Dynamic stability of elastomeric seismic isolation bearings and seismic protection using enhanced adaptive negative stiffness system
Doctor of Philosophy
In this study the issue of stability of elastomeric bearings subjected to extreme dynamic loading is studied in detail and response reduction strategies in structures both fixed base and base-isolated structures using a negative stiffness device (NSD) are evaluated. For response reduction, the composite response spectrum approach is utilized to provide a comprehensive representation of the trade-offs associated with each choice of design strategies. A comprehensive database of strong ground motion is used to study the suitability of NSD for seismic protection. The spectral characteristics of all the ground motions selected are identified using a mathematical model and the characteristics of the ground motion that affect the structural assembly and the response reduction strategies considered are evaluated. The results of the study are organized based on the identified characteristics of ground motion to demonstrate the effectiveness of the negative stiffness device for response reduction. The analytical study takes into account both linear fixed base structures as well as base-isolated structures equipped with linear and nonlinear dampers. For stability of elastomeric bearings, detailed analytical models that capture the nonlinear behavior of the bearings under extreme axial loads and horizontal displacements are developed, calibrated and verified using experimental data. A new analytical model is developed to capture the highly nonlinear horizontal behavior of the bearings under dynamic loading. This includes both the response under normal operating conditions as well as the instability experienced by the bearings under large axial loads and extreme horizontal displacements. The analytical model developed for the horizontal behavior of bearings is enhanced to include the coupled horizontal-vertical behavior of the bearings with the main objective of understanding the actual behavior of the bearings at the instant it experiences loss of stability. An important distinction is made between the geometrical effects and deformation of the bearing that contributes to its vertical response at this instant. The important effect the increased vertical reaction has on the bearing stability is demonstrated and an empirical analytical model is developed to capture this effect. By using the analytical models developed above for elastomeric bearings the response of a six storeyed structure isolated using elastomeric bearings is studied for different ground motions. The effectiveness of the NSD in reducing the response of the structure for different ground motions is demonstrated based on this study and the findings are organized using the identified characteristics of the ground motion. By identifying completely the limitations of the isolation system and subjecting it to actual ground motions the behavior of the system under extreme ground motions is studied. For instances where the bearings fail due to severe ground motions, addition of an NSD not only reduces the response of the superstructure but also protects the bearings by retaining their stability. Stability retention is mainly achieved due to a decrease in the imposed axial loads in the system with an NSD. A fail-safe NSD with mechanical displacement feedback is proposed to prevent accidental failure of isolation systems and its effectiveness is demonstrated for severe ground motion using analytical study.
Elastomeric bearings; Dynamic stability; Seismic protection; Negative stiffness device