Fire Effects on Structural Materials

Introduction

Structural fire safety depends on more than preventing ignition. A building must retain sufficient load-bearing capacity, stability and compartmentation for the required fire-resistance period. Fire changes both the temperature and the mechanical behaviour of structural materials. Steel loses strength rapidly at elevated temperature; concrete may crack, spall and suffer irreversible chemical changes; timber chars progressively; masonry experiences dehydration, cracking and loss of bond; aluminium loses strength at comparatively low temperatures; and composite members are affected by interaction between their constituent materials.

The engineer or fire-safety professional must therefore understand the relationship between fire exposure, temperature distribution, material properties, structural actions and failure modes. Material properties are not single fixed values during a fire. They are temperature-dependent and may also depend on heating rate, moisture, load level, restraint, cross-sectional geometry and cooling history.

Learning Objectives

By the end of this chapter, the learner should be able to:

1. Explain how fire temperature affects the thermal and mechanical properties of major structural materials.
2. Distinguish thermal conductivity, specific heat, density, thermal expansion and mechanical resistance.
3. Describe steel weakening, concrete spalling, timber charring, masonry degradation and aluminium softening.
4. Explain the behaviour of reinforced-concrete, steel–concrete and timber–steel composite members.
5. Interpret temperature-dependent strength and stiffness reductions using reduction factors.
6. Apply material knowledge to fire-resistance assessment and practical fire-safety decisions.

1. Fire Exposure and Material Response

A fire exposes a structural member to convection, radiation and, in some cases, direct flame contact. The gas temperature is not automatically the temperature of the member. Heat must move from the fire environment into and through the material.

For a surface exposed to hot gases, a simplified heat-flux expression is:

where q̇ is net heat flux in W/m², h_c is the convective heat-transfer coefficient in W/(m²·K), T_g and T_s are gas and surface temperatures in °C, ε is effective emissivity, and σ is the Stefan–Boltzmann constant, approximately 5.67 × 10⁻⁸ W/(m²·K⁴). The expression is a simplified idealisation. It assumes uniform surface conditions and does not include flame radiation, openings, shadow effects, moisture evaporation or protective systems explicitly.

Once heat reaches the surface, conduction transfers it through the member. For one-dimensional conduction:

Here q_x is heat flux in W/m², λ is thermal conductivity in W/(m·K), T is temperature in K or °C for temperature differences, and x is distance in metres. The negative sign indicates that heat flows toward lower temperature. In real fire exposure, λ, density and specific heat may vary with temperature.

The principal thermal properties are:

A useful measure of thermal inertia is thermal diffusivity:

where α is thermal diffusivity in m²/s. High diffusivity allows temperature changes to penetrate more quickly, although actual fire behaviour also depends on geometry, moisture and changing properties.

Mechanical resistance is commonly represented by a reduction factor:

where X_θ is a property at temperature θ and X_20 is its reference value near ambient temperature, commonly 20 °C. X may represent yield strength, compressive strength, tensile strength or elastic modulus. Reduction factors are material- and standard-specific; they must not be transferred between materials without justification.

2. Structural Steel

2.1 Thermal behaviour

Steel has relatively high thermal conductivity and therefore heats comparatively quickly when unprotected. A thin steel section can approach the surrounding gas temperature, while a massive or protected section heats more slowly. The temperature is influenced by the section factor, often expressed as heated steel perimeter divided by steel volume. A larger exposed perimeter relative to volume generally produces faster heating.

Thermal expansion is approximately proportional to temperature change over moderate ranges:

where ΔL is free expansion, α_s is the coefficient of thermal expansion of steel, L_0 is original length and ΔT is temperature increase. This equation applies to unrestrained, uniform heating. In a real frame, connections and adjacent cooler members restrain expansion, producing compression, bending and additional connection forces.

