Earthquakes are a reality of life in many parts of the world, and their impact can be incredibly expensive. The Christchurch earthquakes in 2011 cost the economy upwards of $40 billion USD (20% of New Zealand GDP), while the 2004 Chuetsu earthquake in Japan, and the 2008 Sichuan earthquake in China cost $28 billion USD, and $148 billion USD respectively.
Over the last 50 years, structural engineers have developed design methods which drastically reduce the likelihood of loss of life in a building when an earthquake happens. Now we are looking at the more difficult task of minimising costly damage to reduce the economic impact of a seismic event. We can do this by improving building performance to limit building movement – and one of the most effective solutions is to use viscous dampers.
Viscous dampers reduce, or ‘dampen’, the amount of movement a building experiences during an earthquake, and they are effective whatever the height and shape of a building, making them suitable for both new build, and existing structures.
Which is great! Except, viscous dampers don’t come cheap, and earthquakes are difficult to predict. So how do you decide where to place your seismic dampers, and how many does your building really need? If fact, how do you decide whether to invest at all?
This is where the smart technology comes in. At Beca, we use a very sophisticated smart platform called SONA (Structural Optimisation using Nonlinear Analysis) to help us to calculate the quantity and distribution of dampers required to optimise the performance of the building and evaluate the probable future seismic financial loss if a severe earthquake happens.
It does this for a range of damper numbers and capacities, with the beauty of the resulting curve being that stakeholders can make a decision on how and where to invest, based on their risk appetite.
Outputs of SONA framework – relationship between quantity/capacity of dampers and seismic loss
As you move to the right of this curve, there will be more seismic loss, but less initial cost as fewer dampers are needed. Conversely, at the left of the curve, less seismic loss will be incurred, but the number and cost of the dampers increases markedly. SONA calculates the optimal distributions for each and every point on the curve.
This approach is very appealing to stakeholders and decision makers as it addresses the concept of ‘low damage’ in its entirety, taking into account the economic impact of a major earthquake on a building.
Although there are less direct means of achieving low damage – for example, by restricting building characteristics, such as inter-storey deformations and floor accelerations - these miss the real ‘spice in the curry’. The SONA framework comes into its own by directly addressing the low damage design impact of viscous dampers in a holistic manner, minimising both structural and non-structural or architectural damages.
The result is the ability to make an informed decision about how to use dampers to protect your building and your business, based on real data. And that peace of mind is invaluable.
Viscous dampers 101
- When Wellingtonians finally get to see the renovated and earthquake-strengthened Ballet Building, part of the St James Theatre in Courtenay Place, they will see viscous dampers on the outside of the structure.
- If you’ve ever walked over the Millennium Bridge in London, you may have seen the viscous dampers beneath the ends of the bridge. These were installed after its initial completion to stop the unnerving ‘wobbly’ movements experienced by pedestrians on opening day.
- Viscous dampers were installed under the Aurora Terrace bridge in Wellington in the 1980s to dampen the horizontal motion of the bridge deck in earthquakes.
- The shock-absorbers on your car’s front wheels are another form of… you guessed it – viscous dampers!
- In basic terms, viscous dampers work by absorbing some of the energy of the motion between their ends and turning it into heat. They warm up and are full of a viscous liquid. Think of pumping up and down a coffee plunger!
- Viscous dampers are mobilised by very small amounts of movement activated at amplitudes as small as 0.00254mm.
- Paradoxically, the maximum damping from each device occurs when it is at the zero position in its dynamic cycle.
Beca is currently incorporating optimised viscous-damper schemes in a number of new-build and seismic-strengthening projects and using the SONA framework to do so.