Fire engineering in-house, UK wide
Smoke Control and Evacuation Modelling
Smoke, not flame, is what stops people leaving a building in time. Smoke control and evacuation modelling let you prove that the escape routes in your design stay usable for long enough. AL23 Safety carries out that work in-house for clients across the UK.
What is smoke control and evacuation modelling?
Proving the design works rather than assuming it does
Smoke control is the set of systems and openings that keep smoke away from the routes people use to escape and the routes firefighters use to attack a fire. Evacuation modelling calculates how long those people need to get out. Together they answer one question. Do the occupants have more time than they need.
Prescriptive guidance answers by pattern matching. Your building either looks like the example in the guidance or it does not. Modelling answers by calculation, which is what an atrium, a converted mill or a transport interchange needs. This is fire engineering work rather than fire risk assessment work. AL23 Safety holds it in-house.
Who needs one
When the building does not fit the guidance
The trigger is a design that does not fit the standard pattern. It can also be an approver asking for evidence rather than assertion. Typical triggers include:
- Residential buildings with a single stair, extended travel distances or a lobby that departs from Approved Document B
- Atria, malls and other large connected volumes where a smoke reservoir has to be designed rather than assumed
- Enclosed and basement car parks where natural cross ventilation is not achievable
- Buildings adopting phased or progressive horizontal evacuation, including hospitals and care settings
- Higher-risk buildings preparing a Gateway 2 application to the Building Safety Regulator
- Refurbishment where the stair, shaft or ventilation cannot be enlarged and a performance case is the only route left
The regulations that apply
Your legal framework, in plain terms
Fire engineering does not sit outside the law. The Building Regulations 2010 set the functional requirements in Part B. Approved Document B is one way of showing compliance rather than the only way. Depart from it and you are relying on a documented engineering case that your building control body will test. In occupation the Regulatory Reform (Fire Safety) Order 2005 keeps the responsible person answerable for the systems the design relied upon.
Approved Document B
Guidance rather than law. The current publication is the 2019 edition incorporating the 2020, 2022 and 2025 amendments. The 2026 amendment takes effect on 30 September 2026 and recommends a second staircase in residential buildings with a storey at 18 metres or more. Transitional protection runs to roughly 30 March 2028.
BS 9991:2024
Fire safety in the design, management and use of residential buildings. From 30 September 2026 all Gateway 2 applications adopting BS 9991 must use the 2024 version. It is not statutory guidance and carries no deemed to satisfy status.
BS 9999:2017
The current edition for non-residential and mixed buildings. It is the usual home for risk profile based design and management level assumptions.
BS 7974 and the PD 7974 series
The framework for applying fire safety engineering principles, including the qualitative design review that fixes scope, design fires and acceptance criteria before any model is run.
BS EN 12101 series
Product and system standards for smoke and heat control, including pressure differential systems for protected stairs.
What the service covers
From smoke shafts to ASET against RSET
- Natural smoke ventilation, including automatic opening vents and the geometry needed to make a smoke reservoir behave
- Mechanical smoke extract, including car park systems, corridor extract and impulse or jet fan arrangements
- Smoke shaft design for residential corridors and lobbies, covering sizing, damper strategy and the detection interface
- Stair pressurisation, class selection under BS EN 12101-6, air release paths and the door opening force limits that decide whether the system is usable
- Enclosed car park ventilation for smoke clearance and firefighting access, including the daily air quality duty on the same plant
- Computational fluid dynamics where the geometry, the design fire or the ventilation interaction defeats hand calculation
- Evacuation and egress modelling, including pre-movement time distributions, flow through doors and stairs, counterflow from firefighters and phased or progressive horizontal strategies, allowing for occupants covered by personal emergency evacuation plans
- ASET and RSET comparison against tenability criteria for visibility, smoke layer temperature, radiant heat and toxic gas exposure
- Sensitivity analysis on the assumptions that move the result, typically design fire size, growth rate and pre-movement time
Our process
Agree the criteria first, then model to them
- 01
Qualitative design review
We agree scope, fire scenarios, design fires and acceptance criteria with you and with the approving body before modelling starts. Doing this first is what stops a model being rejected after it is paid for.
