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Advances in Performance-Based Design for Structures in Fire

By: Thomas Gernay, Johns Hopkins University, U.S.A.

This article is the short version of the published paper “Performance-based design for structures in fire: Advances, challenges, and perspectives”. The paper was prepared as a result of the author’s receipt of the IAFSS Magnusson Early Career Award and was presented in October 2023 at the 14^th IAFSS Symposium in Tsukuba, Japan.

Gernay T. (2024). “Performance-based design for structures in fire: Advances, challenges, and perspectives”. Fire Safety Journal, 142, 104036 [1]. https://doi.org/10.1016/j.firesaf.2023.104036

Building codes govern the design of structures, including with respect to fire safety. There are two main paths to demonstrate compliance with requirements in building codes: prescriptive and performance-based. While prescriptive is the most common and straightforward approach, performance-based offers benefits for structural fire design that have led to its adoption in multiple types of projects. Performance-based designs require modeling the structure to predict its fire response against an acceptable performance level. Advances over the last four decades have helped defining procedures and developing calculation methods to enable this assessment. Today, even though challenges and barriers to broader adoption persist, the performance-based approach is a key tool to support safe, innovative, and resilient structural fire designs.  

The Performance-Based Structural Fire Design Approach

A Performance-Based Design (PBD) adopts a goal-oriented approach to the design process. Instead of specifying what exact steps have to be taken to achieve a compliant design for each individual element of the structure, the stakeholders identify goals and objectives for the design which must be met for an acceptable outcome. Modeling and calculation is then conducted to evaluate the expected fire performance of the design against the objectives. This approach provides flexibility for structural fire engineers and can deliver benefits, such as providing greater confidence in the system’s behavior through quantification of safety, incorporating sustainability and resilience objectives together with life protection, and enabling innovative and architectural designs. As the PBD approach requires an explicit evaluation of the outcome in case of fire, engineers have developed rigorous frameworks and analysis tools to support this assessment. 

A Very Brief History of PBSFD

Figure 1 shows a timeline with selected milestones in the development of structural fire engineering and performance-based structural fire design (PBSFD). For building fire safety, the concept of performance-based regulations and use of engineering tools has existed at least since the 1980s. Buchanan published a fire engineering design guide in 1994 [2]. Major research projects in Europe in the 1990s such as the ECSC-funded Natural Fire Safety Concept and the Cardington fire tests at the Building Research Establishment (BRE) advanced methods for structural fire design, including on fire models and tensile membrane action. The Structures in Fire (SiF) conferences were started by Franssen on a biennial basis in 2000. Building codes have progressively evolved alongside research to include performance-based design provisions for structural fire design, notably in Britain, New Zealand, Australia, Japan, Europe, and more recently in the United States where the American Society of Civil Engineers (ASCE) 7 standard permits the application of PBSFD since its 2016 edition. Books on structural fire design were published [3-4] to compile the advances in the field and support application of the approach. In parallel, software such as FDS [5] and SAFIR [6] have provided modeling capabilities for simulating realistic fire scenarios and thermal-structural response.

Figure 1. Selected milestones in development of PBD for structures in fire over the last 40 years.

The Value Proposition

As an alternative approach to prescriptive, PBSFD requires additional effort from the stakeholders and structural fire engineers to explicitly define goals and objectives and to evaluate the designs. Nevertheless, PBD has proven to bring benefits that justify this effort in a broad range of situations, as indicated by the review compiled in Table 1.

Table 1 lists possible reasons for adopting a PBSFD approach in a project. Many of the references in Table 1 were written by practitioners based on constructed projects. As shown through these examples, the PBSFD can deliver deeper confidence in fire reliability, enable innovation and optimization, and support consistent multi-hazard risk-informed design. Additional discussion on these examples is provided in Ref. [1].

Table 1. Value proposition of PBSFD with examples of applications. Full reference list is provided in Ref. [1].

Process and Framework

The process to conduct a PBSFD is shown in Figure 2. It includes three major stages: 

i. Definition of the scope, objectives, approaches, and scenarios, in agreement with the stakeholders (including AHJs, client, engineers and design team). This stage determines in a design brief the expectations for the design in terms of performance criteria.

ii. Evaluation of the structural fire performance for the trial designs and comparison of the outcomes with the performance criteria. This part is completed by the structural fire engineer who uses tools and methods to quantify the effects of the selected design fire scenarios on the structure.

iii. Selection of a trial design that satisfies the performance criteria and documentation.

The described process for PBSFD is adapted from the international standard ISO 24679-1:2019 [7]. Its application to concrete structures is discussed in a recent fib bulletin [8]. The PBSFD must be integrated with the overall fire safety strategy for the project, which covers additional provisions for evacuation, detection, fire and rescue service access, and compartmentation. 

