This article examines key fire protection principles used for steel-framed buildings. It explores passive and active methods, their practical applications, and how engineers make design decisions.
Fire poses one of the greatest threats to the integrity and safety of steel-framed buildings. Although steel does not burn, it loses strength when heated. Once the temperature exceeds 500°C, structural steel begins to soften, reducing its load-carrying capacity. Without adequate protection, failure can occur quickly, endangering lives and property.
Engineers must consider how fire affects steel structures during design. They apply protection strategies that maintain strength during fire exposure. Fire resistance is not a secondary issue. It remains central to the structural performance of any steel-framed system.
This article examines key fire protection principles used for steel-framed buildings. It explores passive and active methods, their practical applications, and how engineers make design decisions. It also highlights fire testing standards and the role of modern fire modelling. The article concludes with insights into regulations, case studies, and evolving practices in structural engineering.
Why Steel Requires Fire Protection
Steel can carry heavy loads and form flexible, efficient structures. However, its performance drops rapidly when exposed to heat. At 600°C, the yield strength of steel can fall to 40% of its original value. Without adequate protection, buckling or collapse may occur within minutes of a fire.
Unlike timber, steel will not ignite or produce fuel for a fire. But this does not make it immune. Instead, steel transfers heat quickly and loses stiffness as temperature rises. Engineers must consider this property during the design stage to ensure public safety.
Unprotected steel fails to meet fire resistance ratings required by building codes. Hence, engineers must provide thermal insulation or allow for sacrificial material. Fire resistance is not optional. It is required for structural stability during evacuation and firefighting operations.
Key Fire Protection Strategies
Engineers apply two main types of fire protection: passive and active. Passive protection slows the temperature rise in structural steel. Active systems detect or suppress the fire directly. Each method plays a role depending on the risk profile of the building.
Passive Fire Protection
Passive fire protection methods include:
- Intumescent coatings
- Fire-resistant boards
- Spray-applied fire-resistive materials (SFRMs)
- Concrete encasement
Intumescent coatings expand when exposed to heat. They form an insulating char layer that protects the steel. These coatings are thin and aesthetic. They suit exposed steel in architectural designs.
Board systems consist of layers of non-combustible material, such as calcium silicate or gypsum. Boards offer precise thickness control and can include decorative finishes.
Spray-applied materials form a thick insulating layer. They are often used in hidden or industrial settings where appearance is less critical.
Concrete encasement offers strong protection and durability. It is heavy and may increase construction time and cost. Still, it performs well in severe fire conditions.
Active Fire Protection
Active systems include sprinklers, water mist systems, and fire alarms. These reduce the fire’s temperature or alert occupants. Although useful, active systems cannot replace passive protection. Sprinklers may fail or get blocked. Engineers must not rely on them alone.
Fire Resistance Ratings
Codes require that buildings meet specific fire resistance periods. Ratings are usually given in minutes (e.g., 30, 60, 90, or 120). They indicate how long a member should perform under fire before failure.
Ratings depend on the building type, height, and use. Residential high-rises often require longer resistance than single-story warehouses. Fire resistance is tested by standard heating curves. The ISO 834 time-temperature curve remains the most common.
Steel sections are tested under load in furnaces. The temperature inside the furnace follows a controlled curve. Instruments measure deflection and deformation. The test ends when the member fails or reaches its limit state.
Engineers use either standard fire test data or calculations to demonstrate compliance. Eurocode EN 1993-1-2 offers design rules for structural fire resistance of steel.
Critical Temperature and Design Concepts
The critical temperature is the point at which steel loses enough strength to compromise safety. This value varies with the applied load ratio. Heavily loaded members fail at lower temperatures. Typical values range from 500°C to 620°C.
Designers determine the required fire resistance period, then calculate the heating rate. From there, they determine how long the member can last unprotected. If the time is too short, they apply insulation to delay the temperature rise.
The section factor (Hp/A) is critical in fire calculations. It is the ratio of the heated perimeter (Hp) to the steel cross-sectional area (A). Members with large surface areas heat faster. Hollow sections often have higher section factors.
Protection thickness depends on the section factor and required fire duration. Intumescent manufacturers provide tables that match section factors to coating thicknesses.
