Experimental Study of Unbonded Fiber Reinforced Elastomeric Bearings

Abstract

<p>Multilayer elastomeric bearings are a type of seismic isolation device that mitigates seismic damage by lengthening the fundamental period of a low-rise structure. Carbon fiber reinforced elastomeric isolators (FREIs) have been identified as a cost effective alternative to bearings reinforced with steel shims. The stable unbonded fiber reinforced elastomeric isolator (SU-FREI) is an evolution of the FREI. In an attempt to reduce costs even further, FREI bearings of a specific aspect ratio and shape factor have been investigated in an unbonded application with successful results. SU-FREI bearings have shown potential as a viable solution for a more affordable and efficient method of seismic base isolation. Several experimental test procedures were employed in this thesis to further investigate the performance of SU-FREI bearings.</p> <p>The first objective of this research was to compare the dynamic properties of unscragged SU-FREI bearings at parallel and diagonal orientations. Square 1/4 scale SU-FREI bearings were subjected to cyclic excitation under design axial load at 0° (parallel) and 45° (diagonal) orientations. Square SU-FREI bearings achieved acceptable base isolation characteristics at both orientations despite subtle differences in their mechanical properties. Stable rollover (SR) deformation was observed for both orientations.</p> <p>The stability of SU-FREI bearings under dynamic excitation was the next topic investigated in this thesis. To achieve this, 1/4 scale square SU-FREI bearings were subjected to cyclic testing under incrementally increasing lateral displacement amplitudes and axial loads. It was found that the critical buckling load under dynamic excitation decreases with increasing lateral excitation amplitude. SU-FREI bearings exhibited acceptable performance at axial loads well in excess of expected design axial loads.</p> <p>In addition, an ultimate shear properties test was performed in order to investigate rollout instability in SU-FREI bearings. Rollout was not observed in bearings tested in this study. Test results did however highlight the stiffening effect of vertical facial contact throughout roll over deformation.</p> <p>Finally, SU-FREI bearings underwent cyclic testing under serviceability and fatigue conditions. Serviceability tests were performed on 1/4 scale SU-FREI bearings at lateral displacement amplitudes corresponding to those expected from a 1 in 10 year return period wind pressure. Fatigue testing was performed on 1/4 scale bearings at a lateral displacement amplitude equal to the total design displacement (D_TD) as required by ASCE 7-05. SU-FREI bearings displayed adequate scragged performance under both serviceability and fatigue testing. Both effective stiffness and damping remained within acceptable limits throughout these tests.</p>