- 02
Method selection
We choose the simplest method that answers the question. Hand calculation and zone models are used where they are defensible. CFD is used where the geometry or the ventilation interaction genuinely demands it.
- 03
Modelling and analysis
We run the scenarios, compare available safe egress time against required safe egress time and test how sensitive the margin is to the inputs we were least certain about.
- 04
Reporting
We issue a fire engineering report stating the assumptions, the limitations and the residual design constraints in plain language alongside the numerical output.
- 05
Support through approval
We answer the technical queries that come back from the approving body and revise the analysis where the design changes.
What you get
A case your building control body can accept
- A fire engineering report setting out the analysis, the acceptance criteria and the conclusion, ready for a building control application
- A qualitative design review record showing what was agreed with the approving body and when
- Smoke control design intent, covering system type, performance targets and the interfaces the installer must meet
- Evacuation modelling output with ASET and RSET results scenario by scenario, plus the sensitivity testing behind them
- A schedule of design constraints and management assumptions to carry into occupation so nothing disappears at handover
- A cause and effect narrative that the commissioning and verification process can be tested against
What we need from you
Drawings, occupancy and any approver correspondence
- Current architectural drawings including sections, plus the mechanical and electrical layouts where they exist
- The fire strategy where one exists. Where it does not, an instruction to write it first
- Occupancy numbers and type, plus the intended management arrangements and who is on site out of hours
- Any correspondence already exchanged with building control or the fire and rescue service
- For existing buildings, access to survey the as-built condition rather than the as-designed drawings
Why AL23 Safety
Modelling kept in-house, limitations written down
Fire engineering is in-house
We do not subcontract the modelling and relay the answers. Health and safety, fire safety and fire engineering sit in one team so the strategy, the model and the risk assessment agree with each other.
We say what the model cannot do
A model is only as good as its input assumptions. Where the result rests on a management assumption or a design fire we cannot fully justify, we write that down rather than bury it.
Practical, not just compliant
We will tell you when a simpler prescriptive solution is cheaper than the modelling needed to justify a clever one.
UK wide
We are Manchester based and work on projects across the UK, in every sector.
Test the strategy before it is built
The cheapest change is the one made on a drawing
If the escape strategy is still being argued over, a conversation now is worth more than a report later. Tell us the building and the sticking point and we will say whether modelling is the right answer.
Common questions
Answers, up front
Cannot see your question? Get in touch and we will answer it directly.
Contact usWhen the geometry, the ventilation interaction or the design fire cannot be resolved by hand calculation or a zone model. Atria, complex car parks and unusual smoke shaft arrangements are the common cases. Where a simpler calculation gives a defensible answer, CFD adds cost and programme without adding certainty. We will say so.
No. It is an alternative route to demonstrating compliance with the functional requirements of Part B. Approved Document B is guidance rather than law. BS 9991:2024 and BS 9999:2017 both accommodate performance-based design. The building control body still has to accept the case, which is why we agree the acceptance criteria before the work starts.
The mathematics is reliable. The inputs are where the uncertainty lives. Design fire size, growth rate, pre-movement time and occupant characteristics all move the answer. A margin can turn on one assumption. We test sensitivity on those inputs and report the range rather than a single flattering number.
The building control body, which for a higher-risk building is the Building Safety Regulator. The fire and rescue service is consulted and its views carry weight on firefighting provisions. Agreement should be sought at the qualitative design review stage. Modelling agreed late is modelling redone.
Cost is driven by the number of scenarios, the complexity of the geometry, whether an existing building needs surveying and how many rounds of comment the approver generates. We scope it against your drawings and quote a fixed fee before starting. We also produce the design intent and review the specialist contractor design against it, since we are consultants rather than installers.
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