2.2 Mechanical degradation

At elevated temperature, steel loses yield strength and stiffness. The elastic modulus generally decreases earlier and more rapidly than yield strength. Consequently, deflection and second-order effects may become serious before the member reaches its ambient-temperature yield stress.

Typical qualitative behaviour is:

• Below approximately 400 °C, many structural steels retain a substantial proportion of ambient resistance.
• Between approximately 400 and 600 °C, strength and stiffness reduction becomes increasingly important.
• Around 550–600 °C, unprotected steel may have insufficient resistance for ordinary design loads, depending on utilization and restraint.
• At higher temperatures, rapid loss of stiffness, buckling and connection failure become dominant concerns.

These temperatures are indicative, not universal design limits. The actual resistance depends on load ratio, steel grade, buckling length, restraint and the adopted fire-design standard.

Steel does not normally burn in a building fire, but it can lose load-bearing capacity through yielding, local buckling, lateral-torsional buckling, connection deformation and catenary action. Fire protection may be provided by boards, sprays, intumescent coatings, concrete encasement or other tested systems. Protection must be continuous, correctly applied and compatible with the substrate.

3. Concrete and Reinforced Concrete

3.1 Thermal and chemical response

Concrete is a heterogeneous material containing cement paste, aggregates, pores and usually reinforcement. Its thermal behaviour is affected strongly by moisture. As temperature rises, free water evaporates and chemically bound water is released. Moisture migration produces pore pressure and thermal gradients.

Concrete generally has low thermal conductivity compared with steel and provides useful insulation to embedded reinforcement. However, its moisture content and aggregate type are critical. Siliceous aggregates may undergo significant thermal expansion; calcareous aggregates often show different expansion characteristics. Concrete properties must therefore be selected with reference to the actual mix and design standard.

3.2 Strength and stiffness loss

Compressive strength may remain relatively stable at moderate temperatures but decreases as dehydration, cracking and aggregate–paste incompatibility develop. Tensile strength is lost more rapidly than compressive strength. The elastic modulus also decreases substantially, causing increased deflection.

A simplified fire-design relationship may be expressed as:

where f_c,θ is concrete compressive strength at temperature θ, f_c,20 is reference compressive strength, and k_c,θ is the temperature-dependent reduction factor. This relationship is not a universal material law; values of k_c,θ must be obtained from the applicable design code, test data or validated analysis.

Reinforcing steel is protected by concrete cover. If the reinforcement temperature remains relatively low, it may retain much of its resistance. However, thin cover, cracking, spalling or severe thermal gradients can expose reinforcement and accelerate heating. Prestressing steel is especially sensitive because loss of strength and prestress can occur at elevated temperature.

3.3 Spalling

Spalling is the detachment of concrete from the surface during heating. It can be superficial or explosive. Important mechanisms include:

1. Pore-pressure spalling: vapor pressure develops when moisture cannot escape readily.
2. Thermal-stress spalling: restrained expansion creates tensile stress near the heated surface.
3. Aggregate-related spalling: mineral transformations or differential expansion weaken the matrix.
4. Load-induced spalling: applied compression increases stress in already weakened concrete.

High-strength concrete is often more vulnerable to explosive spalling because its dense microstructure restricts vapor movement. Polypropylene fibres may reduce risk by melting and creating channels for vapor release, but they do not guarantee prevention.

Spalling reduces cross-sectional area, exposes reinforcement, changes the thermal boundary and may cause sudden loss of capacity. Mitigation includes appropriate concrete mix design, moisture control, adequate cover, reinforcement detailing, fire protection boards or sprayed protection, and validated testing.

3.4 Reinforced-concrete failure modes

Potential failure modes include crushing of heated concrete, yielding or rupture of reinforcement, bond deterioration, excessive flexural deflection, shear failure and instability of columns. Restrained thermal expansion can produce beneficial redistribution in some circumstances, but it can also impose severe forces on connections and surrounding structure.

