Why Performance-Based Design?
Our modern design specifications strive to achieve some level of performance by defining minimum design requirements that ensure safety of occupants during specific design events. Design conformance to prescriptive criteria on materials, configuration, detailing, strength, and stiffness are implicitly taken as evidence that desired performance will be achieved.
However, numerous disasters have prompted that significant damage could occur even when buildings are compliant with the building codes. We believe our structures are safe, but we rarely know the true safety margins or whether other design solutions would provide better performance. To make matters worse, we rarely state or even know what performance levels we should strive to achieve.
In hindsight, we are not able to apply our full capabilities to the design process. 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. The societies we serve are not getting maximum value from their limited resources of time, money, energy, and materials. Instead, they are getting designs that are constrained by prescriptive codes that attempt to address all conditions for all structures serving all purposes, with uncertain reliability because design by prescription neither quantifies nor directly evaluates performance.
Performance-based design is founded on the premise that structural systems must meet specific performance objectives. Specific performance expectations are set for the completed design, and processes are prescribed in minimal terms. Performance-based design, therefore, reverses the design process by defining the end goal as the starting point. The engineer then engages creativity and innovation to identify optimal solutions to multiple, and sometimes competing, objectives. The design is completed by demonstrating complying performance through analysis, simulation, testing, or a combination thereof.
The recent boom in high-rises construction has offered structural engineers the opportunity to advance the state of the art in seismic engineering. While the concept of performance based design is not new, its application to the design of newly constructed high-rise buildings in regions of high seismicity is relatively recent.
Prescriptive v/s Performance-Based Designs
A central difference between the traditional prescriptive design method and PBD is in the design objectives as illustrated in the figure below. While prescriptive design objectives require achieving an acceptable demand-to-capacity (D/C) ratio, the objective of PBD is to achieve a specific level of performance, as correlated to appropriate consequences, which may be measured in several ways including a monetary cost. Each of these methods requires design iterations until either an acceptable D/C ratio (for prescriptive design) or a desired performance level (for PBD) is achieved.
Another difference between the prescriptive design and PBD paradigms lies in their computational underpinnings. For prescriptive design, this relates to capacity and demand, and is based on structural reliability methods. PBD is based on risk methods that consider hazards, vulnerabilities and consequences.
The third major difference between the two approaches lies in the steps that are taken in addressing the design considerations. For traditional prescriptive methods, the seismic hazard level and the acceptable level of damage in the structure is determined by prevailing building and design codes. In performance based design, both of these considerations are addressed during the design process, along with anticipated consequences and uncertainties in the design and analysis process. These decisions are based on a desired level of performance, rather than a predetermined set of codes.
The performance-based design approach is not proposed as an immediate substitute for design to traditional codes. Rather, it can be viewed as an opportunity to enhance and tailor the design to match the objectives of the community’s stakeholders.
Technical Challenges
After completion of a number of Tall Buildings using PBD, a few of the apparent challenges observed so far are as follows:
Response Modification Factors:
The first generation of PBD high-rise buildings largely adopted a traditional approach to establishing minimum required design strength. A design basis earthquake (DBE) was defined for the specific site, then reduced somewhat arbitrarily by traditionally prescribed response modification factor, R. This approach was intended to provide some form of consistency with traditional prescriptive designs. However, these factors were not developed considering unique response characteristics or framing systems common to high-rise buildings. In addition, the approach conflicts directly with the premise of PBD: rather than defining demands and directly evaluating building performance, artificial modifications to demand levels lead to a design based on something not much better than a numbers game.
Higher modes effects:
It is common for the response of a tall building to be heavily influenced by higher modes of vibration when subjected to strong ground shaking. However, traditional engineering practice has focused on only the first translational mode when setting strength requirements and lateral force distributions. For tall buildings, the second or even third mode of vibration can be equally, if not more, important to the overall design.
The influence of these higher modes of vibration can result in significantly higher flexural demands, well above a buildings base, as well as shear demands three to four times greater than those anticipated by a typical prescriptive design. Failing to recognize and incorporate these demands into a towers design can lead to undesirable performance.
