The secret of the swaying columns
Completed in 1420, the Forbidden City is the world's largest collection of preserved ancient wooden structures. For 600 years, its nearly 1,000 buildings have survived fires, weather, and at least 200 recorded earthquakes, including the devastating 1976 Tangshan earthquake. The structural resilience comes from a sophisticated system of joinery that uses no nails or glue in its main supports. The main element is the dougong, an interlocking wooden bracket that connects the tops of columns to the massive roof beams.
This design allows the structure to behave more like a flexible skeleton than a rigid box. During an earthquake, the mortise-and-tenon joints of the dougong can flex, shift, and rotate. This movement absorbs and dissipates huge amounts of seismic energy through friction. The main structural columns, crafted from whole logs of precious Phoebe zhennan wood, are not anchored into their stone plinths. Instead, they are designed to rock and shift on their foundations, preventing the columns from snapping under stress. The heavy, ornate ceramic tile roofs, far from being a liability, press down on the frame, adding a stabilizing weight that helps reset the joints after the shaking stops.
Tested on the shake table
The historical resilience of the Forbidden City's architecture is more than a matter of record; it has been confirmed by modern scientific testing. Researchers built a 1:5 scale model of a typical palace hall, using traditional materials and construction techniques, and placed it on a seismic simulator, or shake table. These tables are designed to replicate the ground motions of specific earthquakes.
The engineers subjected the model to a series of simulated quakes with increasing intensity. The structure easily handled simulations of recorded events. The team then pushed the limits, programming the shake table to simulate an earthquake of magnitude 9.5—powerful as the strongest earthquake ever recorded—and then a magnitude 10.1. Video of the test shows the model's columns swaying violently, moving as much as 20cm, but the dougong system held the structure together. The building flexed dramatically but did not collapse, demonstrating the effectiveness of a 2,500-year-old engineering principle.