Vibration isolation performance of quasi constant natural frequency isolation pads associated with test verification

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The vibration and noise induced by the operation of subway trains cause disturbances to the adjacent building structures. The serviceability of building structures (i.e., offices and residences) in terms of the annoyance of humans is sensitive to vibration and noise; therefore, it is necessary to conduct an effective vibration control strategy for building structures in terms of the structure-born noise and vibration protection. In serviceability-related vibration control, two issues are considered herein: (a) the irregularity of the controlled structure and (b) the structure uncertainties, such as insufficient knowledge about the structural mass redistributed in the isolation system. A quasi-constant natural frequency (QCNF) pad, mixed with rubber and foam material, is introduced herein to tackle these two issues. Such a system is nonlinear, and its stiffness is associated with the structural mass distribution. In this paper, a single-degree-of-freedom (SDOF) system is employed first to illustrate the property of a QCNF system. Inspired by the ideal load-displacement property of a QCNF system, a streamlined rubber element, an unconstrained cylinder rubber element, and two constrained cylinder rubber elements (including rigidly constrained and flexibly constrained) are designed and simulated with a finite element model. The physical properties of such four designed elements are obtained based on a static testing and an ambient field test. In the end, a quasi-constant frequency isolation pad mixed with rubber and foam material has been comprehensively discussed regarding the finite element simulation and an on-site ground-borne vibration test validation. The results indicate that the flexibly constrained QCNF rubber pad, using a combination of rubber and foam materials, can maintain a nearly constant frequency in the range of 67–1000 kPa load. The test validation based on a real-world application of the QCNF rubber pad indicates its feasibility in serviceability control of a building structure.