Peer Review:
The complexities of high-rise design, coupled with evaluation by advanced mathematical modeling have led building officials to require detailed peer reviews of these projects. These reviews are an integral part of the successful implementation of a PBD, as they ensure appropriate consideration of important design parameters.
However, these same reviews can vary widely in their focus and thoughtfulness. It is imperative that selected peer reviewers have experience in design of tall buildings. It is further important that reviews be led by senior staff members and not delegated to those less experienced. While there is greater rigor to the numerical side of PBD, interpretation of analytical results remains an art form, requiring thoughtful consideration.
Elements of Performance-Based Design
The three basic steps of PBD are the estimation of hazard, the evaluation of vulnerability, and the computation of consequences, shown schematically in the figure below. When using PBD, determining the design hazard level requires evaluation of the seismic event and the probability of occurrence. This can range in complexity from choosing only the hazard level and the shape of the design spectra to a more involved process, such as generating a group of seismic acceleration time histories. In most situations, the designer needs to address issues such as return period and maximum ground acceleration. In the second generation seismic PBD effort, the probability of the chosen seismic hazard is an integral part of the design input needs. Another feature of second generation seismic PBD is that it can be based either on a single scenario, such as a unique earthquake level, or on multiple earthquake levels with varied return periods. This latter approach is obviously more time consuming, since design calculations must be performed for each of the scenarios. However, the advantage of the multiple scenario approach is that it gives a more complete picture over the total life of the building. As noted earlier, prescriptive design methods do not address probabilities of occurrence or consequences, as these are implicitly addressed through the development of the design codes.
After the seismic input is defined, the building design process starts. The key differences between the two design approaches are in the acceptance criteria, the analysis techniques, and the analysis objectives. In traditional prescriptive design, the acceptance criteria is generally prescribed simply to ensure life safety, while PBD allows for varied acceptance criteria based on the determination of an acceptable level of earthquake damage to the structure.
Performance-Based Design Process
The PBD process explicitly evaluates how building systems are likely to perform under a variety of conditions associated with potential hazard events. The process takes into consideration the uncertainties inherent in quantifying the frequency and magnitude of potential events and assessing the actual responses of building systems and the potential effects of the performance of these systems on the functionality of buildings. Identifying the performance capability of a facility is an integral part of the design process and guides the many design decisions that must be made.
Acceptable risks are typically expressed as acceptable losses for specific levels of hazard intensity and frequency. They take into consideration all the potential hazards that could affect the building and the probability of their occurrence during a specified time period. The overall analysis considers not only the intensity and frequency of occurrence of hazard events, but also the effectiveness and reliability of the building systems to survive the event without significant interruption in the operation.
Summary
Performance-based design offers several advantages over prescriptive design. First, properly executed performance-based approaches enable desired performance to be attained with greater confidence and expectations of reliability mainly because of the focus on the damage states. Second, since the performance objectives for the design are explicitly defined, the stakeholders can select the expected performance levels that are appropriate and satisfy their own needs. Third, since performance is evaluated directly as part of the engineering process, engineers need not be limited by requirements to conform to prescriptive solutions, thereby allowing for innovation and creativity using new materials and systems and using existing materials and systems in new ways. These advantages of PBD make meeting all the associated challenges a worthwhile goal.
Performance-based design approaches are not needed for most structures. In the future, we could easily have dual code approaches for structural design. Design of routine structures could default to prescriptive requirements, with a performance-based option for those interested in exploring its benefits. However, performance-based design processes should become an accepted protocol for complicated, high-value, and mission-critical structures (e.g., hospitals, emergency facilities and shelters, high-rise and iconic buildings, etc.), since the communities they serve will benefit from the innovation and creativity, performance-based approaches foster.
References:
“Performance-Based Design with Application to Seismic Hazard” in STRUCTURE Magazine by Margaret Tang et al.
“Performance-Based Seismic Design-Rising” in STRUCTURE Magazine by Ron Klemencic.
“Performance-Based Design is the Future” in STRUCTURE Magazine by Donald Dusenberry.
“PBD: A Component in the Future of Structural Engineering” in STRUCTURE Magazine by Stephen Szoke.
“Chapter 2: Performance-Based Design” by Risk Management Series Publication.
“Why Performance-Based Seismic Design?” by Jinal Doshi