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Performance-Based Design at the Wildland-Urban Interface

By: Greg Baker, Fire Research Group Ltd, New Zealand; Greg Penney, Department of Fire and Emergency Services, WA, Australia; Andres Valencia, University of Canterbury, New Zealand; Dan Gorham, Insurance Institute for Business and Home Safety, SC, USA.

Introduction

The interface between wildlands and urban areas (known as the wildland-urban interface or WUI) is increasingly expanding, which in turn increases the potential for wildfires to impact the built environment. One notable example of this impact is the Victorian ‘Black Saturday’ wildfires in 2009 [1]. The same trend, at a larger scale, is also apparent in the USA where the WUI has grown rapidly in recent decades along with increasing wildfire impacts [2]. These trends are even becoming apparent in countries such as New Zealand, which have not traditionally had a perceived wildland fire problem [3].

This article provides a summary of a paper by the authors entitled The CAED Framework for the Development of Performance-Based Design at the Wildland-Urban Interface that was published in a recent edition of the FIRE journal [4]. The objective of the paper was to define a fire safety engineering consultation process appropriate for the wildfire context by translating well-established performance-based design (PBD) frameworks and approached. The product from this exercise was termed the CAED Framework.

Development of CAED Framework

Methodology

The first step to develop a framework that was suitable for application at the WUI was to review traditional fire engineering PBD frameworks that were available internationally. The second step in the development process was to then distill and streamline the elements from international best practice into a framework that was relevant for application of PBD at the WUI. Finally, the framework was ‘tested’ by contextualizing it through a comparative case study or both urban and WUI development.

Existing Frameworks

Four different publications were reviewed by the authors to identify common approaches to the process steps involved in PBD.

The existing[1] SFPE PBD Guide [5] provides a ten-step process to be followed for PBD, consisting of: 1. Defining the project scope; 2. Identifying the fire safety goals of the project; 3. Development of objectives; 4. Defining the performance criteria; 5. Development of fire scenarios and design fire scenarios; 6. Develop trial designs; 7. Evaluation; 8. Selecting the final design; 9. Design documentation; and 10. Completing the final report.

The corresponding International Standard ISO 23932-1:2018 [6] provides a similar process, albeit in this case with 12 discrete steps. One difference between the SFPE and ISO approaches is that in the latter, risk analysis is more explicitly identified, where at Step 4, the process flowchart requires the selection of a risk analysis approach. The ISO standard states that risk analysis will typically consist of a comparison of the estimated risk to the tolerable risk, and that the tolerable risk is either absolute (i.e., is explicitly stated) or is comparative (i.e., it is implicit). With deemed-to-satisfy (prescriptive) provisions, the tolerable level of societal risk is implicitly defined in the regulations by the jurisdiction’s regulatory authority, whereas in PBD, the tolerable risk must be defined explicitly in numerical terms. The risk analysis approach that is selected is defined in terms of how the uncertainty in the risk analysis is treated, with the lowest level of treatment being a qualitative analysis, and intermediate level being a (quantitative) deterministic analysis, and the highest level of treatment being a (quantitative) probabilistic analysis.

The International Fire Engineering Guidelines (IFEG) [7] was developed collaboratively by various code development organizations and federal regulatory agencies. The corresponding PBD process flowchart has 14 discrete steps. One key focus for the IFEG is agreement from all relevant stakeholders being required, via the so-called fire engineering brief, prior to any PDB analysis being undertaken.

The recently-published Australian Fire Engineering Guidelines (AFEG) [8] provides a four-step procedure where PBD is involved, namely:

  • A performance-based design brief is prepared in consultation with the relevant stakeholders.
  • Engineering analysis is carried out using suitable assessment methods.
  • The results of the analysis are evaluated against the acceptance criteria agreed in the performance-based design brief.
  • A final report to document the process is prepared.

CAED Framework

The SFPE, ISO and IFEG PBD procedures have between 10 and 14 discrete steps, which gives the appearance of greater complexity and detail, compared to the four-step AFEG process. However, the authors identified upon further analysis that the former three frameworks could all be distilled down to the same four key elements from the AFEG process, which the authors termed the CAED Framework, namely:

  • Consultation.
  • Analysis.
  • Evaluation.
  • Documentation

A number of fundamental components associated with each of the four elements in the CAED Framework were identified. The authors identified four components for the Consultation element of the CAED Framework: 1. Scope; 2. Stakeholders; 3. Objectives; and 4. Performance metrics. A further five components were identified for the Analysis element: 5. Methods of analysis; 6. Initial design; 7. Analysis; 8. Refinement; and 9. Finalization. For the Evaluation element of the framework, a further three components were identified: 10. Evaluate; 11. Confirmation; and 12. Signoff. For the fourth and final Documentation element of the CAED Framework, an additional four components were identified: 13. Design documentation; 14. Quality control; 15. Regulatory approval; and 16. Construction documentation.

Contextualization to the WUI Context

Description of Case Studies

In order to illustrate the application of the framework, and to clarify the differences between traditional fire engineering and wildfire engineering, the CAED Framework was applied to two comparative cases, Case A (urban) and Case B (WUI). Case A consisted of a proposed multi-storey apartment building in an existing central city setting, while Case B consisted of a proposed new residential subdivision, inclusive of a school, shops, holiday chalets, and a petrol station, on the outskirts of an urban area that bordered a wildland area.

