Seismic isolation is an efficient technology that, due to its cost has mainly been applied in high importance projects in rich countries. The cost originates from both the devices per se and the extra slab that is needed to work as a diaphragm at the isolation level. The development of practical and affordable isolation devices could lead to the more widespread adoption of isolation and improved seismic protection of larger numbers of structures.
In the ETH Zurich, together with international colleagues, Dr. Dario Candebat Sanchez, Head of the Earthquake Engineering Group of the National Center for Seismological Research (CENAIS) of Cuba, and Professor Marcial Blondet, Pontificia Universidad Catolica del Peru (PUCP), we are exploring an affordable isolation device that has the potential of being so affordable that it will remove the cost barrier for the use of seismic isolation in low-income countries for lightweight structures and low-cost construction. It is a rolling variation of the well-known spherical, or alternatively named pendulum, sliding isolation concept: a rubber ball rolling in a concave spherical concrete surface. Rubber is proposed for two reasons: a) to reduce the stress concentrations that harder materials would cause and b) to increase energy dissipation by increasing the rolling friction. Notably, the rolling friction coefficient does not depend on the properties of the interface between the rolling sphere and the concrete, as the mechanism of rolling friction is entirely different than that of the sliding one. Instead, energy dissipation is due to the continuous deformation of the sphere as it rolls and therefore depends primarily on the properties of the rubber material and is not much influenced by the adjacent interface. Hence, we expect that assuring the manufacturing quality of such bearings will be much easier than assuring the coefficient of friction of sliding bearings, and thus a reliable and affordable isolation mechanism would be achieved. For the spheres and the compression loads tested, the rolling friction coefficient is on the order of 5%.
Professor Michalis Vassiliou
Chair of Seismic Design and Analysis
Institute of Structural Engineering
ETH, Zurich
Dr. Antonios Katsamakas
Chair of Seismic Design and Analysis
Institute of Structural Engineering
ETH, Zurich
Sergio Reyes
Chair of Seismic Design and Analysis
Institute of Structural Engineering
ETH, Zurich
Dr. Dario Candebat Sanchez
National Center for Seismological Research (CENAIS) of Cuba
Professor Marcial Blondet
Pontifica Universidad Catolica del Peru (PUCP)
Figure 1. (a): Force deformation loops of an elastomeric sphere rolling on a spherical surface, carrying different compressive loads (Katsamakas and Vassiliou, 2023); (b): Schematic representation of creep and roll of a rubber sphere.
The behavior of such rolling systems and their modelling, however, is more complex than the almost perfect bilinear behavior of sliding bearings: under gravity loading over a period of weeks the rubber spheres creep to an oval shape, but under seismic motion that lasts for a few seconds the oval object does not have the time to recover its original spheroidal shape (Figure 1b). This means that in the seismic loading condition, essentially, it is an oval-shaped body (and not a sphere) that is experiencing rolling. This results in a fluctuating restoring force. Figure 1a shows this restoring force for a solid sphere rolling on a flat and a spherical concave surface. The fluctuations cannot be seen, as the displacement capacity of the setup was not large enough to induce a 180o rotation of the spheres. However, the negative stiffness of the flat bearings (for this range of displacements) can be seen clearly and is due to the change of shape of the sphere. When a spherical surface is used as a support, the positive stiffness contribution from the geometry of the rolling surface counteracts the negative stiffness due to the oval shape of the rolling sphere. Such behavior a) is harder to describe phenomenologically and b) leads to an unexplored behavior of the superstructure. To reduce this creep-related phenomenon a steel core can be used within the sphere. We have found that the addition of a rigid core significantly reduces creep without dramatically increasing the stress concentration at the region of contact or reducing the energy dissipation. Moreover, a sufficiently large steel core reduces the change of shape of the sphere and leads to almost bilinear hysteresis loops – with the coefficient of friction still close to 5%. The rubber spheres that we tested cost only about $20 (100 mm diameter), meaning that numerous spheres can be placed continuously under the isolated structure, thus minimizing the amount and cost of reinforcement required for the diaphragm above the plane of isolation.
Figure 2. (a): Manufacturing process of grout-filled tennis balls; (b): Tennis balls between concrete plates; (c): Loops for flat and spherical (isolation period T=2 sec and T=3 sec) concrete plates
We have also tested a variation of the rubber-steel roller bearing, where instead of a rubber sphere a tennis ball filled with cement grout is used. The tennis ball can support up to 30 kN of compressive load and the lateral force deformation loops are bilinear (with a coefficient of friction between 5% and 7%), due to the low thickness of the rubber layer (Figure 2).
We are currently using a finite element model of the rolling spheres to investigate the design of the steel core. At the same time, we are designing shake table tests of a masonry house isolated on such a system (Figure 3). The tests are expected to take place in 2024 using the shake table of the National Technical University of Athens.
References and additional reading:
Cilsalar, H., & Constantinou, M. C. (2019). Behavior of a spherical deformable rolling seismic isolator for lightweight residential construction. Bulletin of Earthquake Engineering, 17(7), 4321-4345.
Katsamakas, A. A., Chollet, M., Eyyi, S., & Vassiliou, M. F. (2021). Feasibility study on re-using tennis balls as seismic isolation bearings. Frontiers in Built Environment, 7, 768303.
Katsamakas, A. A., Belser, G., Vassiliou, M. F., & Blondet, M. (2022). Experimental investigation of a spherical rubber isolator for use in low income countries. Engineering Structures, 250, 113522.
Katsamakas, A. A., & Vassiliou, M. F. (2023). Experimental parametric study and phenomenological modeling of a deformable rolling seismic isolator. Journal of Earthquake Engineering, 1-30.
Katsamakas, A. A., Del Giudice, L., Reyes, S. I., Candebat-Sanchez, D., & Vassiliou, M. F. (2023). Experimental and numerical assessment of grout-filled tennis balls as seismic isolation bearings. Engineering Structures, 294, 116716.
Reyes, S., & Vassiliou, M. F. (2023). Recent Advancements in Rolling Isolation Systems Using Elastomeric Spheres: Numerical and Experimental Results. In 18th World Conference on Seismic Isolation (18WCSI).
Robinson W. (1999), International Patent application, WO 99/07966.
