Stability of the “Tensyudai” of a Japanese castle under long-term load

Tensyudai is the term used for the truncated masonry base that supports the main tower (Tensyukaku) of a Japanese castle. In response to concerns regarding the structural integrity and conservation of Tensyudai masonry walls, this study seeks to investigate the mechanism of their collapse and load capacity. A reduced scale model of Tensyudai, based on Osaka Castle, was constructed and subjected to a vertical load test to evaluate its three-dimensional mechanical properties. The paper postulates a collapse mechanism, and the equilibrium of forces for the Sumiishi and Hiraishi sections of the Tensyudai are analysed, considering the arch effect. Experimental results are compared, using a series of equations to calculate the collapse burden (q c ). Additionally, the safety factor is determined, based on the active earth pressures at various masses. The findings indicate that the safety factor of a standard Tensyukaku weight is approximately 25, which rises with increased Tensyukaku weight. Moreover, this study contributes to future research and preservation efforts for historical structures, by shedding light on the structural stability and safety of Tensyudai masonry walls. It can also provide basic data for considering the protection of other masonry wall structures from earthquakes.