Isolating an above grade MRI from traffic induced vibration



MRI installations require that ambient floor vibration levels not exceed limits defined by the manufacturer. In this study, vibration measurements for a second-floor installation exceeded the allowed levels.  A road adjacent to the building experienced heavy traffic and was in poor condition with potholes and asphalt patches.  The second-floor exhibited resonances in the same 8-15Hz frequency range as ground-borne vibration due to traffic close to the building.  The MRI comes supplied with resilient pads that effectively isolate above 20 Hz.  For isolation of the floor resonances excited by vehicular traffic, an additional stage of isolation pads was designed that lowered the system resonance frequency of the MRI on the floor to 6.5Hz.  The added pads, with greater thickness and softer material, raised concerns of the MRI collapsing sideways like a thin, tall column above its critical buckling load.  Lateral snubbers were incorporated where their compressive stiffness constrained the rails laterally and a low shear stiffness did not significantly increase the system resonance frequency.  Vibration measured on the MRI after installation showed the novel system provided effective isolation of traffic induced vibration, satisfying vibration requirements for the MRI.  Model predictions of MRI vibration relative to floor vibration agreed well with measurements.