Project Overview & System Type
This project involved the performance-based design and CFD validation of an engineered mechanical smoke control strategy serving a listed luxury London hotel. The study covered the wider building smoke control response, including the protected firefighting shaft, evacuation lift shaft, stairs, associated lobby interfaces, the basement bar area, and the ground-floor restaurant. The system features a sophisticated dual-fan arrangement comprising independent extract and supply fan sets (Shaft 1 and Shaft 2) serving multiple levels.
The Engineering Challenge & Regulatory Framework
Prescriptive compliance under BS EN 12101-13 focuses primarily on demonstrating specific pressure differentials across closed doors. However, for high-density commercial spaces with complex geometries, a purely prescriptive approach may not account for real-world fire plume dynamics, smoke movement during occupant escape, or the massive thermal forces generated by a developed fire. The challenge was to engineer an alternative, performance-based mechanical smoke control system that guarantees a completely smoke-free staircase and maintains tenable conditions in the firefighting lobby during both the means-of-escape and active firefighting phases, all while ensuring that door opening forces never exceed the statutory 100 N limit due to excessive system over-pressure.
CFD Modelling & Analysis Methodology
A rigorous, transient CFD analysis was performed using Fire Dynamics Simulator (FDS). To eliminate numerical instability often associated with heavy HVAC interactions in fire models, actual fan performance curves were directly integrated into the FDS solver. Structural background leakage and door gap leakage were explicitly modeled using calibrated open vents. The characteristic fire diameter ($D^*$) was calculated to establish a highly refined, localized computational mesh (ranging between 0.1 m and 0.2 m) in the immediate vicinity of the fire, ensuring accurate resolution of buoyancy-driven plumes and thermal gradients.
Simulation Scenarios & Operational Timelines
The smoke control system was subjected to multiple high-severity design fire scenarios, including a localized fire within an upper-level office compartment and a fire within a mezzanine-level space. The operational timeline captured two critical phases:
- Means of Escape (MoE) Phase: Simulating transient door openings as occupants evacuate, evaluating soot visibility and temperature along the escape routes.
- Active Firefighting Phase: Simulating doors held fully open to replicate firefighters deploying hose lines from the protected shaft into the incident lobby.
Results & Performance Outcomes
The CFD simulations demonstrated flawless containment of products of combustion, with zero smoke leakage or migration into the protected staircase during all operational phases. The integrated fan curve and pressure-balancing logic successfully modulated air supply and extract rates, maintaining the pressure differential across the stair door well within safe limits and ensuring door opening forces remained comfortably below the 100 N threshold. Tenability criteria including maintaining a minimum visibility of 10 meters and low temperature thresholds were successfully achieved within the firefighting lobby, ensuring safe operational conditions for emergency services.
Value Delivered & Compliance Impact
The performance-based fire engineering approach successfully demonstrated that the engineered mechanical system provided a level of safety exceeding prescriptive codes. The CFD analysis satisfied the third-party technical review and secured formal approval from the Authority Having Jurisdiction (AHJ). It allowed the client to maintain architectural flexibility, protect the open-plan spatial design, and optimize fan selections based on realistic operational performance rather than conservative theoretical assumptions.