When a fire engineer's computer simulation predicted a fatal bottleneck on the 28th floor, they had 6 months to redesign the escape route. Without the model, they'd have discovered the problem during a real fire.. The Model That Changed Everything In late 2024, a fire engineering team was commissioned to review the evacuation strategy for a 52 storey residential tower in east London — 2,400 residents across 800 apartments, served by two staircases and four lifts. The prescriptive approach (Approved Document B) suggested the design was compliant. Travel distances were met. Staircase widths complied. The 'stay put' strategy was documented. But the fire engineer wasn't satisfied. They ran the building through Pathfinder evacuation simulation software, modelling a scenario where stay put failed and simultaneous evacuation was required. The result was terrifying. What the Model Revealed Scenario: Full Simultaneous Evacuation Assumptions: Fire on floor 15, compromising the corridor Stay put abandoned at T+8 minutes All 2,400 residents begin evacuating 12% of residents have mobility impairments 6% require wheelchair evacuation Pre movement time: 3 8 minutes (residential, night time) Results: Total evacuation time: 87 minutes (target: 60 minutes) Fatal bottleneck at floor 28: Staircase 1 reached capacity at T+22 minutes Merging flow problem: Residents from floors 28 32 entered the staircase simultaneously, creating a density of 4.2 persons/m² — above the crush threshold Wheelchair users stranded: 14 wheelchair users could not access the staircases Predicted casualties: 23 41 persons (smoke inhalation in stalled staircase flow) The Redesign Changes Made Based on Modelling 1. Staircase width increased from 1,100mm to 1,300mm on floors 25 52 2. Refuge areas added at every 5th floor (floors 10, 15, 20, 25, 30, 35, 40, 45, 50) 3. Evacuation lift installed with dedicated power supply and fire rated shaft 4. Voice alarm system upgraded with floor specific messaging for phased evacuation 5. Smoke control system redesigned to maintain tenable conditions for 90 minutes 6. Wayfinding lighting installed in staircases (photoluminescent + powered) Result After Redesign Total evacuation time: 54 minutes (target: 60 minutes) ✅ No bottlenecks — maximum density 2.8 persons/m² All wheelchair users evacuated via evacuation lift within 40 minutes Predicted casualties: 0 under modelled scenario Cost of Changes: £1.8 million Cost of 23 41 lives: Incalculable Why Every Tall Building Needs Evacuation Modelling Prescriptive Codes Can't Handle Complexity Approved Document B was written for simple buildings. It uses lookup tables for travel distances and staircase widths based on occupancy numbers. It doesn't account for: The interaction between multiple escape routes Pre movement behaviour (people don't evacuate immediately) Merging flows from different floors The impact of mobility impaired occupants Smoke spread affecting escape route viability Human behaviour under stress (counterflow, wayfinding errors) The Technology Available Today Pathfinder — agent based simulation with 3D visualisation STEPS — validated for high density evacuation scenarios MassMotion — crowd simulation for complex geometries FDS+Evac — coupled fire evacuation simulation buildingEXODUS — risk based evacuation modelling These tools can model thousands of agents with individual characteristics (age, speed, mobility, pre movement time, familiarity) moving through detailed building geometry. Magnus Opifex uses advanced evacuation modelling on every tall building project. For evacuation analysis, contact us.