4. Timber

4.1 Pyrolysis and charring

Timber is combustible. When heated, it dries, undergoes pyrolysis and produces combustible gases. A visible flame may develop when gases mix with oxygen and ignite. Behind the char layer lies a heated pyrolysis zone, followed by a relatively cooler residual section.

The char layer has low structural value and acts as partial insulation. The effective residual cross-section is estimated by removing the char depth and an allowance for the heated zone. A simplified expression is:

where d_eff is effective charring depth, β_n is the notional charring rate in mm/min, t is fire exposure time in minutes, and d_0 is an allowance representing the heated zone near the char boundary. The values depend on species, density, moisture, orientation, protection and the design standard.

The residual area is then assessed geometrically. For a rectangular beam with initial width b and depth h, assuming charring from three sides and equal effective depth d_eff on each exposed side, an idealised residual section is approximately:

where d_eff,h and d_eff,v are effective losses in the horizontal and vertical directions. This is only a geometric illustration; actual charring may be one-sided, corner-rounded or affected by joints and protection.

4.2 Mechanical behaviour

Timber strength and stiffness decrease in the heated layer, while the uncharred core may retain useful capacity. Fire design therefore considers both reduced dimensions and temperature effects. Connections are often critical because metal fasteners conduct heat into the timber and may lose strength or cause splitting.

Large timber members can perform well in fire because the char layer develops gradually and insulates the core. Small sections, exposed edges, concealed cavities and combustible linings can behave less favourably. Adhesive performance, delamination in engineered wood products and protection continuity require specific assessment.

5. Masonry

Masonry includes clay brick, concrete block, calcium-silicate units, mortar and sometimes grout or reinforcement. It is generally non-combustible, but non-combustibility does not mean that it retains all structural capacity during fire.

Heating produces moisture loss, differential expansion and cracking. Unit–mortar bond may deteriorate, and hollow units may experience internal thermal gradients. Masonry walls can lose capacity through:

• cracking and separation of units;
• reduction in compressive and shear strength;
• out-of-plane bowing due to thermal gradients;
• loss of restraint at floors and roofs;
• premature failure of ties, anchors or lintels.

The behaviour of masonry depends on unit type, density, moisture, workmanship, wall thickness, slenderness and restraint. Load-bearing masonry should be assessed using approved fire-resistance tables, tested assemblies or engineering calculations rather than relying only on material classification.

6. Aluminium

Aluminium has high thermal conductivity and a melting point substantially lower than that of steel. More importantly, its strength and elastic modulus decline sharply at temperatures commonly reached in building fires. Thus an aluminium member may become structurally ineffective long before it melts.

Aluminium expands significantly when heated. In framed systems, this can cause joint movement, loss of bearing and distortion. Protective measures include insulation, shielding, water cooling in specialised applications and limiting exposure. Fire assessment must consider alloy, temper, section geometry, connection details and the possibility of rapid local buckling.

Aluminium’s thin oxide layer does not eliminate the fire risk. It protects against ordinary corrosion but does not preserve structural strength at elevated temperature.

7. Composite Materials and Composite Members

Composite construction relies on interaction between different materials. Examples include steel beams with concrete slabs, concrete-filled steel columns, timber–steel connections, fibre-reinforced polymer strengthening systems and sandwich panels.

Fire behaviour is governed by compatibility and differential response. Steel heats quickly while concrete provides insulation. A concrete slab may act as a heat sink and enable membrane or catenary action, but cracking and connection demands must be considered. In a concrete-filled steel column, the steel tube may restrain concrete and the concrete may protect the steel core; loss of bond, moisture pressure and load redistribution still require evaluation.

Composite panels can contain combustible cores. The fire performance of the complete assembly—including joints, facings, fixings and cavities—must be tested or assessed. A facing material with good surface classification does not automatically make the complete panel safe.

