Adaptive Characteristics of Fiber-Reinforced Elastomeric Isolators

Abstract

Seismic base isolation has become an increasingly common approach to reduce earthquake induced losses. Base isolation aims to decouple structures, such as buildings or bridges, from strong ground motions through the introduction of a flexible layer, typically located at the foundation. Base isolation is a well-established concept and accepted as an effective method of protecting both the structure and its contents from damage due to earthquakes. Elastomers are ideal for base isolation due to their soft material properties and ability to undergo large recoverable strains. Steel-reinforced elastomeric isolators (SREIs) have been widely applied as base isolators; however, the weight and cost of SREIs have been perceived as barriers to the widespread application of base isolation. In order to alleviate these concerns, it has been proposed that the steel reinforcement could be replaced with lighter fiber reinforcement with similar tensile properties as steel. Recent investigations have demonstrated that fiber-reinforced elastomeric isolators (FREIs) are viable and have desirable characteristics. An additional proposed cost saving measure was to place the FREI unbonded between the upper and lower supports. The combination of the flexible fiber reinforcement and the unbonded application resulted in a unique rollover deformation under horizontal displacement. Rollover causes a nonlinear force-displacement relationship characterized by a softening and stiffening phase. This nonlinear relationship is believed to be advantageous and to allow the performance of the device to be tailored to the earthquake hazard level. This work investigates the adaptive characteristics of unbonded FREIs. It is demonstrated that the softening and stiffening characteristics of the isolator can be altered through modifications to the isolator or to the surrounding support geometry. Equations are developed to predict the horizontal force-displacement relationship. Furthermore, simple expressions appropriate for use in building and bridge design codes are proposed for critical isolator properties. Potential limitations introduced due to the unbonded application are identified and addressed through the development of a new partially bonded hybrid isolator. It is demonstrated that unbonded FREIs are highly versatile and a potentially competitive device appropriate for widespread application in developed and developing countries.