The utility of ring springs in seismic isolation systems

Abstract

Ring springs are frictional devices consisting of inner and outer ring elements assembled to form a spring stack. External load applied to the spring produces sliding action across mating ring interfaces. Large amounts of energy, as much as 60-70% of the total cycle energy, may be absorbed in overcoming frictional forces. This thesis details the characteristics and dynamic behaviour of ring spring systems and describes the design and testing of a seismic isolation system that uses ring springs. Initially the characteristics and fundamental dynamic behaviour of single-degree-of-freedom mass/ring spring systems are studied. This study uses a model based upon the non-linear force/deflection characteristics of the ring spring. A prototype ring spring cartridge that enables dynamic inputs to be applied to a ring spring was then designed and subjected to short duration shock excitation. Experimental results are compared with those given by computer simulation and are seen to be in good agreement. Ring springs have been identified as suitable devices for use in earthquake-resistant structures. A particularly attractive candidate for use of ring springs is in protecting columnar structures during earthquakes. To enable further study, a pivotal rocking seismic isolation system was developed. So that computational analyses of these systems could be undertaken, the ring spring model has been incorporated within the computer program RUAUMOKO. Dynamic analyses using RUAUMOKO show that pivotal rocking isolation systems can significantly reduce structural loads during short period type earthquakes. Subsequently, a pivotal rocking seismic isolation system was designed and manufactured. Shaker table tests were then carried out on the rocking system for a range of earthquake inputs. The experimental results show that for columnar structures with fundamental periods in the range of dominant spectral accelerations, structural loads can be significantly reduced during short period type earthquakes. Experimental results compare well with those given by computer simulation, thus confirming the effectiveness of the isolation system. The work outlined in this thesis has established a basis from which further research can be undertaken. The pivotal rocking seismic isolation system developed has potential application to protecting a wide range of columnar structures during short period type earthquakes.