A major challenge to performance-based seismic design and engineering of buildings is to develop simple, yet effective, methods for designing, analyzing and checking the design of structures so that they reliably meet the selected performance objectives. Needed are analysis procedures that are capable of predicting the demands-forces and deformations-imposed by earthquakes on structures more realistically than has been incorporated in the ATC-40 and FEMA-274 documents (Applied Technology Council, 1996; FEMA, 1997) to determine the displacement demand imposed on a building expected to deform inelastically.