Investigation of the behaviour of seismic isolation systems for bridges.
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The thesis is concerned with the experimental and analytical investigations into the behaviour of laminated elastomeric and lead-rubber bearings as seismic isolation bearings for bridges. Seismic isolation as a seismic design option has become popular over the past decade. During last several years, Japan, Italy and the U.S.A. have been making great progress following the great contribution of New Zealand in this area. Despite new developments in seismic isolation systems, the New Zealand lead-rubber bearings (LRB) still lead with their reputation for reliable performance. The design methods for lead-rubber bearings applied for both bridges and buildings are provided in each of these countries but with differences to allow for variations in codes and design applications. However the basic concepts of these design methods are similar and are partially based on empirical backgrounds. Acknowledging the above status, the experimental and analytical investigations into the lead-rubber and laminated elastomeric bridge bearings were carried out under compression, shear and rotation loading states. A maximum bearing compressive strain of about 6% was reached in the compression tests. In the shear tests, a maximum bearing shear strain of 200% was reached in the load-deflection response and the lead-rubber bearings showed almost the same shear stiffness as the elastomeric bearings after yield of the lead. The rotation tests were performed over the limitations stipulated in the current design codes showing a constant linear stiffness independent of the axial load levels on the bearings. The analytical investigation using the finite element method indicated that the steel shims in the bearings might exceed the yield point somewhere between 100% and 150% bearing shear stains, The shear strains in the rubber due to different types of loading calculated by the existing code relationships, and which are usually the governing factor for the bearing design, were not in reasonable agreement with the analytically obtained values except under compression. The relationships for predicting load-deflection response of the bearings used in the bearing design methods always gave excessive values when compared with the experimentally and analytically obtained values. Based on the findings in the experimental and analytical work, several recommendations for the design of seismic isolation bridge bearings are presented. Moreover, the dynamic response analysis for bridges incorporating seismic isolation systems is carried out under different seismic excitations to confirm the benefits of the seismic isolation systems. As a consequence, for continuous multi-span long bridges the benefits due to seismic isolation in terms of reducing member forces in the bridge and keeping the dynamic behaviour of bridge piers uniform regardless of travelling velocities of the seismic wave are greater when compared with those for standard bridge structures.