Fire Safety Engineering: Performance-Based Design Approaches

What Is Performance-Based Fire Engineering?

Performance-based fire engineering — also known as fire engineering design or fire safety engineering — is an approach to fire safety design in which the engineer demonstrates, through quantitative analysis and engineering judgement, that a building provides an acceptable level of fire safety, rather than by mechanically applying prescriptive code provisions. It is the alternative to prescriptive design, and it is essential for buildings where the standard codes cannot provide a workable solution.

The UK's principal fire safety codes — Approved Document B, BS 9991, and BS 9999 — are prescriptive documents. They set out tables of travel distances, compartment sizes, fire resistance periods, and other parameters that, if followed, are deemed to provide an adequate level of fire safety. For straightforward buildings, prescriptive compliance is the right approach. For complex or atypical buildings, prescriptive codes can produce impractical or disproportionate requirements, or may simply not address the building type at all.

When Is Performance-Based Design Used?

Performance-based fire engineering is used where prescriptive codes cannot accommodate the building's design intent, or where a design departure from prescriptive requirements can be justified by engineering analysis. Common applications include:

• Large atria and open-plan spaces — where prescriptive travel distance limits would preclude open-plan floor layouts, smoke modelling can demonstrate that adequate smoke-free height is maintained over escape routes for the required time
• Tall mixed-use towers — where multiple occupancy types with different fire safety requirements occupy the same building, performance-based analysis can optimise the design
• Underground and basement spaces — where standard means of escape provisions cannot be applied, performance-based analysis of the specific egress challenges is required
• Heritage and historic buildings — where the installation of fire safety systems would damage historic fabric, performance-based analysis can identify alternative measures that achieve an equivalent level of safety
• Sports stadia and large public assembly buildings — where the occupancy is large, transient, and potentially unfamiliar with the building, crowd modelling and evacuation analysis are required
• Innovative architectural forms — curved floor plates, voids between floors, unconventional escape routes, and other design features that fall outside the scope of prescriptive codes

Tools and Techniques

Performance-based fire engineering draws on a range of analytical tools:

• Computational fluid dynamics (CFD) fire modelling — simulates the growth and spread of fire and smoke within a space, predicting temperatures, visibility, and toxic gas concentrations at different points and times. Used to assess smoke control system performance and tenability of escape routes.
• Evacuation modelling — simulates the movement of occupants through a building during evacuation, predicting evacuation times and identifying bottlenecks. Used to assess whether escape routes are adequate for the occupancy.
• Structural fire engineering — analysis of the structural behaviour of building elements under fire conditions, allowing more efficient structural fire protection specifications than prescriptive tables permit.
• Probabilistic risk assessment — quantification of fire risk using statistical data on fire frequency, ignition sources, and outcomes, allowing risk-based decision making for complex or novel buildings.

Acceptance by Building Control and the BSR

Performance-based fire engineering solutions must be accepted by the relevant authority — building control or, for higher-risk buildings, the Building Safety Regulator. The BSR applies rigorous scrutiny to performance-based solutions and expects them to be developed by chartered fire engineers with relevant competence and experience.

The engineer must demonstrate that the design fire scenarios used in the analysis are appropriately conservative, that the modelling tools are used correctly, and that the conclusions are robust. Peer review by an independent fire engineer is increasingly expected, particularly for Gateway 2 submissions.