Date of Award:

12-2009

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Civil and Environmental Engineering

Advisor/Chair:

Keri Ryan

Abstract

Current design codes generally use an equivalent linear approach for preliminary design of a seismic isolation system. The equivalent linear approach is based on effective parameters, rather than physical parameters of the system, and may not accurately account for the nonlinearity of the isolation system. The second chapter evaluates an alternative normalized strength characterization against the equivalent linear characterization. Following considerations for evaluation are included: (1) ability to effectively account for variations in ground motion intensity, (2) ability to effectively describe the energy dissipation capacity of the isolation system, and (3) conducive to developing design equations that can be implemented within a code framework.

Although current code guidelines specify different seismic performance objectives for fixed-base and isolated buildings, the future of performance-based design will allow user-selected performance objectives, motivating the need for a consistent performance comparison of the two systems. Based on response history analysis to a suite of motions, constant ductility spectra are generated for fixed-base and isolated buildings in chapter three. Both superstructure force (base shear) and deformation demands in base-isolated buildings are lower than in fixed-base buildings responding with identical deformation ductility. To compare the relative performance of many systems or to predict the best system to achieve a given performance objective, a response index is developed and used for rapid prototyping of response as a function of system characteristics. When evaluated for a life safety performance objective, the superstructure design base shear of an isolated building is competitive with that of a fixed-base building with identical ductility, and the isolated building generally has improved response. Isolated buildings can meet a moderate ductility immediate-occupancy objective at low design strengths whereas comparable ductility fixed-base buildings fail to meet the objective.

In chapter four and five, the life cycle performance of code-designed conventional and base-isolated steel frame buildings is evaluated using loss estimation methodologies. The results of hazard and structural response analysis for three-story moment resisting frame buildings are presented in this paper. Three-dimensional models for both buildings are created and seismic response is assessed for three scenario earthquakes. The response history analysis results indicate that the performance of the isolated building is superior to the conventional building in the design event. However, for the Maximum Considered Earthquake, the presence of outliers in the response data reduces confidence that the isolated building provides superior performance to its conventional counterpart. The outliers observed in the response of the isolated building are disconcerting and need careful evaluation in future studies.

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