clay. For each jian (building module) the outer rim of clay is longer than that of wood, which faces the courtyard. Further, a given amount of material gives a 41 % larger courtyard and approximately a 13 % larger building area in the circular than in the rectangular tulou.
Clay versus wood
Circular versus rectangular
3. A circular building has greater static stability. Analysis of the outer wall alone indicates that a cylindrical shell is more stable (ring and restraining moments).The cylindrical shell is further strengthened considerably by the rigid, horizontal and circular decks of each floor (membrane forces). If additional vertical elements are built, such as fire walls (as illustrated in the case of the Zhenchenglou) the rigidity and strength is further improved, as the cylinder surfaces are fixed in all four directions. (Ole Vanggaard, engineer and lecturer at The Royal Academy of Fine Arts, School of Architecture, in Copenhagen, kindly analysed the tulou's static stability for this article).
Static stability
4.The circular tulou has a more uniform room division - As the main source of light is from the courtyard, a corner room would be poorly lit and without adequate ventilation.
5. Local superstition holds that evil spirits are everywhere, especially along roads and in brooks, streams and mountain passes. Every corner in a rectangular building is an opportunity for evil spirits to enter the building as the circular tulou have no corners, spirits are more likely to pass by (Huang Hanmin in Echo (ref: note 12)).
No contemporary sources explain why circular tulou were built. Most counties in China have ‘local gazetteers’ spanning hundreds of years but very few contain information regarding local building costums.
The local history for this part of the country does not contain any interesting information regarding the locality’s architecture and house form (Poul Andersen).
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