A useful conceptual sequence for composite assessment is:

1. Determine the fire exposure and thermal boundary conditions.
2. Calculate or obtain temperature profiles in each material.
3. Apply temperature-dependent strength and stiffness properties.
4. Evaluate bond, connection and interface behaviour.
5. Check member resistance, deformation and stability.
6. Check the surrounding frame for restraint forces and alternate load paths.

8. Temperature-Dependent Structural Assessment

A fire-resistance assessment normally combines thermal analysis and structural analysis. Thermal analysis determines temperature as a function of time and location, T(x,t). Structural analysis then uses those temperatures to evaluate reduced material properties and thermal strain.

The total strain may be represented conceptually as:

where ε_mechanical is load-related strain, ε_thermal is free thermal expansion, ε_creep is time-dependent strain, and ε_transformation represents strain associated with material changes where relevant. Not every term is required in every design method.

At the member level, the design condition is commonly expressed as:

where E_d,fi is the design effect of actions during fire and R_d,fi is the reduced design resistance during fire. Both are time- and temperature-dependent. The inequality is meaningful only when actions, combination factors, material reduction factors, stability and boundary conditions follow the applicable code.

Fire design may use:

• tabulated data for standardised assemblies;
• simple calculation models;
• advanced finite-element thermal and structural models;
• furnace tests or large-scale demonstrations;
• performance-based engineering supported by validated evidence.

9. Worked Application: Unprotected Steel Beam

Consider an illustrative, not code-certified, steel beam exposed on three sides. Its steel temperature after 30 minutes is estimated by a validated calculation as 590 °C. Suppose the adopted design data give a yield-strength reduction factor of 0.47 and an elastic-modulus reduction factor of 0.31 at this temperature.

If the ambient yield strength is 355 MPa:

If the ambient elastic modulus is 200 GPa:

The result indicates a major loss of stiffness and a substantial loss of yield strength. The beam must not be judged safe merely because its temperature is below the melting point of steel. Buckling, deflection, connection capacity, load ratio and restraint must also be checked. The numerical reduction factors are illustrative and must be replaced by values from the governing standard.

10. Illustrative Case Study: Fire in a Composite Floor

Background

An illustrative six-storey office building uses steel beams supporting a reinforced-concrete composite floor. The beams are protected by board systems, but a service penetration has interrupted protection near one beam–column connection.

Problem

A compartment fire produces prolonged heating. Inspection identifies local protection damage, cracking in the slab near the connection and downward beam deflection. The question is whether the floor can maintain its required fire-resistance period.

Analysis

The unprotected region has a larger section factor and heats faster than the protected beam. Reduced steel stiffness increases sagging. The concrete slab limits heat flow to some parts of the steel and may develop tensile membrane action, but the damaged connection is subject to additional rotation and restraint forces. Cracking may reduce the assumed composite interaction. The connection temperature is also important because bolts, welds and surrounding steel do not necessarily have the same temperature.

Approach and solution

The fire engineer should first establish the actual fire exposure and inspect the continuity of protection. A thermal model or approved calculation is used to estimate steel and reinforcement temperatures. The member is then checked using temperature-dependent resistance, stability and connection properties. If the required resistance is not demonstrated, remedial options include replacing the protection, installing a tested encasement, strengthening the connection, reducing fuel load, improving compartmentation or implementing a managed evacuation and monitoring strategy where permitted by the governing authority.

Outcome

The assessment demonstrates that a composite slab can provide beneficial redistribution but cannot be credited automatically. Protection defects at connections may control performance even when the main beam appears adequate.

Learning points

1. Fire resistance belongs to the complete structural system, not an isolated material.
2. Local defects can create critical thermal hot spots.
3. Stiffness loss and connection deformation may govern before material rupture.
4. Tested protection details and inspection records are essential.
5. Performance-based claims must be supported by validated calculations or tests.

11. Practical Design and Inspection Guidance

Good practice includes identifying the material and grade, obtaining reliable temperature-dependent properties, checking moisture and protection conditions, and examining interfaces and connections. Fire stopping should prevent hidden flame and hot-gas paths. Penetrations, joints, cavities and supports require particular attention.

