The issue of seismic vulnerability assessment and rehabilitation of underperforming existing buildings is a very important and complex problem, especially in seismic-prone areas for buildings that were constructed before modern seismic codes.

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Performance-based seismic design (PBSD) is a concept that is currently being applied in seismic design on a variety of buildings and bridges. Its main goal is to produce structures that will have predictable results in the event of an earthquake.

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A defining parameter in PBSD is its performance objective: the acceptable level of damage selected for a specified earthquake intensity level. A building may be designed based on one or multiple performance objectives.  For example: a residential building could be designed for two performance objectives: 1) to be fully operational (no damage, continuous service) based on an earthquake of low intensity and high probability of occurrence and, 2) to achieve collapse prevention (extensive damage while not endangering the lives of its occupants) for a severe but low probability earthquake events. 

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The selected performance objectives will depend on the intended use of the structure; for example safety-critical buildings, such as hospitals and fire halls, are required to remain operational (light damage, most operations can resume immediately) after a severe earthquake event (1).

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The recent building standards (ATC, FEMA) for seismic design and evaluation have applied performance-based design criteria that estimate the nonlinear response of the building. Performance-based design is started by making a structural model and then simulating its performance against the expected seismic excitation. Each simulation provides the level of damage so that a structural engineer can manage the risk of damage in terms of recovery cost (2).