Submitted to the Department of Civil and Environmental Engineering.
Thesis (Ph.D.)--Stanford University, 2012.
Performance-based earthquake engineering (PBEE) quantifies the seismic hazard, predicts the structural response, and estimates the damage to building elements, in order to assess the resulting losses in terms of dollars, downtime, and deaths. This dissertation focuses on the ground motion selection that connects seismic hazard and structural response, the first two elements of PBEE, to ensure that the ground motion selection method to obtain structural response results is consistent with probabilistic seismic hazard analysis (PSHA). Structure- and site-specific ground motion selection typically requires information regarding the system characteristics of the structure (often through a structural model) and the seismic hazard of the site (often through characterization of seismic sources, their occurrence frequencies, and their proximity to the site). As the ground motion intensity level changes, the target distribution of important ground motion parameters (e.g., magnitude and distance) also changes. With the quantification of contributing ground motion parameters at a specific spectral acceleration (Sa) level, a target response spectrum can be computed using a single or multiple ground motion prediction models (GMPMs, previously known as attenuation relations). Ground motions are selected from a ground motion database, and their response spectra are scaled to match the target response spectrum. These ground motions are then used as seismic inputs to structural models for nonlinear dynamic analysis, to obtain structural response under such seismic excitations. This procedure to estimate structural response results at a specific intensity level is termed an intensity-based assessment. When this procedure is repeated at different intensity levels to cover the frequent to rare levels of ground motion (expressed in terms of Sa), a risk-based assessment can be performed by integrating the structural response results at each intensity level with their corresponding seismic hazard occurrence (through the seismic hazard curve). This dissertation proposes a more rigorous ground motion selection methodology which will carefully examine the aleatory uncertainties from ground motion parameters, incorporate the epistemic uncertainties from multiple GMPMs, make adaptive changes to ground motions at various intensity levels, and use the Conditional Spectrum (CS) as the new target spectrum. The CS estimates the distribution (with mean and standard deviation) of the response spectrum, conditioned on the occurrence of a target Sa value at the period of interest. By utilizing the correlation of Sa values across periods, the CS removes the conservatism from the Uniform Hazard Spectrum (which assumes equal probabilities of exceedance of Sa at all periods) when used as a target for ground motion selection, and more realistically captures the Sa distributions away from the conditioning period. The variability of the CS can be important in structural response estimation and collapse prediction. To account for the spectral variability, aleatory and epistemic uncertainties can be incorporated to compute a CS that is fully consistent with the PSHA calculations upon which it is based. Furthermore, the CS is computed based on a specified conditioning period, whereas structures under consideration may be sensitive to multiple periods of excitation. Questions remain regarding the appropriate choice of conditioning period when utilizing the CS as the target spectrum. To advance the computation and the use of the CS in ground motion selection, contributions have been made in the following areas: The computation of the CS has been refined by incorporating multiple causal earthquakes and GMPMs. Probabilistic seismic hazard deaggregation of GMPMs provides the essential input for such refined CS computation that maintains the rigor of PSHA. It is shown that when utilizing the CS as the target spectrum, risk-based assessments are relatively insensitive to the choice of conditioning period when ground motions are carefully selected to ensure hazard consistency. Depending on the conditioning period, the structural analysis objective, and the target response spectrum, conclusions regarding appropriate procedures for selecting ground motions may differ.