Seismic isolation: Motivation, basic principle, and new solutions

by Nikolin Hima

With almost a third of human population living in areas that are prone to seismic events, it comes as no surprise that on average 3.5 million people every year are affected by the earthquakes globally. That alone makes them one of the most devastating forces in nature, capable not only to directly posing serious threat to people’s lives, but also causing massive economical loses due to the damages of building stock, infrastructure lines and in some cases the chain of events that they ignite can trigger ecological catastrophes (Fukushima Earthquake 2011).

Moreover, the past year brought to focus new devastating scenarios of a combination of an ongoing global pandemic with seismic events, such in the case of the earthquakes of Croatia in 2020, that lead to loss of human lives and to a disruption of fundamental everyday activities such as health care services, which were especially needed in such circumstances.

Therefore, it is necessary to find ways to mitigate or even avoid the effects of such events, that have the potential to turn upside down the lives of millions of people in a matter of seconds. One way to achieve a better protection of buildings and structures, is to seismically isolate them. Unlike the traditional design, which focuses in making the buildings strong enough to withstand an earthquake (or at least to prevent their total collapse), seismic isolation manipulates the impact that the earthquake imposes to the buildings. In principle, that is achieved by simply “detaching” the building from the ground, allowing for it to have relative horizontal flexibility, rather than being fixed on it (Figure 1). This ensures the reduction of the impact that the earthquake imposes on the buildings and most importantly minimizes the perception of the inhabitants over the seismic event, which reduces the panic and uncertainties and enables for the structure to remain in operational phase all the time.

However, the current technologies of seismic isolation have their drawbacks, mainly related to the range of protection they provide. Thus, seismic isolation is usually feasible in regions with high seismic hazard and is mainly used in developed countries (and quite often even there it is used in buildings of high importance such as hospitals and strategic facilities). ESR 08 is tackling these drawbacks of seismic isolation by developing new concepts based on bifurcation phenomena and post buckling behavior to be exploited in the design of mechanical metamaterials. More specifically, the geometry of these elementary structures is designed in such a way that they return a behavior that can be tuned to range from a positive stiffness (requiring increasing load to extend the horizontal displacements) to an effective zero stiffness behavior (null resistance to horizontal displacement), with a target activation force that can be designed to be low enough to enable protection even for frequent seismic events, but high enough to remain undeformed when exposed to other lower accidental loads. Moreover, these mechanical metamaterials can be designed to have a perfectly controlled post buckling behavior, avoiding any risk of global stability that is often present in the current technologies. The present work is under review and will be soon published and presented in ICTAM 2020+1 and ICONHIC 2022.

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