ABSTRACT
A major issue in the development of performance-based design standards is the selection of suitable design performance objectives. As defined in the FEMA-273 (ATC-1996a) and Vision 2000 (SEAOC-1994) projects, a performance objective consists of the specification of one or more design ground motion levels and corresponding limiting damage states, termed performance levels. In both the Vision 2000 and FEMA-273 projects, performance levels are defined in terms meaningful to the lay community. These include the risk to life safety during the event, the ability to repair and re-occupy following the event, and similar measures of performance. These are not particularly useful for design purposes. Consequently, the FEMA-273 project linked specific component-based performance measures such as yielding and deformation of frames, cracking of walls, etc., to these qualitatively defined performance levels. In turn, acceptance criteria for these component level damage states, expressed as permissible ductility capacities, were set.
Although it is an expedient approach, the evaluation of performance, on a component level, without consideration of global behavior has a number of disadvantages. For one, it is difficult to provide a consistent margin of safety at the different performance levels, using a component based approach. Also, the definition of acceptable levels of component damage, tied to the various qualitative definitions of performance, is highly arbitrary. Evaluation of performance acceptability should be based on a global evaluation of the structure’s response and the margin against collapse provided for various levels of ground motion.
