Understand how different structural materials behave in fire and the engineering approaches to achieving fire resistance without excessive passive protection.. Introduction to Structural Fire Engineering Structural fire engineering is the discipline of designing buildings to maintain structural stability during and after fire exposure. Unlike prescriptive approaches that simply specify fire resistance periods, performance based structural fire engineering analyses actual structural behaviour under realistic fire conditions, often enabling more efficient and cost effective solutions. Steel Structures in Fire Behaviour Steel loses strength and stiffness rapidly with increasing temperature. At 550°C, structural steel retains only about 60% of its ambient temperature yield strength. At 700°C, this drops to approximately 23%. The critical temperature for most steel members is between 500°C and 620°C, depending on the load ratio. Protection Methods Intumescent coatings — thin film coatings that expand when heated to form an insulating char Board protection — calcium silicate, vermiculite, or gypsum based boards Spray applied protection — cementitious or mineral fibre sprays Concrete encasement — traditional but heavy and space consuming Water filled hollow sections — circulating water cooling system Performance Based Approach The Eurocode method (BS EN 1993 1 2) allows engineers to calculate actual fire resistance based on: Section factor (Hp/A) — heated perimeter to cross sectional area ratio Applied load ratio — actual load vs. ambient capacity Temperature time relationship — natural fire curves vs. standard fire Connection behaviour — moment redistribution and catenary action Concrete Structures in Fire Behaviour Concrete generally performs well in fire due to its low thermal conductivity and non combustibility. However, key concerns include: Spalling — explosive detachment of concrete cover, exposing reinforcement Reinforcement temperature — steel reinforcement loses strength above 300°C Prestressed concrete — strand relaxation at lower temperatures than mild steel Design Considerations Minimum cover to reinforcement (typically 25 50mm depending on fire period) Aggregate type — calcareous aggregates perform better than siliceous Moisture content — higher moisture increases spalling risk Polypropylene fibres — added to concrete mix to reduce spalling risk Mass Timber (CLT) Structures in Fire The Char Layer Concept Cross laminated timber (CLT) and glulam structures form a protective char layer when exposed to fire, insulating the underlying timber. The charring rate for softwood CLT is approximately 0.65 mm/min under standard fire exposure. Design Methods Reduced cross section method — deducting charred timber plus a zero strength layer Reduced properties method — applying strength and stiffness reduction factors Advanced calculation — finite element thermal structural analysis Key Challenges Delamination — adhesive failure in CLT causing char fall off and increased burning Concealed spaces — fire spread within cavities and service zones Self extinction — ensuring the timber structure stops burning after fire decay Regulatory acceptance — navigating evolving guidance on tall timber buildings Natural Fire Engineering Rather than using the standard fire curve (ISO 834), natural fire engineering considers the actual fire development in a specific compartment: 1. Parametric fire curves (Eurocode Annex A) — based on fire load density, ventilation, and thermal properties 2. Localised fires — Hasemi method for fires that don't reach flashover 3. Travelling fires — for large, open plan compartments where uniform burning is unlikely 4. CFD FEA coupled analysis — FDS fire simulation linked to SAFIR or ABAQUS structural analysis Magnus Opifex SEVEN LTD's Structural Fire Engineering Our team combines fire dynamics expertise with structural engineering knowledge to deliver performance based fire resistance solutions. We have worked on major steel framed commercial buildings, concrete high rise towers, and innovative mass timber projects, consistently delivering designs that optimise fire protection costs while maintaining the highest safety standards. Magnus Opifex SEVEN LTD — UK's Leading Fire Safety & Fire Engineering Consultancy 🌐 magnus opifex.co.uk 📞 +44 7486 691724 ✉️ office@magnus opifex.co.uk Founders: Nicoleta Vasile, Baroness of Brattleby — CEO, Lawyer and Barrister, Legal & Administrative Director Alina — Technical Director & Expert Fire Engineer (BEng) Head Office: Ealing Cross, 85 Uxbridge Road, London W5 5BW Magnus Opifex SEVEN LTD delivers engineering led fire engineering, fire risk assessments, CFD modelling, and building safety consultancy across the United Kingdom and internationally. With over 20 years of combined experience and a UK portfolio spanning healthcare, residential and infrastructure, we bring truly engineered solutions with a personal touch. © 2026 Magnus Opifex SEVEN LTD. All rights reserve