Feasibility of Seismic Isolation with Low-Cost Fiber-laminated Recycled Rubber Bearings

Italy surely holds an important record in the development and manufacturing of structural bearings for bridges and other civil applications on a worldwide scale. Moreover, it is on the cutting edge in the seismic isolation field exporting rubber-steel isolators all over the world. Traditional rubber isolation bearings consist of a superposition elastomeric layers and thin steel plates connected through a vulcanization process. Even though these bearings are today made of new natural or synthetic rubber, the New Italian Code for Civil Construction does not exclude the possibility to use other type of rubber or reinforcement, provided that the overall bearing satisfies specified mechanical characteristics. In this paper the feasibility of using fiber-laminated recycled rubber bearings as low-cost seismic isolators is studied, focusing on two important aspects: experimental characterization of the material, theoretical and experimental study of the bearings. The material is composed by small granules or short fibers obtained from the grinding of new or recycled rubber (inert component) connected through each other by a polyurethane glue (the binder). The rubber is derived from used tyres and rubber factory leftovers. Experimental campaign on the rubber material includes compression, tensile and shear tests with the final aim to compare the mechanical properties with those required by the Italian Code and to define the best rubber compound for the specific application. The distinctive feature of the rubber isolator is that its vertical stiffness is much larger than the horizontal one (even several hundred times horizontal stiffness). This is usually obtained using steel plates as reinforcement which is normally considered as infinitely rigid in the analysis. The adoption of glass or carbon fibers as reinforcement requires its flexibility to be taken into account when the vertical stiffness of the bearing is calculated. Starting from well-known results for typical rubber-steel bearings new expressions of vertical stiffness are derived and compared with the experimental results