Seismic Response and Collapse Analysis of Composite Ancient Pagodas with Internal Soil and External Brick: The Dayan Pagoda

The Dayan Pagoda, originally built in the Tang Dynasty and repeatedly repaired, has a distinctive outer-brick and inner-soil composite structure. A three-dimensional 3DEC discrete-element model was established to investigate its seismic response and potential instability, and its dynamic characteristics were calibrated against in-situ test results. El-Centro, Taft, and Artificial Xi’an ground motions were applied at PGA levels of 0.196 g and 0.4 g. Under the fortification-level earthquake, the pagoda exhibited pronounced top amplification and whipping effects; the brick masonry and rammed soil in the middle and lower parts generally remained compatible, whereas local sliding and vertical separation were more evident near the top. Under the rare-earthquake condition, local detachment of the top rammed soil preceded global instability, followed by progressive damage in the lower first to third stories and increasing brick-soil relative movement, ultimately accompanied by overall instability of the upper pagoda. Although collapse timing differed among the three motions, the predicted macroscopic instability evolution was similar. The results indicate a close association among internal-soil deformation, degradation of brick-soil interaction, and damage development in the lower brick walls.