From cycle paths to seismic maths

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Scotland may not be a hotbed of seismic activity but Edinburgh Napier is at the epicentre of innovative research that could save lives in earthquake-affected parts of the world. And it all started with cycle paths

In April 2015, Nepal was struck was struck by an earthquake which killed 9,000 people. It also destroyed many of the country’s historic structures, including the 16th-century Kasthamandap temple after which the capital city Kathmandu is named. Now researchers at Edinburgh Napier are working on an innovative method of retrofitting existing buildings and monuments to protect them from future earthquakes - and the genesis of the project can be traced back to work at the Mountain Bike Centre of Scotland.

Dr John McDougall, Director of Research for the School of Engineering and the Built Environment, who is leading this project, explains: “We were looking at the use of shredded rubber to improve the resilience of the trails and the more we worked on the project, the more it became apparent that this could have some significant application in managing the dynamic properties of the ground to cope with seismic disturbances.

“It’s quite a small step for us but at a public level I can see that it’s a huge leap from Glentress mountain biking to somewhere on the tectonic margins of the earth where buildings are falling down and people are dying in large numbers.”

Up until now, work in this area has focused on protecting new-build structures but Dr McDougall and PhD student Juan Bernal-Sanchez, together with geotechnical engineering colleagues Dr Daniel Barreto, Dr Marina Miranda and Dr Katerina Marinelli, believe they are developing a novel way of installing a shredded rubber and sand mixture that will allow existing structures to be protected.

Bernal-Sanchez says: “The UK is producing 40 million tons of old tyres every year. You cannot put them in landfill so you have to do something with this rubber and many companies are trying to come up with new uses, such as backfill for retaining walls. One of the other applications could be a seismic isolator.”

While famous skyscrapers tend to have built-in earthquake protection and are designed to sway slightly above ground, smaller high-rises and historic buildings are relatively rigid and still experience movement below ground. If the agitation of the earthquake is in sympathy with the natural properties of the ground then they accentuate each other, which causes a slightly more forceful shaking or vibration at the bottom of the building known as resonance. If the natural frequency of the ground does not match that of the earthquake then they’re not working together and instead offset each other.

“Doing some numerical analysis, we have seen that we can change the natural frequency of the soil, moving it away from the frequency of the earthquake. It’s not just about damping the vibration or the acceleration but also moving it away from the frequency that’s going to affect the structure,” says Bernal-Sanchez.

Work is set to continue in the laboratory, including experiments on large shaking tables, and the team hope to have a field test of their installation method in the ground within three years. 

Dr McDougall says: “It’s really exciting because I’ve never before been involved in a project where there is such a significant life-saving dimension to it. We might be a little way from it just yet, but there is the potential for something that is environmentally friendly, sustainable, affordable in countries where it matters, and has the potential to save historic buildings and lives.”

It’s really exciting because I’ve never before been involved in a project where there is such a significant life-saving dimension to it.

How it works

Dr John McDougall explains that their innovative earthquake damping technology can be illustrated using the example of garden swing. 

“The solution we’re developing can be visualised in the motion of a garden swing.  An earthquake creates cyclic motions in the ground – like a garden swing oscillating back and forth.  There are two ways you can stop a swing: abruptly or gently. 

An immovable object in the path of a swing will bring it to an abrupt stop.  And with inevitable damage to the swing and rider as a result of the instantaneous deceleration.  It is sudden accelerations and decelerations of ground motions that are responsible for the damage to buildings caused by earthquakes.

If you want to stop the swing gently, bring your hands to move with the swing, take hold and gradually decelerate the swing.  You use your flexibility and mass to absorb gently the energy of the oscillating swing and bring it comfortably to rest.  By the introduction of rubber shreds, we are changing the dynamic properties of the ground, enabling it to absorb the energy of ground motions and so minimise their impact on buildings. 

 “We can measure the dynamic properties of the ground through tests and we can assume something about the natural frequency of the buildings from their size and what they’re made from, for example timber, concrete or masonry. From those, we can ascertain meaningful, credible values that point you in a certain direction in terms of the installation and the rubber-sand mixture you’d need to offset the potential damage from an earthquake.”

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