Response of vibration-isolated object to ground motions with intense vertical accelerations

This paper reports the results of shaking table tests performed to assess the response of an object placed on a vibration isolator to intense ground motions whose peak accelerations are close to or over the gravity acceleration in both vertical and horizontal directions. The passive vertical and horizontal vibration isolator, developed by the authors, has rolling load-bearing elements and constant-force springs to provide piecewise-constant restoring forces. First, we performed shaking table tests, wherein only the horizontal vibration was isolated. An unexpected finding from the tests is that, when the vibration isolator was subjected to ground motions with intense vertical accelerations, not only vertical but also horizontal accelerations were amplified. This led to a large rocking response or overturning of the object placed on the vibration isolator. A likely reason for the amplification is rocking of the upper portion of the vibration isolator above the rolling load-bearing elements induced by intense vertical accelerations. Next, we performed shaking table tests, wherein both vertical and horizontal vibrations were isolated. In this case, the vertical as well as horizontal accelerations were significantly reduced and the rocking response of the vibration-isolated object was suppressed. These results suggest potential risks in conventional horizontal vibration isolators having rolling load-bearing elements when they are subjected to ground motions with intense vertical accelerations. The results also demonstrate the effectiveness of vertical vibration isolation to avoid such risks. (C) 2011 Elsevier Ltd. All rights reserved.