Three‐Dimensional Analytical Model of a Podium Structure With Rocking Columns of Square Cross Section

Allowing structural systems to uplift and rock during seismic excitation has emerged as an effective and inherently self‑centering form of seismic isolation. This paper develops a computationally efficient 3D analytical model suitable for the understanding of the behavior of rocking bridges. The model comprises a rigid slab supported by rocking columns of square cross section that can uplift and rock in two horizontal directions, while not sliding, twisting or stepping out of their original position. The equations of motion are derived, impact damping is determined, and rocking spectra are generated under 176 bidirectional pulse‐like ground motions. The study proves the existence of an equivalent solitary rocking column with identical slenderness yet larger size whose dynamic response closely matches that of the podium structure—similarly to the planar case. A statistical evaluation of the ground motions shows that PGV / T p is a more efficient acceleration scale than PGA for the description of the 3D rocking motion. A displacement‑based assessment further proves that, provided the system does not approach overturning, the displacement demand depends only on slenderness and is independent of column size. The above supports the development and use of unified, size‑independent 3D rocking spectra, simplifying the design parameters of 3D rocking structures.