Sliding Instability of Fiber-reinforced Elastomeric Isolators in Unbonded Applications

Fiber-reinforced isolators not anchored to the structure represent a new development in seismic isolation. Their light weight, low cost and great ease of installation, in comparison with conventional elastomeric isolators, could allow to extend the use of these valuable devices to seismic protection of housing and commercial buildings, particularly in developing countries. As demonstrated in studies available in the literature, fiber-reinforced isolators match the behavior of steel-reinforced ones. However the friction behavior between such new devices and the structure has not been investigated yet. This paper describes an experimental study on elastomeric isolators reinforced by bi-directional carbon fiber fabrics performed to investigate the static friction at contact surfaces. The isolators were placed in contact with concrete surfaces and subjected contemporaneously to compressive stress and shear strains. Friction behavior was analyzed in terms of isolator sliding with respect to the concrete sub- and super-structure. Influence on friction behavior of different parameters, such as level of compressive stress, rubber typology, concrete roughness, aging and loading rate, was investigated. The tests showed that, in relation to the value of the applied compressive stress, uncontrolled sliding of the isolator can occur by increasing the shear force. This unstable condition is called sliding instability herein. The experimental results are explained and the limit values of the compressive stress needed to avoid sliding instability is provided both for new and aged isolators, under quasi-static loading conditions. It is demonstrated that quasistatic tests are reasonably conservative with regard to safety. Patterns of stresses and relative displacements based on the experimental results are also proposed.