Base isolators – “Quake brakers”
Base isolators are like giant shock absorbers for buildings. In an earthquake, they cause the building to shake with less severity than if it was fixed directly to the ground.
Lead-rubber base isolators, invented by New Zealand scientists Dr. William Robinson, are now in use around the world, saving lives and millions of dollars.
New Zealand’s national museum, Te Papa, in the capital, Wellington, stands on 152 lead-rubber base isolators. Visitors can go underground to see a display a cutaway of some of the isolators as well as a display explaining how they work:
In the 1960s engineers experimented with steel bars or “dampers.” While these succeeded in absorbing the shock, they can fatigue after being stretched, so they would have only a limited life.
Earthquake scientists at the Department of Scientific and Industrial Research (DSIR) in Wellington, led by Dr. William Robinson, experimented with various materials including lead. Pure lead has some interesting properties: When subjected to strain it does not snap, but deforms. Could this property be useful in an earthquake?
After further experimentation, Dr. Robinson invented lead-rubber bearings, which could provide isolation and damping at the same time.
The ingenious “lead-rubber bearing” base-isolation system consists of rubber bearings laminated with steel, with pure lead poured inside. The rubber provides isolation and flexibility, the laminated steel adds strength and allows the base isolator to support large weights, and the pure lead controls the sideways movements of the isolators and transfers some of the energy of the earthquake to heat.
As well as absorbing some of the shock, the flexible isolators change the short, fast shake of most earthquakes into a longer, gentler one. This causes isolated buildings to move further than a firmly fixed one, but because it shakes more gently, less harm is caused to the people and contents inside.
Because an isolated building will move further in the earthquake than one that is firmly attached to the ground, the foundations have to accommodate additional movement. To do this, a small gap is left around the base of the isolated building for it to sway into during an earthquake. Te Papa’s “seismic gap” is 500mm (19.6 inches), which is how far engineers estimate Te papa will move in the sort of earthquake that occurs once every 2,000 years.