Towards creating seismic-proof cities (Meta-Cities)

by Constantinos Kanellopoulos

With the view to protect a large number of existing structures from potentially severe earthquake events, seismic resonant metamaterials (multiple hollow concrete boxes, embedded into the ground, with an internal mass vibrating horizontally, see Fig. 1) are used to reduce the earthquake footprint on a single existing building first. 

In general, metamaterials can be natural or artificial materials designed in such a way that they exhibit extraordinary properties, or, in other words, properties that cannot be met in natural materials. To easily understand what an extraordinary property could be, the following illustrative example is given: Imagine that you hold with your hands a hamburger filled in with some sauce. If you squeeze it, as you would expect, the sauce will start running out. However, if you somehow were able to “design” a hamburger in a way that while squeezing it the sauce is concentrated in the middle instead of running out, that would make it a meta-hamburger, due to its unnatural behaviour.

Figure 1: Resonant unit-cell metamaterial

And why are scientists interested in those extraordinary properties that metamaterials exhibit? Because, by using them, one can manipulate and guide waves in the entire frequency spectrum, from the very high-frequency ones (e.g. electromagnetic waves) to the low frequency ones (e.g. seismic waves), in unprecedented ways.

Therefore, to preliminary evaluate the efficiency of seismic resonant metamaterials in protecting an existing building, a simplified numerical model was developed (Fig. 2). The building is represented by the vertical beam in the middle, with the concentrated mass at the top (accounting for the total mass of the structure) and a foundation at the bottom, while the metamaterials are arranged in two 3×3 groups (18 boxes in total) embedded into the ground, right next to the building.

Figure 2: Overview of the numerical model

In Fig. 3, the accumulated strain energy (damage indicator) in the building is plotted for the case with and without the metamaterials, after the model was shaken with a particular seismic pulse. Close to 50% reduction is observed due to the presence of metamaterials; a rather promising result. Finally, in the video above, the main mechanism behind this reduction can be witnessed, which is the out-of-phase movement (“extraordinary” property) of the internal masses with respect to the surrounding ground (notice at 25 sec, that while the ground moves rightwards, the internal masses are pushing leftwards, reducing the building’s response).

Figure 3: Accumulated normalized strain energy in the beam for the case with and without metamaterials

In conclusion, the ultimate goal of this project would be to implement this seismic protection concept at a large scale (e.g. within a city) to mitigate the damage in structures and subsequently enhance the seismic resilience of communities. 

More information on the use of the Domain Reduction Method to study this problem is openly available in a recent publication (Kanellopoulos et al., 2020):
The following video contains the associated conference presentation in the XI International Conference on Structural Dynamics (EURODUN2020).

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