High-friction sliding seismic isolation for enhanced performance of light frame structures during earthquakes [electronic resource]
Keywords: Civil and Environmental Engineering.

The primary objectives of this research are to develop an isolation system geared towards light frame structures, and to demonstrate that seismic isolation can be both economical and highly effective for light-frame residential construction. Experience from past earthquakes has shown that light frame residential houses in the United States typically pose a low collapse risk, but they are susceptible to damage that can lead to significant financial loss and displacement of residents. This dissertation develops a low-cost seismic isolation system that balances the tradeoffs between isolator displacement demands and base shear force demands through a high-friction sliding system. The isolation system consists of high-density polyethylene (HDPE) sliders on a flat or an upward concave galvanized steel sliding surface, with a sliding coefficient of friction between 0.15 and 0.25 at the isolation interface. The inherent strength and low mass of light frame residential construction allows for a high-friction isolation system to effectively eliminate superstructure damage during even large earthquakes. Parametric studies show that high friction interfaces reduce isolation displacement demands. Isolator sliding material and component tests have been conducted to characterize velocity and pressure dependent interface friction properties of the sliding isolators. New data is provided on low-pressure (2 MPa to 7 MPa) friction coefficients for HDPE and polytetrafluoroethylene (PTFE) sliding on different finished steel surfaces. Proof-of-concept shaking table tests of a full-scale light framed isolated two-story house have been conducted. Results of these tests have (a) confirmed that the isolation system can effectively eliminate damage to the light frame superstructure under repeated severe earthquake ground shaking (b) validated nonlinear computer models to determine sliding isolator demands and superstructure force demands, and (c) demonstrated the constructability and economic practicality of the proposed details for the foundation and wood-framed first floor isolation platform. Numerous findings regarding the behavior of isolated light frame structures are reported herein. The superstructure of an isolated house has a dynamic response during shaking causing higher normalized forces in the superstructure than at the isolation level. Superstructure seismic coefficients are recommended to be 1.8 times the coefficient of friction in the flat sliding system, and 1.2 times the normalized isolation force (assuming rigid body response) in the dish system, to achieve a 20% probability of the induced forces greater than design forces at MCE intensity shaking. The inclusion of the vertical component of the ground motion is shown to not significantly affect isolation response in high friction sliding systems, however, there can be large increases in superstructure and isolator force demands. While using velocity and pressure-dependent friction models can affect the sliding isolation response of systems subjected to individual ground motions, Coulomb friction is found to be sufficient for response prediction when looking at a suite of records. The controlling design variable for seismic isolation systems is the peak isolation displacement. Typically, traditional isolation systems with low yield force and high restoring stiffness are assumed to have an effective period, based on the secant stiffness at the expected displacement. This period is used to infer the seismic displacement demands on the isolator, using the spectral displacements at the assumed period. For a high-friction low-restoring sliding system, the effective period is sensitive to the expected displacement. Additionally, these isolation systems accumulate displacement through ratcheting-type behavior, controlled by the friction properties of the isolation system, rather than exhibiting harmonic-type response as would an equivalent linear system with an effective period. Risk-based methods for determining isolation displacements rely on an assumed period of the isolation system as well. Using the conditional spectrum when scaling records to have the same spectral acceleration at the assumed period and matching the expected median and dispersion of spectral acceleration away from the assumed period results in large dispersion in isolation displacements because spectral ordinates, which are based on linear analysis, do not correspond to nonlinear isolation response. Alternative intensity measures that account for the aspects of the ground motion and sliding system that lead to large isolation displacements are required to make robust predictions of isolation displacements. A new ground motion intensity measure for high friction sliding systems is derived herein from closed-form solutions for the peak sliding displacement of a rigid block subjected to simple pulses. The new intensity measure is analogous to the extra incremental ground velocity of the pulse beyond what it would take to yield the system times the duration of the pulse and is termed the Effective Incremental Ground Velocity (EIGV). EIGV is effective for predicting peak sliding displacements for high friction systems because the large sliding excursion in high friction systems is typically similar in magnitude to the overall peak isolation displacement, which can be captured by a pulse intensity measure. A first generation set of ground motion prediction equations are offered for conducting probabilistic seismic hazard analysis using EIGV, which in term can be used to generate isolation displacement demand curves, better informing design decisions.