Fire Safety Engineering · CFD Analysis · Ashbeck pipeline

Residential block, six storeys — fire strategy assessment (specimen)

FDS 6.11.1 · analysis to BS 7974 / PD 7974 framework · generated 26 July 2026

Status: engineering analysis — NOT a certified fire strategy. Requires review & sign-off by a competent chartered fire engineer before any use for design, approval, or regulatory submission (see §8).

Contents
1. Executive summary · 2. Methodology · 3. Design fire · 4. Mesh sensitivity (V&V) · 5. Tenability (ASET) · 6. ASET vs RSET · 7. Sensitivity · 8. Assumptions, limitations & sign-off · 9. References

1 · Executive Summary

1018 kW

Peak HRR (0.2 m grid)

88 s

ASET (visibility)

101.5 s

RSET (PD 7974-6)

-13.5 s

ASET − RSET margin

Simulation of Residential block, six storeys. The governing result is on floor 0, where ASET is 88 s against an RSET of 101.5 s, a margin of -13.5 s. On this basis the life-safety criterion is NOT satisfied for the modelled scenario.

2 · Methodology

Approach

FDS 6.11.1, LES, MPI-parallel.
Tenability to PD 7974-6 at occupied height: visibility, convective temperature and toxic FED.
Detection from a modelled optical smoke detector.
ASET from CFD; RSET by PD 7974-6 hand calculation.
Model geometry confirmed against the drawings before simulation (§9).

3 · Design Fire

Basis

Design fire selected from the referenced library for the occupancy; t² growth per PD 7974-1.

Growth class	per reference (α = — kW/s²)
HRRPUA	— kW/m²
Fire area	2 m²
Peak HRR	— kW at — s
Soot / CO yield	0.1 / 0.024 kg/kg

4 · Mesh Sensitivity (Verification & Validation)

No grid-convergence study was carried out for this assessment. The resolution index below is reported for the single grid used; convergence has not been demonstrated and near-fire quantities should be treated as indicative.

Grid	Cell δx	D*/δx	Peak HRR	ASET	Δ ASET vs fine	Verdict

Selected grid for assessment: 0.2 m — D*/δx = 4.8 (coarse (4-10) — screening only).

5 · Tenability Assessment — ASET (PD 7974-6)

Criterion	Basis	First breached
Visibility < 10 m	PD 7974-6	88 s
Temperature > 60 °C	PD 7974-6	139 s
Toxic FED ≥ 0.3 (design)	PD 7974-6	not breached in the simulated period
Toxic FED ≥ 1.0 (incap.)	PD 7974-6	not breached in the simulated period

Governing criterion: visibility → ASET = 88 s. Peak temperature 84.7 °C; minimum visibility 0.75 m; max toxic FED 0.003.

6 · ASET vs RSET (Life-Safety Margin)

Basis

RSET = detection + alarm + pre-movement + travel (PD 7974-6). Simplified hand calculation, not agent-based evacuation modelling.

Detection (from CFD smoke detector)	— s
Alarm + pre-movement	10 + 60 s
Travel	15.5 s
RSET	101.5 s
ASET − RSET	-13.5 s — INADEQUATE

7 · Findings and recommendations

Raised automatically from the modelled results against the referenced guidance. They are candidate findings for engineering review, not a compliance determination.

LS-01 — Inadequate life-safety margin on 1 of 6 storeys CRITICAL

Evidence: Worst case floor 0: ASET 88.0 s vs RSET 101.5 s → margin -13.5 s. Failing storeys: [0].

Basis: PD 7974-6:2019 — ASET must exceed RSET with an appropriate margin.

Recommendation: Protect the escape route and/or reduce RSET. Priority measures: (a) enclose the stair as a protected shaft with self-closing FD30S doors; (b) provide smoke control (AOV) to the stair head; (c) improve detection to reduce pre-movement time.

MoE-01 — Escape stair is not a protected shaft CRITICAL

Evidence: Stair core modelled open to the common corridor at every storey; smoke propagates vertically by stack effect.

Basis: BS 9991:2015 / ADB Vol 1 — common stairs in blocks of flats should be protected, normally approached via a protected lobby or corridor.

Recommendation: Enclose the stair in fire-resisting construction with self-closing fire doors at every level; consider a ventilated protected lobby between corridor and stair.

SC-01 — No smoke ventilation to the stair/common corridor MAJOR

Evidence: No AOV or smoke shaft modelled; smoke accumulates in the stair enclosure.

Basis: BS 9991:2015 Section 3 — smoke control to common escape routes (AOV at head of stair / smoke shaft to corridor).

Recommendation: Provide an automatic opening vent (min. 1.0 m² free area) at the head of the stair, and smoke shaft/AOV to each corridor, interlocked with detection.

TEN-01 — Tenability governed by smoke obscuration, not heat or toxicity MODERATE

Evidence: Visibility is the first criterion breached on 5 of 6 assessed storeys (storeys [0, 1, 2, 3, 4]); peak toxic FED 0.003 against a 1.0 incapacitation limit (0.3 design).

