The Performance of SU-FREIs (Stable Unbonded - Fiber Reinforced Elastomeric Isolators)

Abstract

<p> Steel-reinforced elastomeric isolator (SREI) bearings are currently the most commonly used type of seismic isolators. However, high manufacturing and associated installation costs prohibit their application in ordinary residential and commercial buildings. Fiber-reinforced elastomeric isolator (FREI) bearings are comprised of alternating layers of elastomer bonded to fiber-reinforcement layers. Research studies have shown that FREI bearings can be used as an alternative to SREI-bearings with comparable performance.</p> <p> FREis are much lighter in weight than traditional SREIs. In addition, their manufacturing cost can be lower, if individual FREI bearings, with the required size, are cut from a large sheet or a long strip, fabricated through mass production manufacturing techniques. An appealing application which simplifies the installation of FREI bearings is when they are placed between the substructure and superstructure with no bonding at their contact surfaces. This specific application is denoted as "unhanded application".</p> <p> When an unbonded FREI bearing is deformed laterally, it shows "rollover deformation" due to lack of flexural rigidity in the fiber-reinforcement sheets. The rollover deformation, as a beneficial feature, reduces the lateral stiffness of the bearing and enhances its seismic isolation efficiency, compared to the same bearing employed with bonded contact surfaces. However, it is important that the bearing is properly sized to maintain its lateral stability and hence exhibit ''stable rollover" (SR) deformation. Such a bearing is termed in this thesis as "stable unbonded" (SU)-FREI bearing.</p> <p> The main objectives of this research were to investigate the influence of geometry on the lateral response behavior of unbonded FREI bearings, and to evaluate the feasibility of employing SU-FREI bearings for seismic mitigation of low-rise buildings. The first objective was accomplished by conducting an experimental study on full-scale square FREI bearings. To achieve the latter objective a shake table study was performed on a 1/4 scale 2-storey steel frame which was seismically isolated with 1/4 scale SU-FREI bearings. The mechanical properties, including vertical and lateral stiffnesses and effective damping, of prototype samples of the 1/4 scale SU-FREI bearings were evaluated by vertical compression testing and cyclic lateral shear (under constant compression) testing. In addition, the influence of parameters such as lateral displacement amplitude and rate, amplitude history, and variations in the vertical pressure on the lateral response of the 1/4 scale SU-FREI bearings, were investigated.</p> <p> It was found that for FREI bearings having identical material properties and shape factor (the plan area of the bearing divided by the perimeter area of a single elastomer layer) the aspect ratio (length to total height of the bearing, also called second shape factor) plays an important role in achieving stable lateral response. All tested prototype 1/4. scale SU-FREI bearings exhibited SR-deformation with sufficient lateral flexibility and damping. Lateral response was found to be nonlinear and dependent on the amplitude and history of lateral displacement. However, due to the application of a relatively low design vertical pressure of 1.6 MPa, the influence of ±50% variation in the design vertical pressure on the lateral response was found to be insignificant. Lateral displacement capacity of the SU-FREI bearings was attained when their originally vertical faces fully contacted the upper and lower horizontal supports. This was accompanied with a significant increase in the lateral stiffness of the bearings which maintains the overall stability of the bearing to unexpectedly large ground motions. Shake table tests clearly demonstrated that SU-FREI bearings were efficient in seismic mitigation of the test-structure.</p> <p> The final component of this thesis involves investigating the applicability of two simplified analytical models in seismic response prediction of a base isolated structure. The two models use different techniques to simulate the lateral load-displacement hysteresis loops of prototype SU-FREI bearings which are obtained from cyclic shear tests (under constant compression). Model 1 includes the rate and the amplitude of bearings' lateral displacements to simulate the hysteresis loops through a multi-parameter curve fitting function. Model 2 uses bilinear idealization to simulate the hysteresis loops. Due to the highly nonlinear lateral response of SU-FREI bearings, these models utilize an iterative time history analysis approach to improve their accuracy. Comparisons with shake table results of a 1A scale structure show that both models can be used in response prediction of ordinary structures which are seismically isolated with SU-FREI bearings.</p>.