Performance-based Seismic Evaluations
by Ardalan Sabamehr, P.Eng., Ph.D.
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.
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.
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.
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).
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).
Base shear and roof displacement based on FEMA criteria (2)
Performance design levels and their descriptions (2)
Why performance-based seismic assessment?
Existing buildings are expected to be non-conforming to the prescriptive rules in the seismic codes and the earlier generation of seismic assessment guidelines. By applying the modern seismic standards to these buildings will only yield a large number of ‘non-compliance’ buildings without necessarily an understanding of their actual seismic risk or performance. Similarly, code writers struggle in providing a prescriptive set of rules that could capture all the possible variation of non-compliant existing buildings.
One key principle is that the understanding of how the structural components behave and respond under earthquake shaking becomes more important than necessarily meeting specific code clauses. Similarly a thorough understanding of the underlying principles of various code clauses is required such that ensure that contravening a rule does not unduly jeopardize the minimum levels of resilience that are inherently provided for in a new building.
Performance-based seismic assessments offer a consistent framework for engineers to evaluate how a building may behave in an earthquake, in particular when a building that does not meet a number conventional prescriptive requirements and responds in a highly non-linear manner with potentially mixed-ductility response (3).
Major advantages of performance-based seismic design (4)
Overall, performance-based seismic evaluations are a reliable method for seismic evaluation and seismic damage prediction. Our 3D-SAM software in combination with our rigorous ambient vibration testing solutions are useful tools able to extract the engineering demand parameters needed for these kind of evaluations. This analysis can be done on a global scale or at a component-level to get as very specific details on the predicted performance of the structure.
2. Hakim, R.A., Alama, M.S. and Ashour, S.A., 2014. Seismic assessment of RC building according to ATC 40, FEMA 356 and FEMA 440. Arabian Journal for Science and Engineering, 39(11), pp.7691-7699
4. Advocating for Performance-Based Design, Report to the Structural Engineering Institute Board of Governors, April 2018
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