Fire exposes the weaknesses of materials, turning concrete brittle, steel soft, and timber vulnerable
Fire remains one of the greatest threats to the safety and durability of buildings. Unlike other hazards, fire exposes structural elements to high temperatures that fundamentally alter their strength, stiffness, and stability. Every construction material responds differently to heat, and engineers must understand these behaviours to design safe structures. The fire resistance of timber, steel, and concrete varies significantly, and these differences dictate design strategies and safety measures in modern buildings.
The ability of a structural element to resist fire does not depend on the material alone. It also involves geometry, loading, connections, and protective measures. A slender steel column, for example, loses strength far faster than a heavily encased concrete column under the same fire exposure. A deep timber beam chars slowly on the outside, maintaining strength for longer than many expect. Concrete, though often considered inherently fire resistant, can spall explosively if moisture and aggregates behave unfavorably.
Comparing timber, steel, and concrete in fire gives practical insight into safe design. Each material has strengths and vulnerabilities that engineers must balance. Understanding these behaviors helps structural engineers specify member sizes, fire protection methods, and detailing practices. More importantly, it underscores that fire resistance is never absolute; it is achieved only through deliberate design and careful engineering judgment.
Fire Resistance: Concepts and Design Framework
Fire resistance describes the ability of a structural element to withstand fire exposure while retaining load-bearing capacity. Codes define resistance periods, typically ranging from 30 minutes to 120 minutes, depending on building function and risk level. These values are not arbitrary. They reflect the time required for safe evacuation and firefighting efforts, not indefinite endurance.
Standard fire tests underpin resistance ratings. The most widely used is the ISO 834 fire curve, which prescribes a controlled rise in temperature over time. Elements such as beams, slabs, and columns are placed in furnaces, loaded, and observed until failure. Failure criteria may involve collapse, excessive deformation, or temperature rise on the unexposed surface. Though artificial, these tests provide benchmarks for design.
Performance-based approaches are also gaining ground. Instead of relying on prescriptive ratings, engineers model fire exposure, heat transfer, and structural response. Advanced methods simulate real fire scenarios, including localized heating, ventilation effects, and fire growth rates. These approaches highlight that structural fire safety is both material science and applied engineering.
Timber in Fire
Timber behaves uniquely in fire because it chars rather than melts. When exposed to flames, the outer layer of wood undergoes pyrolysis and forms a charred surface. This char acts as insulation, slowing heat penetration to the inner core. As a result, the residual section beneath the char retains much of its strength and stiffness, allowing timber members to carry load for extended periods.
The charring rate of timber is relatively predictable. Eurocode 5 suggests a one-dimensional charring rate of 0.65 mm/min for softwoods under standard fire exposure. Engineers can calculate residual cross-sections by deducting the charred depth from original dimensions. This allows design of fire-resistant timber members without applied fireproofing.
Large timber sections perform better than small ones. A heavy timber beam chars on the surface while preserving a strong core. In contrast, small sections may burn through entirely, leading to early failure. This explains why historic timber-framed halls often survive fires longer than expected, while modern light-timber constructions can collapse quickly without protection.
Connections are a weak point in timber construction. Metal fasteners, plates, or adhesives lose strength faster than the surrounding wood. Charring can also undermine fastener embedment depth, reducing joint capacity. Engineers must, therefore, detail joints with fire in mind, often by embedding fasteners deeper or using fire-protected connectors.
Steel in Fire
Steel is strong and versatile, but it performs poorly in high temperatures. Unlike timber, steel does not char or insulate itself. Heat penetrates immediately, and the material softens as temperature rises. At around 400°C, steel loses significant stiffness. By 600°C, it retains only about 50% of its yield strength. Without protection, slender steel members can buckle or fail within minutes of fire exposure.
The exact response depends on cross-section and loading. A massive steel column absorbs more heat before reaching critical temperatures than a slender beam. However, steel’s high thermal conductivity ensures rapid heating throughout the section. This makes unprotected steel among the most vulnerable structural materials in fire.
Fire protection is therefore essential for steel structures. Common methods include intumescent coatings, fire-resistant boards, or encasement in concrete. Intumescent paints swell when heated, forming an insulating char that delays steel heating. Spray-applied fireproofing materials are also widely used in industrial and commercial buildings.