Common Fire Protection Materials
Engineers select protection based on performance, appearance, cost, and installation needs. Each system has benefits and drawbacks.
Intumescent Paints: Thin and lightweight. Best for visible steel. They require precise application and regular inspection.
Fire-Resistant Boards: Offer fast installation and good thermal resistance. Suitable for large members or retrofitting.
SFRMs: Low-cost and easy to apply. However, they can be messy and lack finish. Often used in industrial settings.
Concrete Encasement: Offers excellent protection and durability. However, it adds significant weight and may complicate detailing.
All materials must meet fire testing standards. Engineers rely on certifications to ensure that products perform under real fire conditions.
Fire Modelling
Modern designs use fire modelling to simulate fire growth and temperature distribution. Computational tools model fire dynamics, smoke spread, and thermal loads. These tools provide better insight than standard curves alone.
Fire modelling can consider different fire sizes, locations, and ventilation rates. It allows performance-based design. This approach tailors protection to real risks rather than assuming worst-case fires.
However, fire modelling requires expertise and calibration. It supplements, not replaces, standard design codes.
Regulatory Framework and Compliance
Most countries enforce building codes that set fire protection requirements. In the UK, the Approved Document B and BS EN 1993-1-2 govern fire design. In the US, the International Building Code and ASTM E119 apply.
Engineers must understand local laws and test standards. Fire resistance must be demonstrated through tests, calculations, or engineering judgment.
Third-party certifications help validate products and ensure consistent quality. Designers should always confirm the manufacturer’s data and system compatibility.
Integration with Structural Design
Fire protection is part of the structural design, not an afterthought. Engineers must allow for added thickness, weight, and detailing changes.
Coordination with architects ensures that coatings and claddings meet aesthetic goals. Detailing must avoid gaps or thermal bridges that reduce effectiveness.
For composite structures, engineers must protect both steel and concrete. Connections, columns, and bracing require special attention.
Maintenance and Durability
Fire protection must last the life of the structure. Some systems, like concrete, require little maintenance. Others, like intumescents, may degrade over time.
Inspections help detect damage from impacts, corrosion, or water. Building owners must include fire protection in their maintenance schedules.
Neglect can reduce fire performance and lead to code violations. Engineers must provide maintenance instructions with their designs.
Case Studies
The World Trade Center Collapse (2001)
The towers had fireproofing sprayed on steel. The plane impact dislodged this material. Exposed steel weakened quickly, leading to collapse. This event highlighted the need for robust protection that can resist mechanical damage.
Broadgate Phase 8 Fire (1990, London)
An intense fire lasted over four hours in an office building under construction. Despite extreme temperatures, no collapse occurred. Protected steel framing retained integrity, showcasing the effectiveness of fire-resistive materials.
Windsor Tower Fire (2005, Madrid)
This concrete and steel high-rise burned for over 20 hours. The steel framing above the 17th floor failed where no protection was applied. Below that level, concrete protection preserved the structure. The event proved how material choice and protection level affect outcomes.
Conclusion
Steel-framed buildings must resist fire to ensure safety and structural stability. Engineers cannot ignore how fire affects steel. Protection methods such as intumescent coatings, boards, and sprays delay temperature rise and maintain strength.
Designers must consider critical temperatures, section factors, and required resistance periods. Fire modelling, testing, and compliance support sound engineering judgment.
Ongoing maintenance ensures long-term performance. Lessons from case studies reveal what happens when protection fails—or succeeds. Emerging practices continue to improve safety, performance, and sustainability.
Fire protection remains a core responsibility in structural design. Engineers must treat it with the same importance as load calculations or detailing.
Also See: Fire Safety in Modern Timber Buildings
Sources & Citations
- Institution of Structural Engineers. (2020). Fire safety of steel-framed buildings. The Structural Engineer, Vol 98, Issue 3. Read here
- EN 1993-1-2: Eurocode 3: Design of steel structures – Part 1-2: General rules – Structural fire design.
- Buchanan, A. H., & Abu, A. K. (2017). Structural Design for Fire Safety. Wiley.
- Ghojel, J. (2022). Fire performance of structural steel in buildings. Fire Safety Journal, Volume 130.
