Graded Seismic Metamaterials with Structural Steel Sections for Low-Frequency Isolation of Surface Wave

By introducing the concept of a graded structure to seismic metamaterials, a new type of graded seismic metamaterial assembled using four steel sections with different graded levels is proposed to investigate its attenuation performance for surface waves. The dispersion curves and vibration modes are obtained using the finite element method and the sound cone method. A comparative analysis of the band gap characteristics of the four graded seismic metamaterials shows that an increase of the graded level is beneficial for widening the total band gap to a much larger relative bandwidth in the range of 0.1–13.07 Hz. In addition, a detailed analysis of the vibration modes reveals that local resonance is the main mechanism for the generation and change of the three band gaps. Moreover, the filling materials in the cavities, material and geometric parameters of the structure play important roles in the distribution and relative bandwidth of the band gaps. Finally, frequency–domain analysis is carried out on a finite system, and the agreement with the bandgaps is verified. This study paves the way for the design of graded seismic metamaterials. This concept allows flexible manipulation of the surface wave propagation by adjusting the graded level, fillers, geometric parameters of the steel sections, and soil materials to achieve seismic wave attenuation in low-frequency broadband.