###### Abstract
Their work showed that the inclusion of nonlinear local resonators resulted in wider bandgaps than their linear counterparts. The aforementioned efforts demonstrate the growing interest in lowering frequency bandgaps and generating multiple transmissibility bandgaps in metamaterial structures. To achieve this goal, researchers have resorted to a variety of design changes in the beam structure or local resonator. The pioneer work done by [PERSON] et al. demonstrated the possibility of bandgap generation in a simple Timoshenko beam with evenly spaced local resonators [7]. The single Timoshenko beam developed had eight unit cells and a total length of 0.6 m. The model of an application of white noise to the left end of the free-free single beam within a frequency range of 0-800 Hz showed the existence of a single bandgap from 310 to 498 Hz. While the work of [PERSON] et al. paved the way for more recent advancements, research interests in developing lower frequency bandgaps and multiple frequency bandgaps still exists [1; 4]. This is a crucial issue because most mechanical vibrations occur at frequencies lower than 300 Hz. Thus, lower frequency bandgap structures are desirable in engineering practice and real-world applications [21]. To this end, in order to shift the bandgap frequencies of a single beam, [PERSON] et al. developed a design of a single-beam metamaterial structure with high-static-low-dynamic-stiffness local resonators [18]. The design parameters adopted by [PERSON] et al. [18] were based on the single-beam structure pioneered by [PERSON] et al. [7]. Their resonator model combined a vertical spring with two horizontal springs that added negative stiffness in the vertical direction. The physical Euler-Bernoulli beam model developed by [PERSON] et al. consisted of eight unit cells [18]. Each unit cell had a vertical local resonator with oblique springs connected to the local resonator. The results of the work done by [PERSON] et al. showed the existence of a single transmissibility bandgap which agreed with the work done by [PERSON] et al. [7; 18]. Moreover, [PERSON] et al. showed that the single-beam bandgap shifted to a lower frequency region when the negative stiffness from the horizontal oblique springs was factored into the forces from the local resonators. In essence, [PERSON] et al. successfully showed, through numerical modeling, the possibility of shifting the bandgap of single Euler-Bernoulli beams with local resonators to lower frequencies. This was achieved by adding negative stiffness elements to the resonators. Nonetheless, it is worth noting that only a single transmissibility bandgap was observed in this study. The aforementioned studies have focused on studying single-beam metamaterial structures and lowering their bandgaps to lower frequencies. Recent interests in developing metamaterial structures as dual-purpose vibration attenuation energy harvesting systems show the importance of beam-type metamaterial structures [1; 4; 9; 14; 16; 21; 22; 23]. The use of single-beam metamaterial structures in dual-purpose structures has been reported in multiple recent articles, particularly, as a simultaneous vibration isolation energy harvesting [8; 23]. [PERSON] et al., for example, used rainbow trapping effect in a metamaterial beam to isolate vibrations and concurrently harvest these free vibrations [16]. In addition, [PERSON] et al. converted structure-borne energy on a metamaterial single-beam configuration into electric power using piezoelectric materials [29]. In addition to the aforementioned efforts, more recently, researchers have given more attention to layered-beam metamaterial structures and subsequent applications in vibration attenuation energy harvesting structures [14; 21]. Layered metamaterial structures are metamaterial structures with multiple horizontal beams arranged in layers and linked up with each other through vertical beams. For such layered metamaterial structures, the local resonators are replicated on each beam in a continuous pattern.