Seismic engineering is a branch of engineering that searches for ways to make structures, such as buildings and bridges, resistant to earthquake damage.
Seismic engineers aim to develop building techniques that will prevent any damage in a minor quake and avoid serious damage or collapse in a major shake.
A visitor to the seismic engineering laboratories and test centres at the University of Canterbury, Christchurch, is likely to meet researchers and students of many nationalities. They are attracted by its world-wide reputation for innovative thinking in the field of seismic engineering.
Smarter, not stronger
The testing equipment at Canterbury is impressive, but it would be dwarfed by the huge laboratories in some other countries, where whole buildings can be tested to see how they perform in an earthquake. New Zealand may not have testing equipment on this scale, but it does have very creative scientists who are carrying out world-leading research.
These scientists aim to think smarter rather than stronger. It is tempting to create buildings that are rigid and strong,but one day, an earthquake will come along that is stronger than the building, and it will break.
A ‘smarter’ approach is to allow the building to deform in an earthquake in a way we want it to. This idea – called ductile design – was pioneered at Canterbury University from the 1970s by Bob Park, Tom Paulay and Nigel Priestley. The concept of ductile design has since spread around the world, and research is constantly expanding its usefulness.
Since this article was written, ductile design has been tested in action. The endoscopy building at the Southern Cross Hospital in Christchurch was built using concrete pillars with steel cables providing flexible joints. This building did not suffer structural damage during the September 2010 and February 2011 earthquakes. More recently, major buildings constructed using timber frames with flexible joints include the Arts and Media Building at the Nelson Marlborough Institute of Technology (opened 2011, also using seismic energy dampers) and the Massey University creative arts building Te Ara Hihiko in Wellington (opened 2012). The Alan MacDiarmid Building at Victoria University of Wellington (opened 2010) has a concrete frame with flexible steel joints.
The weakest link
One approach of ductile design is to restrict damage to places where it can be controlled. This is at the weakest parts of large buildings – the joints between columns and beams. In many modern concrete buildings, columns and beams are cast on site, in one piece. This means the frame of the building is rigid. When this frame is forced to move by an earthquake, there is no flexibility, so something breaks – it may be the beams, the columns, or where they meet.
A new approach is to create a flexible joint where a beam meets a column. Steel cables tie the parts together – in an earthquake the cables stretch, which lets the joint open slightly rather than break.
This is just one way that is being investigated to tackle the problem. Base isolation, also developed in New Zealand, has been used in buildings around the world, but it is expensive and best suited to large structures. At Canterbury University, research is being carried out on cheaper alternatives.
The beam-column joints include parts that are made of materials that are designed to break or plasticise in a controlled way, rather than the beams and columns breaking. These parts are then replaced after an earthquake. This is something like an electrical fuse, which is designed to blow and be replaced, rather than allowing electrical equipment to get damaged.
Dampers are incorporated at beam-column joins. They are made of materials that disperse an earthquake’s energy, reducing the chance of breakage.
Nature of science
Running a full-scale structural test is very expensive, so there is a lot of preparation. Scale models are made and tested, and hundreds of tests are done using computer models. Only when a full prediction has been made is a full-scale test carried out. The results are used to refine computer models so that pre-test development is improved.
A lot of research goes into finding ways of using new materials - often those developed in other fields.
- Carbon fibre polymers from the aeronautical industry are very strong and light and do not need the constant maintenance that steel needs as it corrodes. Experiments are being carried out on how to retrofit carbon fibre ‘bandages’ around the joints of existing buildings to make them stronger.
- Shape-memory alloys can be made to go back to their original shape after being deformed in an earthquake.
- New types of concrete contain thousands of short steel or PVA fibres rather than the steel bars used at the moment. The fibres stretch and heat, using up an earthquake’s energy. This results in fewer cracks and less damage.
These are just some current approaches being investigated. In many cases, a combination of techniques will be best for new buildings.