In 2019, I read and was inspired by the structural efficiencies and improvements described in Don Dusenberry’s editorial in this publication titled “Performance-Based Design Is the Future.” Here we are, seven years later, and that future is now.
Performance-based design (PBD) is more relevant and critical today than it has ever been. This is evidenced by the Structural Engineering Institute’s (SEI’s) recent decision to establish PBD as one of four new “Focus Initiatives” (alongside Education and Leadership, Young Professionals, and SE 2050) to help prioritize resources and activities for the immediate future. SEI selected PBD as a Focus Initiative because of its proven ability to improve design, reduce material quantities, and streamline construction offering a solution for some of the most significant challenges society faces related to sustainability, cost of construction, and affordable housing.
What Has Transpired?
Since 2019, PBD has increased in popularity and impact. From its origin with design firms on the west coast of the United States in the 1990s as a way to design better structures in seismic zones, performance-based seismic design (PBSD) has expanded around the world. Because it is recognized as state-of-the-art design methodology, it is even being employed in areas of low or moderate seismic hazard. In many cities, PBD is no longer considered an exotic design approach, but rather the standard of care for tall or unique buildings.
The expansion of PBD principles to other hazards beyond earthquakes has also gained traction. With the publication of ASCE’s Prestandard for Performance-Based Wind Design (PBWD) in 2019, and an updated version in 2023, the first project designed and built using PBWD principles was recently completed: ATX Tower in Austin, Texas. Due to the direct consideration of nonlinear structural behavior, the design resulted in significant cost savings for the project as compared to prescriptive design.
Creativity Shines
Prescriptive code-based design has its advantages and disadvantages. Its advantages include simplicity, uniformity, broad applicability, and ease of implementation; however, those advantages come with inherent drawbacks. Because prescriptive design must be capable of being written down in “mandatory language” in a building code, it is necessarily limited. It cannot address all cases and therefore must be a simplification and approximation of reality.
On the other hand, PBD flips the narrative. Rather than defining a precise step-by-step process, it creates a framework. This framework puts the end first, starting with defining the intended performance objectives for a given hazard (e.g. collapse prevention or reoccupancy) and works backwards from there. By reframing the problem in this way, it allows and even encourages structural engineers to get creative. PBD creates the opportunity for structural engineers to do the following:
- Extend prescriptive structural systems beyond their rigid code limitations. A common example is the Special Reinforced Concrete Shear Wall system, which for earthquake design is limited to 160 feet (or 240 feet with certain regularity requirements) for most high seismic sites. PBSD has been used on hundreds of tall buildings to demonstrate that the structures performed as intended by the building code without conforming to every prescriptive requirement.
- Introduce new structural systems. Opportunities for innovation abound with PBD, which creates a framework to introduce new systems or construction techniques like SpeedCore into the industry.
- Enhanced performance targets. PBD provides a detailed and comprehensive understanding of the performance of engineering designs. With input from stakeholders (e.g. architects, owners, and the public), this allows us to set enhanced performance targets that may exceed the building code. This is a common discussion with “long term” building owners such as universities, hospitals, and government agencies who may desire structural performance better than “code minimum.”
Shift in Mindset
PBD is sometimes misunderstood as a synonym for “advanced analysis” or “nonlinear modeling.” While PBD may include advanced design techniques as part of the process, those techniques are merely tools rather than being fundamental.
A mindset shift is needed within the profession to embrace probabilistic thinking and move beyond binary pass/fail design approaches. Instead, we must arm our clients with information to make risk-informed decisions. PBD can be best summarized as a shift in focus from compliance to intent. In other words, rather than ticking boxes within a checklist of building code requirements, engineers should aim to explicitly prove that a design achieves the intent of the building code.
Let’s Be Leaders
As Dusenberry succinctly warned in his 2019 editorial, “We are evolving into masters of the Code, who add value by being able to navigate the complexity of prescriptive provisions rapidly, rather than by developing creative and innovative solutions to multi-faceted problems.” This is a danger collectively to our profession, made much more critical by the advent of Artificial Intelligence, which will inevitably soon surpass structural engineers with ability to instantly recall every aspect of the building code and perform basic engineering tasks.
PBD presents an opportunity to bring increased value to society as thought leaders rather than facilitators. Engineers are key to solving many of the industry’s biggest challenges by reducing project cost, building structures faster, and tailoring performance objectives to client needs. As structural engineers, we must embrace this opportunity for leadership, not only because it is what is best for society, but it is also what is best for our profession. ■
About the Author
Kevin Aswegan, PE, SE, is a Senior Principal at Magnusson Klemencic Associates, a structural and civil engineering firm headquartered in Seattle. Kevin serves as the Chair of the SEI Advancement of Performance-Based Design Committee.
