Experimental and Numerical Simulation of Seismically Isolated Critical Facilities under Extreme Seismic Loading
Seismic isolation can be an effective strategy to protect critical facilities including Nuclear Power Plants (NPPs) from the damaging effects of horizontal earthquake ground shaking. For critical facilities, the isolation system should demonstrate a high-confidence of low-probability of failure at the design level and the load carrying capacities should be maintained under beyond design earthquake shaking (BDBE). Experimental evaluation of seismic isolation bearings is important to fully understand their behavior and capacity for reliable performance. Safety mechanisms such as a stop can be imposed to prevent excessive displacement of the isolation system under BDBE, however, this raises concerns for detrimental effects of pounding against a stop or moat wall. Methods of analysis are presented in this dissertation to evaluate both seismic isolation system behavior under extreme earthquakes and the potential effects of pounding by imposing displacement restraints.
The dynamic response of an isolated NPP depends on the combined characteristics of the ground motion, bearings, and structure while the seismic isolation bearings themselves can exhibit complex nonlinear behavior that depends on several factors, including the scale size, axial load, temperature, and rate of loading especially under strong earthquake shaking. With a specific interest on the in-structure response of seismically isolated NPPs, hybrid simulation is shown to be a viable approach to examine bearing behavior at full scale under realistic earthquake loading. The adaptation of a full-scale bearing test machine (SRMD testing facility at UC San Diego) and developed toolsets for the implementation of fast hybrid simulation to study the dynamic response of base isolated NPP using full scale lead plug rubber bearings under realistic earthquake loading conditions are presented. Results from these tests validate the effectiveness of seismic isolation technology for application in nuclear facilities and provide valuable data towards improving numerical models of seismic isolation bearings.
In a seismically isolated NPP, a surrounding moat wall can function as a stop to limit isolation system displacements and prevent bearing failure for beyond design basis shaking. Impact of isolated structures against a moat wall is of concern due to potential amplification of superstructure response. A moat wall model able to capture impact forces is proposed and used in numerical simulations to capture the effects of impact on the response of seismically isolated NPPs. Variable clearance to the stop and a range of properties for the impact model, moat wall and isolation system are considered to identify parameters that influence the response. Results indicate that large NPP plants as considered here can have significant penetration into the moat wall, not fully limiting displacements in the isolation system, while causing considerable increases in accelerations throughout the NPP. A simplified methodology to estimate impact response parameters including penetration is proposed towards developing design tools that consider these effects.