The control of bending resonance of vibration isolation system

Abstract

Structure-borne noise radiation from concrete box infrastructures, such as railway viaduct, buildings and ductwork, cause considerable nuisance to neighbours. Floating system is generally used on these box structures to reduce the vibration transmission into the receiver. Some studies have found that the bending resonances on floating slab can affect the vibration isolation performance. However, there is still a lack of comprehensive experimental and theoretical results to confirm this situation and no published guideline for the design of the floating slab. This study was an attempt to establish the responses of the global/local modes for any rectangular box structures and identify their interaction with dynamic response of floating slab/floor and, in addition, to control the effects of bending mode of floating slab/ floor on the isolation efficiency. The extensive results in this study confirmed that the bending vibration modes of isolation system can degrade the vibration isolation performance. The effects becomes more significant when the bending resonance frequencies of floating slab match the supporting structure resonant frequency, passage support frequency or vertical isolator natural frequency. Two types of interaction modes between rectangular concrete box and floating slab have been established. The first one is between the local distortion mode of box structure and the rotation mode of isolated slab. The second one is between the local combined mode of box structure and the first bending mode of isolated slab. The results suggest that decreasing the length of floating slab/floor can increase the bending resonance frequency, which can improve the vibration isolation performance. To reduce the effect of bending resonance of floating system, isolators should be placed at the nodal points of the lowest symmetric bending mode of isolated panel. The research establishes the additional rules for the design of floating system; (1) the overlap of floating slab resonant mode with those of supporting structure and passage support frequency of the track should be avoided; 2) the installation of smaller floating slab/floor and nodal point support can reduce the bending modes effects on vibration isolation.