Explain performance-based seismic design philosophy and its implementation.

Performance-based seismic design (PBSD) explicitly links structural design to desired performance levels under specific earthquake intensities. Performance levels include Operational (minimal damage), Immediate Occupancy, Life Safety, and Collapse Prevention. Implementation involves: (1) Site-specific hazard analysis for multiple return periods, (2) Nonlinear analysis (pushover or time-history), (3) Evaluation against acceptance criteria for drift, plastic rotation, and component capacities, (4) Risk assessment using fragility functions and loss estimation. PBSD allows innovative designs beyond prescriptive codes with documented performance.