Lead-rubber hysteretic bearings suitable for protecting structures during earthquakes

Lead-rubber hysteretic bearings provide in a single unit the combined features of vertical load support, horizontal flexibility and energy absorbing capacity required for the base isolation of structures from earthquake attack.The lead-rubber hysteretic bearing is a laminated elastomeric bearing of the type used in bridge structures, with a lead plug down its centre.Since the invention of the lead-rubber bearing, a total of eleven bearings up to a diameter of 650 mm, with lead plugs ranging from 50 to 170 mm in diameter, have been tested under various conditions, including vertical loads to 3.15 MN, strokes to ±110 mm, rates from 1 mm/h to 100 mm/s, and temperatures ofIn all of these tests, the lead-rubber bearings behaved satisfactorily and the hysteresis loops could be described reasonably well by assuming that the lead behaved as an elastic-plastic solid with a yield stress in shear of 10.5 MPa.The bearings showed little rate dependence at ∼100 mm/s, though at creep rates of ∼1 mm/h the force due to the lead dropped to 30 per cent of that at typical earthquake frequencies.The effect of many small displacements has been tested with 11 000 cycles at ±3 mm.A total of 92 lead-rubber bearings have been used in New Zealand to base isolate one building and three bridges.They have yet to be used overseas.This paper describes the tests on the lead-rubber bearings, the results and a design procedure for selecting the size of the lead plug.