Seismic engineering
Seismic engineering is a branch of engineering that searches for ways to make structures, such as buildings and bridges, resistant to earthquake damage.
What is seismic engineering?
Dr Stefano Pampanin from the University of Canterbury believes seismic engineering relies on the merging of knowledge from many scientific areas, using an interdisciplinary approach and combining the best tools and know-how to obtain high level results.
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.
What skills do seismic engineers need?
Dr Stefano Pampanin from the University of Canterbury explains that seismic engineers need a good understanding of physics, mathematics and other sciences, as well as a broad general knowledge and a scientific approach.
Smarter, not stronger
Engineers and 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.
Ductile articulated structures
Dr Stefano Pampanin from the University of Canterbury explains how the ‘smarter’ idea of ductile design originated at the University in the 1960s and is currently under further developments and refinements. Making buildings stronger is not the answer for earthquake protection.
A ‘smarter’ approach is to allow the building to deform in an earthquake in a way we want it to – this is called ductile design.
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. Learn more about this and elements of ductile design such as fuses and dampers in the article Seismic engineering at Canterbury University.
Base isolation and dampers
Base isolation is a technique where a building is built away (isolated) from the ground, resting on flexible bearings or pads known as base isolators. Base isolators were developed by New Zealand scientist Dr Bill Robinson.
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.
Is base isolation always appropriate?
Dr Stefano Pampanin from the University of Canterbury believes buildings can be protected from earthquakes by a combination of different available technologies, such as ductile design, base isolation, bracing or rocking systems.
UPDATE
This video was recorded prior to the 2010 and 2011 Canterbury earthquakes. In it Stefano mentions the Christchurch Women's Hospital. This building performed well during these earthquakes with staff reporting that the base-isolated building did not shake wildly but instead moved gently from side to side. It was operational immediately after the large quakes, unlike neighbouring hospital buildings that sustained significant damage.
Seismic dampers are a method for dissipating the energy of an earthquake. Dampers are incorporated at beam-column joins. They are made of materials that disperse an earthquake’s energy, reducing the chance of breakage.
Learn more about how base isolation works and the difference between base isolation and dampers here.
Buildings with base isolators
One of the world's first base-isolated structures is the William Clayton building in Wellington, built in 1982, which uses about 80 lead rubber bearings. The number used in a building depends on how engineers want to distribute the load.
William Clayton building
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 William Clayton building in Wellington is one of the original base-isolated structures. It was enlarged and retro-fitted with seismic dampers in 2015.
Research on building performance during large magnitude earthquakes in the 1990s, including the Kobe earthquake, showed that buildings in close proximity to active faults were likely to exceed their maximum allowable displacements with base isolation alone. This led to the retrofit of the William Clayton building with seismic dampers during enlargement of the building, and 40% of the base isolators were also replaced to accommodate the larger building. The moat was also enlarged from 150 mm to 400 mm.
Just 2 weeks after staff began to move back into the building in 2016, Wellington was strongly shaken by the Kaikōura earthquake. The building performed well and became a refuge for workers displaced from other buildings at that time.
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 using big bearings with a few columns, while others prefer lots of little bearings.
The Christchurch Justice and Emergency Precinct was built following the Canterbury earthquakes. Opened in 2018, the precinct is made up of four buildings. A variety of systems have been used to mitigate the effects of a large earthquake. The foundations have been designed to reduce the risk of liquefaction. All four buildings in the precinct are connected by a first-floor podium. The base isolation and PFTE sliders have been installed at this level, meaning all four buildings will move together in an earthquake event.
Christchurch Justice and Emergency Precinct
The Christchurch Justice and Emergency Precinct has four buildings connected by a podium at level one, where the base isolators have been installed. The red lines show this isolation plane. The Tx and Tz values are the building structural periods of vibration in each horizontal direction east-west/north-south, i.e. how long the building takes to sway from one side to the other side and back again to the start
Image from: Pettinga, D and Oliver, S. Aspects of design for the base isolated Christchurch justice and emergency services precinct [online]. SESOC Journal, Vol. 29, No. 2, Sep 2016: 29-35. ISSN: 0114-2879.
PTFE sliders are Teflon sliders. They are essentially a steel puck that slides between two flat discs. The puck is steel but has Teflon outer layers to reduce friction. The sliders are typically used when gravity loads from the structure above need to be supported but an engineer does not want to use too many lead rubber bearings. In a large structure, it is common to use a mix of lead rubber bearings, rubber bearings (without lead) and/or flat sliders.
Each of the four buildings have their own period of vibration, generally less than one second, and then the seismic isolation of the whole precinct has a separate and significantly longer period of vibration at over three seconds. This longer period of vibration is what provides the seismic protection to the buildings on top of the isolation plane. It is important for effective base isolation protection that the structures built on top of the base isolation plane are comparatively stiff (i.e. low period of vibration) compared to the base isolation in order that the majority of displacement during an earthquake is confined to the base isolators.
Related content
A number of buildings in New Zealand now utilise seismic engineering technology. Learn more about these buildings and other methods of protecting buildings in Base isolation and seismic dampers.
Activity idea
The activity Best base isolator uses a physical model to investigate the effectiveness of different properties for base isolators.
Useful link
This Linkedin blog post, Wellington: an active earthquake engineering laboratory by Matthew Plummer from Beca, explores the cutting edge seismic engineering developments in Wellington, including at the Jerningham Apartments (1968), the Beehive (1979), The William Clayton Building (1982) and 8 Willis Street (1987/2021).
