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.
During an earthquake, a building can move around 300 mm or more relative to the ground. Therefore, the use of base isolation also means there must be a way for movement during an earthquake to be accommodated. This usually means a rattle space or moat has to be put in place around the building so that the building doesn’t crash into something nearby. Building services such as water, sewerage and electrical all need to be designed to accommodate this movement without being damaged.
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 such as taller buildings, as base isolators have a limited ability to cope with tension, meaning a taller building could overturn or topple during an earthquake. The building site will also be an important consideration when looking at base isolator use, for example, there may not be sufficient space to incorporate a moat around the building. Furthermore, base isolation is designed for hard soil, not soft.
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.
There is a lot of mass in a building, and after a strong earthquake, a building could continue to sway back and forth on the isolators. Lead cores were added to base isolators as an energy dissipation mechanism.
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.
Mechanical engineer Associate Professor Geoff Rodgers from the University of Canterbury won the Kiwinet 2017 Emerging Innovator Award for work that included developing a simplified seismic damper for buildings. View the Kiwinet video: Dr Geoff Rodgers – Seismic damping solutions for buildings and joint implant diagnostics.
The difference between base isolators and seismic dampers
A base isolator predominantly provides a way to prevent a structure having to move and follow the ground as the ground shakes during an earthquake, while a seismic damper absorbs energy when the structure moves.
Sometimes base isolation is combined with seismic dampers, which provide an additional form of energy dissipation to prevent the structure moving too far relative to the ground.
By adding a damper into the structure with base isolators, seismic energy can be further absorbed as the building moves, which will help to limit the amount a building sways, helping to better protect the building from damage and to reduce the inconvenience to occupants and damage to contents.
Read about other work to earthquake-proof structures in Seismic engineering.
In the activity, Best base isolator, students use a physical model to investigate the effectiveness of different properties for base isolators.
To learn more about present developments and thinking around seismic engineering, listen to Dr Geoff Rodgers in this RNZ podcast.