Figure 2. Process for performance-based design of structures in fire (adapted from ISO 24679-1:2019 [7]).

Recent Advances in Methods for Evaluation

To complete the process from Figure 2, appropriate data, tools, and methods are needed to enable evaluation of specific projects. The evaluation of the structural fire performance is a complex multi-physics problem that involves fire dynamics, heat transfer, and structural mechanics. Research has resulted in much progress to provide these components, including on the coupling between the fire and thermal-structural models, the characterization of the material behavior at elevated temperature, numerical modeling of structures subjected to fire, probabilistic risk assessment, and cost-benefit analyses. Advanced analysis based on numerical finite element (FE) modeling has established itself as a central tool for this purpose (Figure 3). FE software packages for structures in fire can incorporate the latest elevated temperature material models, interface with an array of fire modeling techniques including CFD-based, and enable running probabilistic analyses for robust assessment of the structural fire performance. Capturing the role of uncertainties and assessing the probability of various damage states and losses, as is done for other hazards, offers increased confidence and allows tackling objectives of resilience. An overview of recent advances in models and methods for simulating the predicted response of structures in fire is provided in Ref. [1].

Figure 3. Some of the elements and models used to quantify the performance of structures in fire.

Challenges and Looking Ahead

Despite the aforementioned advances and use of PBSFD in real projects, barriers to broader adoption remain. On the technical level, some phenomena remain poorly understood, such as the fire behavior of timber structures. The complexity and computational cost of modeling approaches also limit their accessibility. Another type of challenge lies in education and competency. The successful execution of these designs and, on the part of the AHJs and reviewers, the evaluation of their adequacy, requires a robust pool of well-educated structural fire engineers, a need that is currently underserved due to insufficient training opportunities. On a political level, the PBD approach generally carries a more complicated path to approval. The lack of awareness and risk perception (in terms of process and approval) result in missed opportunities in situations where the PBSFD could have brought benefits on a safety and technical level.

Table 2. Challenges and perspectives for broader adoption of PBD for structural fire design.

It is the author’s opinion that PBSFD will continue in its growing adoption and acceptance within the profession. It is now permitted in many jurisdictions. It has already been implemented in numerous projects worldwide, adopted by structural engineers (Table 1) to quantify safety and performance, optimize design and use of fire protection, enable designs that are not covered by prescriptive codes, conduct forensic analyses, study existing structures to preserve heritage, control the failure mode, or demonstrate stability to burnout. With this approach, engineers can develop optimal solutions for high-value structures through application of science-based methods and demonstration of expected performance. Importantly, as innovation brings new materials and structural systems which might sometimes erode the conservatisms embedded in the prescriptive fire resistance design paradigm, the PBSFD provides a framework to examine the effect of these innovations on structural fire safety. These benefits, coupled with continued research and education and development of the profession, are expected to result in a broader adoption of PBD for structural fire design, following the general trend observed in structural engineering for other hazards.

Fire hazard governs many aspects of design in a building, with impacts on cost, aesthetics, sustainability, and resilience. The performance-based design provides a goal-oriented approach rooted in principles of fire science and structural mechanics to deliver project-specific solutions that meet stakeholders’ objectives.  There is value in empowering structural engineers with the flexibility to choose between the prescriptive and performance-based design approach, as evident from numerous real projects reviewed herein. 

The author is grateful to the IAFSS for the invitation to submit this paper at the 14^th Symposium.

References

[1] Gernay T. (2024). Performance-based design for structures in fire: Advances, challenges, and perspectives. Fire Safety Journal, 142, 104036.

[2] Buchanan A (1994) Fire engineering design guide, Centre for Advanced Engineering, University of Canterbury, Christchurch, New Zealand

[3] Wang Y, Burgess I, Wald F, Gillie M (2013) Performance-based fire engineering of structures. CRC Press, Boca Raton

[4] LaMalva K, Hopkin D (eds) (2021) International handbook of structural fire engineering. Springer AG, Switzerland. https://doi.org/10.1007/978-3-030-77123-2

[5] McGrattan KB, et al. (2000). Fire dynamics simulator--Technical reference guide. Gaithersburg: National Institute of Standards and Technology, Building and Fire Research Laboratory.

[6] Franssen JM, Gernay T. (2017). Modeling structures in fire with SAFIR®: Theoretical background and capabilities. Journal of Structural Fire Engineering, 8(3), 300-323

[7] ISO. (2019). ISO/TR 24679-1:2019. Fire safety engineering – Performance of structures in fire – Part 1: General. International Organization for Standardization, Geneva, Switzerland.

[8] Gernay T, et al. (2023). fib Bulletin 108. Performance-based fire design of concrete structures. Bulletin from the International Federation for Structural Concrete CEB-FIP, 115 pages, ISBN 978-2-88394-169-4.