Buildings don't just burn — they can collapse. Structural fire engineering ensures that the structure maintains its integrity long enough for everyone to escape. Here's how it works.. Why Structures Fail in Fire Fire affects structural materials in fundamentally different ways, but the result is the same: loss of strength and stiffness that can lead to partial or complete structural collapse. Steel loses approximately 50% of its yield strength at 550°C and 90% at 800°C Concrete suffers surface spalling, exposing reinforcement to heat, and loses strength progressively above 300°C Timber chars at approximately 0.7mm/minute, reducing the cross section available to carry load Masonry generally performs well but can crack and spall, particularly in rapid heating scenarios The Prescriptive Approach Approved Document B specifies minimum periods of fire resistance for structural elements: Building Type Height Minimum Fire Resistance Residential <5m 30 minutes Residential 5 18m 60 minutes Residential 18 30m 90 minutes Residential 30m 120 minutes Office <5m 30 minutes Office 5 30m 60 minutes Office 30m 120 minutes These periods are achieved through applied fire protection (intumescent coatings, boards, sprays) or inherent fire resistance (concrete cover, timber section size). Performance Based Structural Fire Engineering Performance based design goes beyond prescriptive requirements to analyse actual structural behaviour in fire: Natural Fire Models Instead of assuming the standard fire (ISO 834 curve), engineers model realistic fire scenarios: Parametric fires (Eurocode 1 Annex A) — considering ventilation, fuel load, and compartment geometry Localised fires — for large compartments where flashover is unlikely Travelling fires — for open plan floor plates where fire moves across the space CFD derived thermal exposure — using computational fluid dynamics for complex geometries Structural Response Analysis Finite element analysis models the structural response to fire loading: Thermal analysis — heat transfer through structural members Mechanical analysis — structural behaviour under combined gravity and thermal loading Connection behaviour — often the critical failure mode in steel structures Membrane action — beneficial load carrying mechanism in composite floor systems Global behaviour — understanding how the whole structure responds, not just individual elements Material Specific Considerations Steel Structures: Section factor (heated perimeter / cross sectional area) determines heating rate Connection design is critical — connections often govern overall stability Thermal expansion can induce significant forces in restrained beams Cooling phase can be more dangerous than heating due to tensile forces Concrete Structures: Cover to reinforcement is the primary fire resistance mechanism Spalling risk assessment — high strength concrete is more susceptible Axis distance rather than cover thickness determines performance Post fire assessment can often demonstrate residual capacity Timber Structures: Charring rate is predictable and well characterised Mass timber (CLT, glulam) performs well due to charring protection of inner core Connection design in timber is critical — metal connections conduct heat into the timber Engineered timber products require specific fire testing data Case Study: Open Plan Office — Performance Based Design A 12 storey commercial office building in London: Challenge: Client wanted exposed structural steel for architectural effect — no intumescent coating Approach: 1. CFD modelling of realistic fire scenarios on open plan floor plates 2. Travelling fire analysis showing that steel temperatures remained below critical thresholds due to localised nature of fires 3. Finite element analysis demonstrating membrane action in the composite floor system 4. Sprinkler reliability analysis confirming very low probability of uncontrolled fire Result: Approved design with no applied fire protection to internal steelwork — saving £2.5 million in construction costs while maintaining equivalent safety through a combination of sprinklers, detection, and inherent structural robustness. Post Fire Structural Assessment After a fire, the question is often whether the structure can be retained: Visual inspection — surface damage, discolouration, cracking, spalling Non destructive testing — rebound hammer, ultrasonic pulse velocity, cover meter Core sampling — laboratory testing of material properties Structural analysis — assessment of residual capacity against design loads Many concrete and steel structures can be repaired and returned to service after fire For structural fire engineering consultancy, contact Magnus Opifex.