設計想定を超える地震動に対して高い耐震安全性を有する免震構造の提案

 Since seismically isolated (SI) buildings are thought to be fragile when subjected to extreme input ground motions beyond design level, various measures such as shock absorbers on moat walls and stoppers put in isolation level have been developed. Those technologies should be helpful to increase seismic safety margin of SI buildings, while they cannot avoid the damage to moat walls, superstructure, or isolation devices. In this paper, the authors propose a new lead rubber bearing (LRB) which has much higher seismic safety compared to general isolation bearings including conventional LRBs.<br> The proposed bearing named “FSLRB” or “Fail-Safe LRB” consists of a conventional LRB and a newly developed slider system in series. Friction factor of the slider system is optimally provided to start sliding just before LRB gives a hardening property with the shear strain of around 250%. Then, the bearing behaves as a general LRB which is just friction connected to the building structure at either of upper or lower end, when subjected to design level earthquakes or smaller, while the sliding behavior keeps LRB away from hardening and shear break in severe earthquakes beyond expectations. Hence, supporting a superstructure only by the proposed bearings will enable the whole SI building to protect from any structural damage regardless of the input ground motion levels. A preliminary time history analysis for two SI building models, one applying only FSLRBs, the other applying only conventional LRBs, shows the advantages of the system with the proposed bearings.<br> Realizing the above high seismic performance, firstly the most suitable slider material, which was a certain PTFE, was selected by conducting a series of dynamic loading test for each combination of stainless steel plate (SUS304) and 12 candidate slider materials. A dynamic loading test of a scaled FSLRB applying the selected PTFE showed that the bearing almost behaved as expected in the preliminary time history analysis. Dynamic and static friction factors were evaluated as well as surface pressure and velocity dependence of those two factors were formulated from the test results. Furthermore, a full-scale FSLRB was tested under a rather high compression force with quasi-static horizontal loading, to investigate the maximum allowable conditions regarding surface pressure and the second shape factor of LRB part. Finally, a detailed time history analysis of a FSLRB building model was conducted. As for the analysis model, 256 full-size FSLRBs with the mechanical properties evaluated in the above loading tests supported a deformed nuclear power plant consisted of a reactor building and a turbine building set on a common isolation raft. The analysis result showed that the superstructure remained elastic and the maximum responses were lower than design criteria even when subjected to a long-period ground motion beyond design level. On the other hand, it became clear that both the maximum deformation and residual deformation at isolation level were much larger than conventional LRB building model.<br>  From the above development and analyses, it can be finally concluded that an SI building applying the proposed bearings can realize much higher seismic safety compared to conventional SI buildings, if only provided a large enough isolation gap considering the maximum considered earthquakes.