Rollers offer built-in protection
A new type of isolator protects against large amplitude vibrations such as the sort likely to be encountered in earthquakes. Tom Shelley reports
Smaller structures can be isolated from earthquakes by balls rolling on sheets of rubber.
The technique copes with large vibrations at low frequency the sort most likely to cause damage and suits itself well to structures used in machinery and process plant.
The idea of mounting structures on rolling balls was first mooted around the turn of the century. Earthquakes produce deflections of more than 100mm at frequencies from around 0.5 to 30Hz. The low frequencies tend to cause most damage, because they correspond to the natural frequencies of buildings and large structures. Earthquake isolators need to isolate these low frequencies and be able to cope with large movements.
Dr Alan Muhr has been leading an investigation into these ideas at Rubber Consultants, the consultancy unit of MRPRA at the Tun Abdul Razak Research Centre at Brickendonbury in Hertfordshire. The programme has been funded by ECOEST, the European Commission Earthquake and Shaking Tables project. Other participants have been the Instituto Superior Technico in Lisbon and Italys equivalent of the National Engineering Laboratory. Other participants in the UK include Mohamed Sulong, a PhD student from Malaysia and Professor Alan Thomas at Queen Mary College.
Considerable studies have been made of different types of earthquake shocks and vibrations. Practical tests have been conducted on shaking tables with test structures of concrete blocks mounted above each other on spindly and flexible mounts. If structures were mounted on rolling balls alone, they would tend to run away. If the balls run between layers of rubber, on the other hand, the rubber applies damping to the system when the balls rock at small lateral deflections, and maintain damping when the balls are forced to roll at large deflections.
The result of the studies is that a generic method of design has been established. For example, for a 1.5 tonne structure, ideal mounts should use balls about one inch (25mm) across between layers of 2mm rubber bonded to metal backing plates.
Unlike conventional earthquake isolators, the rolling ball units can protect small structures. Conventional earthquake isolators consist of interleaved layers of rubber and steel, but fail to achieve much for loads of less than about 20 tonnes per isolator. These devices are being particularly tested on their ability to protect smaller items of process plant, and displays of delicate artifacts in museum cabinets. They also provide protection against non-earthquake produced lateral vibrations.