Connections again play a critical role. Bolted or welded joints may fail when steel weakens, particularly in highly restrained frames. Progressive collapse is a major concern, as local joint failure can trigger chain reactions. Engineers must consider not only member resistance but also system stability under fire
Concrete in Fire
Concrete has long been regarded as inherently fire resistant. Its mass and low thermal conductivity slow heat penetration, while its incombustibility ensures it does not contribute fuel. Reinforced concrete elements often achieve high fire resistance without additional protection. However, the behavior of concrete under fire is complex and not universally reliable.
At moderate temperatures, concrete retains much of its compressive strength. Around 300°C, strength begins to reduce. Beyond 600°C, degradation becomes severe, with loss of both strength and stiffness. The presence of reinforcing steel complicates behavior. Steel reinforcement heats more slowly inside concrete, but prolonged fire can still weaken it.
One major risk is spalling. When heated rapidly, moisture inside concrete turns to steam, creating internal pressure. This pressure can cause surface layers to explode outward, reducing section size and exposing reinforcement. High-strength concrete is particularly vulnerable to explosive spalling because of its low permeability. Engineers mitigate this by using polypropylene fibres, which melt under fire and create escape channels for vapor.
Concrete’s fire resistance also depends on cover thickness. Adequate concrete cover delays heat reaching reinforcement. Eurocode 2 provides minimum cover requirements for various fire resistance ratings. Thicker sections and proper detailing significantly improve performance.
Figure 4: Concrete Spalling Mechanism Under Fire Exposure
(illustration showing moisture buildup, vapor pressure, and spalled layers exposing reinforcement)
Comparative Analysis: Timber vs. Steel vs. Concrete
Timber, steel, and concrete each have distinct strengths and vulnerabilities in fire. Timber’s slow and predictable charring gives designers confidence in residual strength, provided members are sized appropriately. Steel, though versatile, requires protective measures for almost all significant fire resistance applications. Concrete performs well under moderate fire but requires careful mix design and detailing to avoid spalling and reinforcement overheating.
Design Codes and Fire Resistance Ratings
Modern design codes address fire resistance comprehensively. Eurocodes provide detailed provisions for timber, steel, and concrete structures under fire conditions. They specify design values, charring rates, reduction factors, and minimum dimensions to achieve standard resistance periods.
- Eurocode 5: Fire design of timber structures. Predicts effective residual cross-section by charring rates.
- Eurocode 3: Fire design of steel structures. Provides reduction factors for steel strength and stiffness.
- Eurocode 2: Fire design of concrete structures. Defines thermal properties, cover requirements, and spalling considerations.
National codes supplement these frameworks. In the US, ACI, AISC, and NFPA provide design guidance. Performance-based fire engineering approaches are also increasingly recognized in both Europe and North America.
Conclusion
Fire resistance remains a central challenge in structural engineering. Timber, steel, and concrete all offer pathways to safe design, but none are invulnerable. Timber provides slow charring and reliable residual capacity. Steel demands fireproofing to prevent catastrophic early failure. Concrete resists fire well but must be detailed to avoid spalling and reinforcement heating.
The choice between materials depends on context. For monumental halls, heavy timber offers both beauty and resilience. For high-rise office towers, fire-protected steel delivers efficiency and speed. For critical infrastructure, reinforced concrete provides durability and inherent protection. The comparative study of fire resistance underscores that materials should not be judged in isolation. Engineers must weigh fire behavior alongside structural, economic, and sustainability considerations.
Also See: Fire Safety in Steel-Framed Buildings: Engineering Approaches and Applications
References
- Eurocode 2: Design of Concrete Structures – Part 1-2: General Rules – Structural Fire Design.
- Eurocode 3: Design of Steel Structures – Part 1-2: General Rules – Structural Fire Design.
- Eurocode 5: Design of Timber Structures – Part 1-2: Structural Fire Design.
- Buchanan, A. H. (2001). Structural Design for Fire Safety. John Wiley & Sons.
- Hertz, K. D. (2003). Limits of Spalling of Fire-Exposed Concrete. Fire Safety Journal.
- Kodur, V. (2014). Properties of Concrete at Elevated Temperatures. International Journal of Concrete Structures and Materials.
- American Institute of Steel Construction (AISC). Steel Design Guide: Structural Fire Protection.
