Hybrid local-resonance/Bragg-scattering metamaterial foundations for broadband seismic isolation: A shake-table study under soil-structure interaction

Metamaterial foundations exploit frequency bandgaps to filter seismic waves, but most experimental studies to date have tested local-resonance (LR) or Bragg-scattering (BS) mechanisms separately, on rigid bases that neglect soil-structure interaction (SSI). This paper reports shake-table experiments on a hybrid LR-BS metamaterial foundation - a 4 × 4 array of lead-rubber-steel resonators embedded in a concrete matrix - supporting a 1:10-scale, three-story RC building model on soft soil. Hybrid, LR-only, Bragg-only, and control foundations were compared under three historical ground motions (El Centro 1940, Kobe 1995, Northridge 1994), sine-sweep excitation, and three soil densities. The hybrid foundation produced the largest roof acceleration reductions (61.4–67.7%), exceeding the LR-only (38.2–46.3%) and Bragg-only (28.7–36.1%) configurations, and yielded two complementary experimental bandgaps (2.0–5.4 Hz and 8.1–12.2 Hz) that agreed with Transfer Matrix Method predictions to within 8.5%. Soft-soil conditions shifted the LR bandgap downward by 0.3–0.6 Hz, improving its overlap with soil-amplified seismic energy. These results provide experimental evidence, under SSI conditions, that combining LR and BS mechanisms broadens the isolation bandwidth beyond either mechanism alone, with implications for the seismic design of metamaterial-isolated buildings on soft soil.