This paper presents an experimental study of a low-cost seismic isolator that can be used for the protection of low-rise houses in low-income countries. The isolator is based on rolling of a rubber sphere on flat or spherical concrete surfaces. Using a closely spaced grid of such spheres may allow for avoiding of the diaphragm slab at the isolation level, or of reducing its thickness. Avoiding the cost of this extra slab is crucial for building seismically isolated low-rise dwellings economically feasible in low-income regions of the globe.The effects of the geometry of the rolling surface (i.e., flat or concave), of the diameter of the rolling sphere (i. e., 25, 50 or 100 mm), and of the applied compressive load on the seismic behavior of such isolation bearings were investigated. Initially, the rubber isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Subsequently, cyclic tests were performed to obtain the lateral force--displacement diagram of the isolation system. Finally, a slab supported by 4 isolators was subjected to a group of 61 ground motions. Eight different configurations were tested, leading to a total of 488 dynamic tests.It was found that the restoring force of such systems can originate not only from the curvature of the concrete surface, but also from the vertical motion induced by the drastic change of the shape of the rubber spheres, as well. In fact, for the configurations tested, the vertical motion sourcing from shape of the deformed sphere was much larger than the vertical motion sourcing from the curvature of the concrete surfaces.Moreover, equations offering the coefficient of friction as a function of the vertical load, the size and the deformation of the sphere were derived. This is crucial for design, as the governing parameter for the design of the rubber spheres is not material failure, but excessive compressive deformation that leads to undesirably high rolling friction. Dynamic tests proved that the proposed low-cost rolling rubber isolators can substantially reduce the accelerations transmitted to the superstructure The cost of the tested 25 mm, 50 mm and 100 mm diameter natural rubber spheres is 0.

No abstract

Global level assumptions of numerical models have received relatively less attention, but have been indicated to be a major source of error in numerical modeling of Reinforced Concrete (RC) structures. In parallel, it has been stated that a statistical approach involving many virgin specimens and ground motions is necessary for model validation. Such an approach would require very small‐scale testing. Then, the reinforcement fabrication becomes a major issue. This paper proposes using additive manufacturing to fabricate the reinforcement cage. It presents the results from cyclic tests on 1:40 RC cantilever members. The cages were manufactured using an SLM 3D printer able to print rebars with submillimeter diameters. Different longitudinal and transverse reinforcement configurations were tested. A numerical model using existing Opensees elements was built and its parameters were calibrated against material level small‐scale tests. It captured the cyclic response of the RC members with a reasonable accuracy. The cyclic behavior of the RC members resembles the behavior of full‐scale RC members indicating that such small‐scale specimens can be used for the statistical validation of the global level assumptions of numerical models.

Rocking motion is notoriously sensitive to the parameters that define it, with experimental tests oftentimes being non-repeatable. Therefore, validating numerical models using a deterministic approach is impossible, since the consistency of any benchmark experimental test is dubious. Three-dimensional rocking is even harder to predict than planar rocking. This paper presents a threedimensional finite element model to predict the statistics of the rocking/sliding response of free-standing cylindrical columns. The response parameters of interest were the maximum displacement at the top of the columns and the residual displacement. Three different columns with varying slenderness and size were examined. The columns were able to slide, rock, and wobble in all directions, with this behavior being representative of building components and monumental structures. The numerical results were statistically compared to a large database of experimental tests, proving the accuracy of the proposed model. The influence of all modeling and physical parameters was elucidated, employing a large number of non-linear time-history analyses. It is shown that, when the numerical parameters are varied within a reasonable range, they do not influence the statistics of the response, even though they influence each individual oscillation. The friction coefficient between the interfaces (physical parameter) can influence the statistics of the response and should be carefully selected. Energy dissipation should be modeled explicitly, following the physics of the problem.

