Chris Gannon from Robinson Seismic explains how lead rubber bearings are tested. The bearings undergo rigorous testing to ensure that structures are properly protected against earthquakes.
The test rig that we’ve got here we designed that about 15 years ago because the retrofit of parliament in NZ was about to start and we knew that every bearing was going to have to be tested and we didn't have a test rig that was suitable for doing those tests. In Wellington the expected movement of the ground is about plus or minus half a metre and so for testing our lead rubber bearings we do pull those out to at least half a metre in any direction to test that the bearings will handle those big distances that the ground’s going to move. So we designed the test rig around that job but also knowing that there would be future jobs for it as well.
When it comes to designing a test rig for lead rubber bearings you have to think about how you are going to load them into the test rig, what building the test rig in going to be in, how fast you want to do the tests, and if you want to do very fast tests, at actual earthquake speed then you need to look at getting a huge amount of horsepower in the order of 2 to 5000 horsepower and that’s not very practical for most places and so a test rig like the one we have designed here we are running at about 1 tenth of the speed that the ground moves or even slower. The loads that we measure on the rubber bearings are not really related to the speed at which the ground moves, but only to the distance. So as long as we can pull the ground out as far as the earthquake is going to move it, it doesn't matter if we do it slowly, so that is the most important part, being able to measure the loads applied to the bearings when the earthquake is there, but you don't have to do it at earthquake speeds.
Parliamentary Service (New Zealand)