Services / Fire Engineering / 04
CFD & Evacuation Modelling
When guidance cannot answer the question, model it.
What it is
Some buildings cannot, or should not, follow prescriptive guidance — atria, large open volumes, unusual occupancies, heritage constraints. Computational fluid dynamics (CFD) models how fire and smoke actually develop in the space under BS 7974 (application of fire safety engineering to the design of buildings); evacuation modelling tests how people move through it. Together they let a design demonstrate safety on evidence rather than rule-of-thumb. Typical commissions cluster around a few situations. Atria and large open volumes, where smoke behaviour rather than travel distance governs escape. Smoke control design, where CFD establishes whether the proposed extract and make-up air arrangement actually keeps escape routes tenable. Extended travel distances or stair arrangements beyond the tables in Approved Document B or BS 9999. Phased or staged evacuation strategies in tall or complex buildings, where the timing assumptions need quantifying. And value-engineering proposals, where modelling tests whether a saving genuinely preserves the level of safety the strategy assumed — before anyone commits to it. Modelling supports a case; it does not manufacture one. Every model rests on assumptions — the design fire, the geometry simplifications, the pre-movement times — and a result is only as defensible as its inputs. We state assumptions explicitly, run sensitivity checks on the ones that drive the outcome, and present results with their limitations. If the analysis shows a proposal does not work, the report says so. That discipline is exactly what makes the cases that do pass persuasive to an approving authority.
When you need it
- A design departs from Approved Document B or BS 9999 and needs justification
- Smoke control, extended travel distances or phased evacuation need quantifying
- An approving authority or fire engineer has asked for analysis under BS 7974
- You want to test a value-engineering proposal before committing to it
What you receive
- CFD smoke and tenability analysis with stated assumptions and sensitivity
- Evacuation modelling with comparison against tenability criteria
- A technical report suitable for submission to approving authorities
How we do it
- Scoping and qualitative review: the question is defined precisely — what the design needs to demonstrate, and whether modelling is genuinely the right tool. Acceptance criteria, design fires and scenarios are set out for agreement with the approving authority before analysis begins.
- Fire and smoke modelling: CFD analysis of the agreed scenarios, tracking smoke movement, visibility, temperature and toxicity in the spaces that matter to escape and firefighting access.
- Evacuation analysis: occupant movement modelled against the developing conditions, comparing the time available for safe escape with the time the population actually needs — with the margin stated rather than implied.
- Reporting and review: a technical report setting out methodology, assumptions, sensitivity and results in full, structured for submission — with support through the approving authority's review until the case is closed.
What drives the cost
Cost depends on the complexity of the geometry to be modelled, the number of fire scenarios and design fires analysed, whether evacuation modelling is needed alongside the CFD or the question is smoke movement alone, the extent of sensitivity analysis the case requires, and the level of engagement needed with approving authorities to agree criteria and respond to review comments. We scope each commission individually and provide a fixed fee before starting.
Common questions
When is CFD modelling worth commissioning?
When prescriptive guidance either cannot answer the question or answers it too conservatively for the design to proceed. Typical triggers: an atrium or large volume where smoke behaviour, not travel distance, governs escape; a smoke control system whose performance needs demonstrating; travel distances or occupancy arrangements beyond the tables in Approved Document B or BS 9999; or a value-engineering proposal that trades away a protection measure and needs testing before anyone commits. Where a building sits comfortably within guidance, modelling adds cost without adding anything — we will say so at scoping rather than run analysis for its own sake.
What is BS 7974?
BS 7974 is the British Standard framework for applying fire safety engineering to the design of buildings. Rather than prescribing solutions, it sets out a structured process: define the design objectives and acceptance criteria, agree the fire scenarios, carry out quantified analysis, and compare the results against the criteria. Its supporting published documents cover fire growth, smoke movement, structural response and evacuation. Working under BS 7974 matters because it gives approving authorities a recognised basis for reviewing a performance-based case — the analysis follows an established methodology rather than an argument invented for the project.
Will building control and other authorities accept a modelled justification?
Yes, where the case is sound and the process is right. Approving authorities routinely accept performance-based justifications under BS 7974, but acceptance depends on more than the model: the design fires, acceptance criteria and key assumptions should be agreed before the analysis is run, not defended after it. We recommend early engagement — a qualitative design review shared with building control and, where relevant, the fire and rescue service — so the modelling answers questions the reviewer has already agreed are the right ones. A model presented as a fait accompli invites scepticism; one built on agreed foundations rarely does.
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