Inspection after a fire should document spalling depth, crack patterns, permanent deformation, exposed reinforcement, char depth, fastener condition, coating damage and residual material properties. Visual inspection alone is insufficient where safety is uncertain. Sampling, hardness testing, rebound methods, ultrasonic testing or laboratory examination may be appropriate when interpreted by qualified specialists.

12. Limitations, Regulation and Professional Responsibility

Material tables are simplifications. They may not represent unusual heating rates, cooling phases, mixed exposures, high-strength materials, recycled products, novel composites or damaged construction. Standard fire curves also do not reproduce every real fire. Natural fires may grow, peak and decay, and ventilation can change the thermal environment significantly.

Designers must use the adopted national building regulations, structural fire-design standards, product test standards and fire-classification procedures. Certification should cover the complete tested system, including thickness, fixings, joints, substrates and installation conditions. It is unethical and unsafe to extrapolate a product rating to an untested configuration without technical justification.

Digital thermal and finite-element models can improve analysis, but software does not replace engineering judgement. Input data, boundary conditions, mesh quality, validation and sensitivity checks must be recorded. Artificial intelligence may assist with document review, defect detection or parametric studies, but final safety decisions require competent professional oversight and traceable evidence.

Contemporary Developments

Current practice increasingly considers performance-based design, natural-fire exposure, resilient structural systems, low-carbon materials and reuse of existing buildings. Mass timber construction has encouraged improved charring models, encapsulation strategies and connection research. High-performance concrete research focuses on moisture migration and spalling mitigation. Digital twins and sensor systems may support post-fire assessment, while advanced protective coatings seek lower environmental impact. These developments do not remove the need for conservative detailing and validated fire testing.

Chapter Summary

Fire changes structural materials through heat transfer, thermal expansion, moisture movement, chemical transformation, strength loss and stiffness degradation. Steel heats rapidly when unprotected and may buckle or deform long before melting. Concrete provides thermal protection but can crack, lose strength and spall, particularly when dense and moisture-rich. Timber forms a protective char layer, but its effective residual section and connections must be assessed. Masonry loses capacity through cracking, thermal gradients and deterioration of unit–mortar interfaces. Aluminium loses strength and stiffness at relatively low temperatures. Composite members depend on interaction, restraint, interfaces and connection behaviour.

Reliable fire assessment combines thermal analysis with structural resistance and stability checks. Temperature-dependent reduction factors are useful only when obtained from appropriate standards or validated evidence. Fire resistance is a property of the complete structural system and its protection, not merely of an isolated material sample. Inspection, correct installation, regulatory compliance and professional judgement are essential to safe practice.

Key Terms

Thermal conductivity: Ability of a material to conduct heat.

Specific heat: Energy required to raise the temperature of unit mass by one degree.

Thermal diffusivity: Ratio of thermal conductivity to volumetric heat capacity.

Thermal expansion: Increase in dimension caused by temperature rise.

Reduction factor: Ratio of a temperature-dependent property to its ambient value.

Section factor: Heated perimeter relative to the volume or mass of a structural section.

Spalling: Detachment of concrete from a heated surface.

Explosive spalling: Sudden violent concrete detachment caused mainly by pore pressure and thermal stress.

Pyrolysis: Thermal decomposition of timber that produces combustible gases.

Charring: Formation of a carbonised timber layer during burning.

Residual section: Remaining uncharred structural cross-section.

Fire exposure: Thermal action imposed on a member by fire gases, flames and radiation.

Load ratio: Relationship between fire design action and available ambient resistance.

Thermal gradient: Variation of temperature through a member or assembly.

Catenary action: Tensile load-carrying action that may develop in a sagging heated member or slab.

Compartmentation: Division of a building to restrict fire and smoke spread.

Intumescent coating: Fire-protective coating that expands when heated.

Composite member: Structural member in which two or more materials act together.