This paper presents an experimental study of a low-cost seismic isolator that can be used for the protection of residential structures in low-income countries. The isolator is based on mortar-filled, used tennis spheres, rolling on flat or spherical concrete surfaces. The tennis spheres serve as permanent, spherical molds to cast mortar, and they are not removed after casting. The thin rubber shell of the tennis sphere offers increased damping and reduces stress concentrations at the contact areas. At the same time, this procedure creates a promising solution for the re-use of tennis spheres. Using a closely-spaced grid of such spheres may allow for avoiding the diaphragm slab at the isolation level, or reducing its thickness. Avoiding the cost of this additional, heavily reinforced isolation slab is crucial for making seismically isolated low-rise dwellings economically feasible in low-income regions of the globe. Initially, the tennis isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Different mixes and low-cost reinforcement approaches to increase their strength were tested. Subsequently, cyclic tests were performed to obtain the lateral force-displacement diagram of the isolation system. The effects of the geometry of the rolling surface (i.e., flat or concave) and of the applied compressive load (i.e., 2.08, 3.23, 4.74, or 8 kN/sphere) on the cyclic behavior were investigated. It was found that the restoring force of such systems mainly originates from the curvature of the concrete surface. However, the vertical motion induced by the compressed sphere and its local casting imperfections is not negligible. When surface imperfections become significant, the force-displacement loops deviate from the bilinear curves that a rigid-body model suggests. When the spheres are properly cast, they experience zero damage even under 8 kN of compressive force, and their loops have a bilinear form. For the tested configurations, the rolling friction (defined as the ratio of lateral to vertical force at zero displacement) was in the range of 4.7–7.2%, thus suitable for seismic isolation applications. The cost of the tested tennis ball isolators was 0.05 $ per sphere.

Seismic isolation is a mature and effective method of reducing earthquake-induced building damage. However, applications of this technology are limited to special and important structures, predominantly in the developed world, due to the associated high costs. An experimental study of a spherical rubber seismic isolator is presented herein. The cost of these devices is sufficiently low that their wide-spread application in low-income countries seems economically viable. Using a closely spaced grid of such spheres may require only a thin, lightly-reinforced diaphragm slab above the isolation level, further reducing construction costs. Avoiding the cost of this extra slab is crucial to make seismically isolated low-rise buildings economically feasible in poor regions of the globe. The examined seismic isolation bearings are based on rolling, with rubber spheres rolling on concave (spherical) or flat concrete surfaces. Concave concrete surfaces provide restoring force to the isolated structure through gravity. In contrast, in the case of flat concrete surfaces, no restoring force is applied. Rubber spheres offer increased damping and better stress distribution in the contact areas than spheres made using stiffer materials. This work investigated the effects of the geometry of the rolling surface (i.e., flat or concave), the diameter of the rolling sphere (i.e., 50 or 100 mm), and the applied compressive load on the seismic behavior of these isolation bearings. Initially, the rubber isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Subsequently, cyclic tests were performed to obtain the lateral force-displacement diagram of the isolation system. It was found that the coefficient of rolling friction depends on the axial load, and the diameter of the spheres. The governing parameter for the design of the rubber spheres is not material failure, but excessive compressive deformation that leads to undesirably high rolling friction. Overall, experimental results proved the efficiency of the investigated system in terms of decreasing the inertia forces transmitted to the superstructure.

This paper presents an extensive experimental study of a low-cost, high-performance seismic isolator comprising a deformable sphere rolling on concrete surfaces. Polyurethane spheres, with and without steel core, rolling on flat or spherical concrete plates, are investigated. Lateral cyclic tests under large displacements demonstrated a rolling friction coefficient between 3.7% and 7.1%. When tested in a shake table under 1170 ground motions, the isolators substantially reduced the acceleration transmitted to the superstructure (to less than 0.15 g) while maintaining reasonable peak and negligible residual displacements. A phenomenological model was calibrated on the lateral cyclic tests and predicted the shake table tests with reasonable accuracy.

No abstract