Basis: PD 7974-6:2019 tenability criteria.

Recommendation: Prioritise smoke control and visibility-preserving measures over thermal/toxicity mitigation; consider way-guidance lighting on escape routes.

QA-01 — Building-scale grid is coarse (D*/δx = 4.8) ADVISORY

Evidence: Resolution index 4.8 below the ≥10 practice target.

Basis: FDS User Guide (NIST SP 1019) mesh-resolution guidance.

Recommendation: Confirm key results on a refined grid before relying on near-fire quantities; smoke transport results are less sensitive.

8 · Sensitivity — Ventilation

No ventilation sensitivity study was carried out for this assessment. Ventilation is normally the dominant modelling uncertainty for compartment fires, and a bounding pair of cases should be run before the results are relied upon.

Case	Peak HRR	ASET	Peak CO	Min O₂
No ventilation sensitivity study was run for this assessment.

9 · Assumptions, Limitations & Competence

Assumptions register

Building modelled as 6 storeys at 3.0 m, footprint 24.0 x 14.0 m.
Cell size 0.20 m; simulated duration 180 s.
Boundaries modelled as inert — no fire-resistance or thermal-mass credit.
No fire-safety measures modelled (as-existing case).
Confirming engineer noted: “Model checked against the architect's GA drawings, rev C.”.

Limitations

Toxicity FED covers CO/CO₂/O₂ only; HCN and irritants are not modelled.
RSET is a hand calculation; no behavioural or agent-based evacuation simulation.
No sprinkler, mechanical smoke control or HVAC systems are modelled.
Single fire location and a single realisation; no scenario ensemble.
Grid convergence was not re-established at building scale for this model.

Competence & sign-off. This document is a computational fire-engineering analysis produced by an automated pipeline. It does not constitute a fire strategy, a competent person's assessment, or a regulatory submission. Under the Building Safety Act 2022 and the BS 7974 framework it must be reviewed, corrected as required, and signed off by a suitably qualified and competent fire engineer (e.g. CEng, IFE member) carrying professional indemnity before any reliance is placed on it.

Prepared by (pipeline)	Ashbeck CFD pipeline (FDS + Python)
Reviewed by	__________________________ (CEng / IFE) Date: __________
Approved by	__________________________ Date: __________

10 · Provenance and confirmation

This section records who accepted each stage of the assessment, and what was visible to them when they did. It is part of the audit trail for this document.

Model geometry confirmed by	A. Engineer on 26 Jul 2026, 16:23 Note: “Model checked against the architect's GA drawings, rev C.”
Warnings visible at confirmation	1
Plan/model conflicts flagged	0
Results interpretation confirmed by	A. Engineer on 26 Jul 2026, 16:23 Note: “Interpretation reviewed; agree the corridor governs on the fire floor.”
Simulation queued by	A. Engineer
Model specification hash	`2a71236a38e17743`
Simulation run id	22
Model currency	The project model is unchanged since this run.

Activity record

When	Action	By
2026-07-26 16:23	issued report	—
2026-07-26 16:23	confirmed geometry	A. Engineer
2026-07-26 16:23	withdrew results confirmation	A. Engineer
2026-07-26 16:23	confirmed results	A. Engineer
2026-07-26 16:07	confirmed results	A. Engineer
2026-07-26 14:30	queued run	A. Engineer
2026-07-26 14:30	queued run	A. Engineer
2026-07-26 14:30	saved geometry	A. Engineer
2026-07-26 14:30	confirmed geometry	A. Engineer

Stage verification record

Each stage of this assessment is checked automatically before it is accepted: deterministic rules that can block progress, and an independent AI review that raises queries for the engineer. The AI never determines correctness and cannot clear a blocking rule.

Stage	Outcome	Checked
Drawing read	1 item raised for review	2026-07-26 21:38
Building model	6 items raised for review	2026-07-26 16:23
Works and measures	3 items raised for review	2026-07-26 21:39
Simulation setup	2 items raised for review	2026-07-26 21:41

What confirmation means here. Confirming the geometry records that a named person compared the simulated model against the drawings and accepted it as a fair representation. Confirming the interpretation records that a named person read the results and accepted the description of them. Neither is a statement of regulatory compliance, and neither substitutes for the competent-person sign-off required in §8.

11 · References

PD 7974-1:2019 — Initiation and development of fire within the enclosure of origin.
PD 7974-6:2019 — Human factors: life-safety strategies, occupant evacuation, ASET/RSET, tenability (FED).
BS 9991:2015 — Fire safety in the design, management and use of residential buildings.
BS 7974:2019 — Application of fire safety engineering principles to the design of buildings.
McGrattan et al. — Fire Dynamics Simulator User Guide (NIST SP 1019) & Technical Reference / Validation Guide.
Approved Document B (Fire safety), Building Regulations (England).

Generated locally on WSL2 (Ubuntu) · FDS 6.11.1 · Python analysis (pandas/numpy/matplotlib/fdsreader) · tenability to PD 7974-6 · narrative assisted by Claude. All numerical results are reproducible from the archived input files.