Use of Performance-Based Design

From a traditional fire engineering perspective, it would be very unlikely that a PBD approach would be taken for a single, standalone residential building in a suburban setting, but an investment in PBD could be justified at a subdivision scale. On the other hand, PBD would be common for a large apartment building in a CBD environment. By way of comparison, conceptually Case A could be envisaged as a ‘vertical suburb’, while Case B is a traditional ‘horizontal suburb’.

Comparison between Case A and Case B – Consultation Element

This comparison by the authors was limited to the Consultation element of the CAED Framework. As an example, in the context of the Scope component of the Consultation element, a comparative discussion for both Case A and Case B was provided with regard to the differences in occupancy types, and well as a wide range of physical parameters, with the latter including the physical scope, Fire Service vehicular access, construction, Building Code provisions, fire hazards, occupant warning systems, occupant egress, Fire Service response, and fire safety systems and procedures.

The key difference identified between Case A and Case B with regard to occupancy was that a Building Code generally deals with a situation where a Case A building has different occupancies, involving different risk profiles, separated into fire-separated compartments that contemplates a fire internal to the building, while Case B deals with the situation where all buildings, regardless of their actual occupancy, face a common external threat from a wildland fire.

With regard to the wide range of physical parameters, some of the key differences between Case A and Case B were as follows:

  • Construction - differences is the type, and hence combustibility, of the construction, with greater variability in the construction of different buildings classes in Case B compared to Case A.
  • Fire hazard – for Case A there is a range of fire hazards, typically within the building (e.g., ignition of combustible contents and construction materials), but the possibility of an external fire would also be considered. This in turns leads to thermal, toxicity, and visibility hazards for internal occupants, as well as the hazard associated with building collapse. In contrast for Case B, the primary fire hazard is an external wildland fire exposure, typically consisting of a fast-moving flame front preceded by burning brands and embers ahead of the flame front. The items ignited in this case are combustible external construction materials, vegetation of allotments, planting, accumulated dead vegetation, and combustible contents such as curtains and roof insulation.  For Case B smoke and subsequent visual obscurity is not as significant as fire spread between the open and non-compartmentalised landscape.
  • Occupant warning systems – for Case A there will be a reasonably sophisticated detection and alarm system to warn building occupants if a fire occurs (timescale of minutes) whereas the key difference for Case B is that occupant warning will generally be further in advance and consist of media coverage and emergency services warning (timescale up to days), although sudden or unexpected changes in weather conditions may reduce this timescale to minutes to hours.
  • Occupant egress – for Case A, occupants would generally be expected to self-evacuate (although some occupants may require assistance from Fire Service personnel), whereas for Case B occupant egress could consist of a suburb-wide self-evacuation (using private motor vehicles typically) or could be the opposite extreme of shelter in place in buildings subject to enhanced wildfire resilient engineering such as AS3959 [9].
  • Fire Service response – Case A would typically involve a fire in a single building and a number of fire appliances would be expected to be in attendance within approximately 5 to 10 minutes, while for Case B a suburb-wide incident would require a multi-agency response and resources are stretched beyond capacity, firefighting water being in short supply, and road access is impeded.

Further Details of CAED Framework

The paper [4] that is summarized in this article provides further details of the differences, where they exist, for the other components of the Consultation element of the CAED Framework, namely stakeholders, objectives, and performance metrics.

The paper did not elaborate of the other three elements of the CAED Framework. However, it is anticipated that the overarching research project being conducted by the authors will produce various other publications which will provide these details.

References

[1] Bushfire and Natural Hazards CRC. (2009). Victorian 2009 Bushfire Research Response Final Report: Bushfire Collaborative Research Council, Melbourne, VIC, Australia

[2] Radeloff, M. et al. (2018) Rapid Growth of the US Wildland-Urban Interface Raises Wildfire Risk. Proc. Natl. Acad. Sci. USA, 115, 3314-3319. https://doi.org/10.1073/pnas.1718850115

[3] Kornakove, M.; Glavovic, B. (2018) Institutionalising Wildfire Planning in New Zealand: Lessons Learnt from the 2009 Victoria Bushfire Experience. Australas. J. Disaster Trauma Stud., 22, 51-61.

[4] Penney, G.; Baker, G; Valencia, A; Gorham, D. (2022). The CAED Framework for Development of Performance‐Based Design at the Wildland‐Urban Interface. Fire, 5, 54. https://doi.org/10.3390/fire5020054

[5] SFPE. (2015). The SFPE Guide to Performance-Based Fire Safety Design: National Fire Protection Association and Society of Fire Protection Engineering: Quincy, MA, USA.

[6] ISO. (2018). International Standard ISO 23932-1:2018 (E), Fire Safety Engineering – General Principles – Part 1: General: International Organization for Standardization, Geneva, Switzerland.

[7] ABCB. (2005). International Fire Engineering Guidelines, Edition 2005: Australian Building Codes Board, Canberra, ACT, Australia.

[8] ABCB. (2021). Australian Fire Engineering Guidelines, Edition 2005: Australian Building Codes Board, Canberra, ACT, Australia.

[9] SA. (2018). Australian Standard AS 3959:2018 Construction of Buildings in Bushfire-Prone Areas: Standards Australia, Sydney, NSW, Australia.

[10] SFPE is in the process of developing a new PBD standard to replace the existing Guide.