Thermal inertia: Resistance of a body to rapid temperature change.

Fire resistance: Ability of an element to meet required load-bearing, integrity and insulation criteria for a specified period.

Review Questions

5 Short Answer Questions

1. Why can a steel beam fail structurally before it melts?
2. Define concrete spalling and identify two mechanisms that cause it.
3. What is the purpose of a timber char layer?
4. Why is aluminium vulnerable in building fires?
5. State the difference between thermal conductivity and specific heat.

5 Long Answer Questions

1. Explain the thermal and mechanical response of structural steel during a standard fire exposure.
2. Describe the causes, consequences and mitigation measures associated with explosive spalling of concrete.
3. Explain how charring is used to assess the fire performance of timber members.
4. Compare the fire behaviour of concrete, masonry and aluminium.
5. Discuss why connections and interfaces are important in composite fire design.

3 Analytical / Application Questions

1. A protected steel beam develops a temperature of 500 °C during fire. Explain the information required before deciding whether its resistance is adequate.
2. A high-strength concrete column shows surface cracking and local spalling during a fire. Analyse the possible effects on reinforcement temperature, load capacity and inspection requirements.
3. A mass-timber floor is protected on its lower surface, but several joints are unprotected. Develop a structured assessment approach addressing charring, concealed fire spread, connections and residual capacity.

Multiple Choice Questions with Answers

Question 1

Which property most directly describes a material’s ability to conduct heat?

A. Specific heat

B. Thermal conductivity

C. Density

D. Emissivity

Correct Answer: B. Thermal conductivity

Question 2

Why may an unprotected steel member lose its load-bearing capacity during a fire?

A. It must melt before losing strength

B. Its strength and stiffness decrease at elevated temperature

C. Its density increases sharply

D. It becomes non-combustible

Correct Answer: B. Its strength and stiffness decrease at elevated temperature

Question 3

Explosive spalling of concrete is particularly associated with which condition?

A. Very open, low-density pore structure

B. Dense concrete that restricts vapour movement

C. Complete absence of moisture

D. Low thermal gradients only

Correct Answer: B. Dense concrete that restricts vapour movement

Question 4

What is the principal structural significance of the char layer in timber?

A. It increases the original cross-section

B. It provides partial insulation while reducing the effective section

C. It prevents all further heating

D. It converts timber into steel

Correct Answer: B. It provides partial insulation while reducing the effective section

Question 5

A reduction factor kθ is generally defined as:

A. Ambient property divided by heated property

B. Heated property divided by ambient property

C. Temperature divided by time

D. Load divided by area

Correct Answer: B. Heated property divided by ambient property

Question 6

Which statement about aluminium in fire is correct?

A. It retains ambient strength until melting

B. It generally loses strength and stiffness at relatively low temperatures

C. It does not expand when heated

D. Its oxide layer provides complete structural protection

Correct Answer: B. It generally loses strength and stiffness at relatively low temperatures

Question 7

Which factor strongly affects the heating rate of an unprotected steel section?

A. Section factor

B. Paint colour only

C. Ambient humidity only

D. The member’s name

Correct Answer: A. Section factor

Question 8

What is spalling?

A. Formation of a timber char layer

B. Detachment of concrete from a heated surface

C. Expansion of aluminium oxide

D. Melting of structural steel

Correct Answer: B. Detachment of concrete from a heated surface

Question 9

For a material with ambient yield strength 355 MPa and reduction factor 0.47, the illustrative heated yield strength is approximately:

A. 75 MPa

B. 167 MPa

C. 355 MPa

D. 755 MPa

Correct Answer: B. 167 MPa

Question 10

Why must composite fire design consider connections and interfaces?

A. They are always cooler than every other component

B. Differential heating, bond loss and restraint can control system behaviour

C. They have no influence on load transfer

D. They eliminate thermal expansion

Correct Answer: B. Differential heating, bond loss and restraint can control system behaviour