NOVEL DEVICES FOR THE ISOLATION OF FLOORS AGAINST EARTHQUAKES AND AMBIENT VIBRATIONS

摘要

Prototype devices able to isolate floors and other light structures with a low center of mass against earthquake inputs and ambient vibrations have been tested. The challenge in designing isolators suitable for light structures is to achieve a very low horizontal stiffness (about 20N/mm) and yet avoid instability. One device, capable of isolating against both vertical and horizontal seismic inputs, uses rolling elements. Its performance has been evaluated in shaking-table tests on a floor mock-up and the results confirmed that the isolator provided strong attenuation in all three directions for most earthquake records. The second type of device is designed to provide isolation against horizontal seismic inputs and vertical inputs associated with ambient vibrations. It is based on a modified fabric-reinforced bag and is relatively simple and economic. It is 600mm in diameter and 375mm in height. The results of characterisation tests on the 2-directional (2D) device are reported in detail here. The static vertical stiffness is sufficiently high to avoid undue rocking of the floor during a seismic input. The dynamic vertical stiffness gives a vertical natural frequency of about 9Hz, a figure low enough to provide a reasonable degree of isolation from ambient vibrations. The shear stiffness decreases from 45 N/mm for a shear displacement amplitude of 10mm to 20N/mm for an amplitude of 60mm, results confirming that the target stiffness for a horizontal isolation frequency of 0.5Hz is achieved. The damping in the horizontal direction is about 16%, so no auxiliary damping device should be necessary. The stability of the device has been checked up to shear displacements of 150mm. The tests confirm that the 2D device has the capability to isolate light structures effectively from horizontal seismic inputs and vertical ambient vibrations.