Analysis of Fiber Reinforced Elastomeric Bearings

Using multilayer springs of rubber and steel is a `state of the art` technique in many areas of application. To achieve the requested combination of horizontal and vertical stiffness it is necessary to influence the shape factor using reinforcement layers. Generally these reinforcement layers consist of steel plates which have several disadvantages and limitations. The development of fiber reinforced elastomeric bearings allows for a substitution of the steel plates by synthetic fabrics with much the same basic mechanical properties. However essential differences and advantages remain on the mechanical and fabrication side. The formulas which describe the mechanical behavior that are given in literature and design codes only give a rough approximation of the bearing behavior, especially for fiber reinforced bearings. They do not take into account effects such as nonlinearity on the material and geometrical side, and do not describe the damping behavior. A deeper knowledge about the mechanical mechanisms of these bearings has to be attained, which is the aim of this project. The given formulas will be evaluated and extended to meet a more realistic description of the bearing behavior.

Unbonded fiber reinforced elastomeric bearings are reinforced with alternating bonded layers of fiber reinforcement. These layers are in general installed between superstructures (e.g. bridges) and substructure (e.g. retaining walls), with no bonding or mechanical fastening at the contact surfaces of the bearing. As a result, the shear loads are transferred via friction between the bearing and its top and bottom contact surfaces. The primary role of the fiber reinforcement in the bearing is to provide sufficient vertical stiffness by limiting lateral bulging of the elastomer layers when the bearing is subjected to vertical compression. Although the fiber reinforcement provides in-plane tensile stiffness to effectively limit the lateral bulging of the elastomer, its effect on flexural rigidity is negligible. Therefore, when an unbonded fiber reinforced bearing is subjected to horizontal loading, its top and bottom faces partially roll off the contact surfaces and the bearing exhibits what is known as rollover deformation. As a result of rollover deformation, the horizontal response characteristics of this particular bearing type are significantly different if employed in a conventional bonded application. Nevertheless, in the current research project, the combination of vertical and horizontal loads, in particular horizontal seismic loads, has been analyzed. In particular, an extension of the existing analytical formulation was developed in order to capture the frequency dependent effects on the vertical and horizontal displacements. An important goal was to capture the horizontal displacements for up to 150% with regard to the bearing height and the considerable nonlinear damping effects resulting from the rollover deformation. The numerical and experimental investigations were carried out both with carbon and with glass fiber reinforced elastomeric bearings. These investigations encompassed: Fiber reinforced full scale bearings (glass-fibers and carbon-fibers) were modeled on a Finite Element basis via hyper-elastic material models in order to capture their behavior under combined vertical and horizontal loads. Comprehensive small scale and large scale experiments were performed in order to create input parameters for (a) the adjustment of the sophisticated hyper-elastic material models required in the Finite Element Analysis and (b) the analysis of the load and frequency dependent damping of glass fiber fabrics, carbon fiber fabrics and elastomers. Finally, the numerical and experimental investigations established the definition of load and frequency dependent damping behavior (of extreme importance in seismic analysis), the definition of fiber reinforced bearing quantities under combined vertical and horizontal loads, and the adaptation of hyper-elastic material laws and of code specific analytical formulations for unbonded fiber reinforced elastomeric bearings.