Unbounded Low Damping Elastomeric Bearings as an Isolation System in Bridges: A State of the Art

Seismic isolation, as an effective strategy to protect bridges against earthquakes, has become prevalent in recent years. Unbounded elastomeric bearings made by thin layers of steel shims bonded by rubber are one of those devices used in this regard, which has been of interest for researchers recently. This paper discusses different characteristics of these bearings and presents related researches done and their concluding remarks. Horizontal stiffness, friction coefficient and damping properties which are three important parameters affecting bearing behavior as an isolation system have been discussed. Relevant models and their assumptions for each parameter is presented. Finally an approximate parametric study is done based on soil type and vertical pressure on the bearing to show the range of applicability of this system. It was shown that the system described above is appropriate for heavy concrete decks like precast or cast in place segmental box girders and rarely in cast-in-place concrete slabs with a period more than 2s. Besides, depending on the soil type and vertical pressure the proper bearing will be of about 800- 1000mm diameter and 250-280 mm elastomer thickness.