In recent years, engineers have been employing “Performance-Based Design” to help achieve client’s requirements for structural safety more effectively and more efficiently. These are answers to five common questions on the subject:
Performance-Based Seismic Design, Defined
In its essence, performance-based design (PBD) is engineering to meet defined performance goals. In seismic design, this typically has a component of providing reliability against the threat to life safety of local or global collapse; it may also have a component of protecting against losses due to damage or facility closure. It extends far beyond designing to meet prescribed building codes. Such rules are intended to provide acceptable performance, but code compliance does not always equate to good performance, and the code provisions can sometimes be ineffective, and other times be wasteful in seismic design.
Although performance-based design almost always employs analysis more sophisticated than typical code-based designs, such analysis only helps to verify and ensure the performance. Applying a sophisticated analysis to a poorly proportioned or poorly configured structure will at best lead to a very inefficient design, and at worst lead to a design optimized for the necessarily constrained parameters of the analysis.
Projects Benefitting from PBD
Projects with high-performance goals, including operational post-earthquake requirements and damage control, are well-suited for this approach, as the building codes offer little in insuring such performance. Projects with innovative architecture that bears little resemblance to the simple, regular building types used to develop code procedures also can benefit, both through providing a better understanding of the likely modes of seismic response and through permitting the use of configurations outside of what the codes address. Performance-based design has been used extensively on analysis of older buildings, which often consist of archaic materials and have details that do not comply with current codes.
Where Engineers Come In
As engineers, we must employ state-of-the-art engineering techniques, including cutting-edge building technologies and sophisticated analytical techniques. Engineers must be capable of adaptive thinking in employing these technologies in innovative ways to address project needs. Ideally, engineers work closely with researchers or specialists who can bring an added perspective to the project. For a successful built project, engineers must have a solid knowledge of the permitting process for performance-based design (a process that varies from city to city), and experience in working on such projects.
Where Clients Come In
Engineers must educate clients so they understand what a “designed-to-code” structure guarantees and what it does not. The entire team must have a willingness to innovate and a willingness to discuss performance expectations. Importantly, clients must be able to work in a review environment that is less subject to black-and-white compliance issues, and discuss risk and probability of failure.
As performance-based design continues to flourish as a means of protecting building assets in seismically-prone areas, we are also seeing PBD being employed on other types of structures prone to extreme conditions, such as wind. We anticipate this shift to PBD in the future as well.