Base isolation is a technique developed to prevent or minimise damage to buildings during an earthquake. It has been used in New Zealand, as well as in India, Japan, Italy and the USA.
A fixed-base building (built directly on the ground) will move with an earthquake’s motion and can sustain extensive damage as a result.
When a building is built away (isolated) from the ground, resting on flexible bearings or pads known as base isolators, it will only move a little or not at all during an earthquake.
The isolators work in a similar way to car suspension, which allows a car to travel over rough ground without the occupants of the car getting thrown around.
Base isolation technology can make medium-rise masonry (stone or brick) or reinforced concrete structures capable of withstanding earthquakes, protecting them and their occupants from major damage or injury. It is not suitable for all types of structures and is designed for hard soil, not soft.
One of the world's first base-isolated structures – the William Clayton building in Wellington, built in 1982 – uses about 80 lead rubber bearings, but this number depends on how engineers want to distribute the load. The Museum of New Zealand Te Papa Tongarewa in Wellington, which opened in 1998, has 135 lead-rubber bearings. Other countries have different ways of doing it, some like big bearings with a few columns, while others prefer lots of little bearings.
How are base isolators constructed?
Lead rubber bearings were developed as base isolators in the 1970s. They consist of three basic components – a lead plug, rubber and steel, which are generally placed in layers.
The rubber provides flexibility through its ability to move but return to its original position. At the end of an earthquake, if a building hasn’t returned to its original position, the rubber bearings will slowly bring it back. This might take months, but it will return to its original position.
Lead was chosen because of its plastic property – while it may deform with the movement of the earthquake, it will revert to its original shape, and it is capable of deforming many times without losing strength. During an earthquake, the kinetic energy of the earthquake is absorbed into heat energy as the lead is deformed.
Using layers of steel with the rubber means the bearing can move in a horizontal direction but is stiff in a vertical direction.
Another method for controlling seismic damage in buildings is the installation of seismic dampers. In this case, the dampening is provided by a lead-based device that looks very similar to a car damper (shock absorber).
Ground movement forces the lead to pass through a narrow gap. When the direction of movement changes, the flow of lead is reversed. The principle is still the same as the lead rubber bearing, with kinetic energy being converted into heat energy, thereby preventing the building absorbing the kinetic energy.