Fire Design of Steel and Composite Structures

Introduction

Steel is a preferred structural material because it is strong, ductile, predictable and relatively light. These advantages do not mean that steel remains safe during fire. As temperature rises, the strength and stiffness of steel reduce, thermal expansion produces restraint forces, connections may weaken, and members may lose stability by buckling. Composite construction—steel beams or columns acting together with concrete slabs—can provide substantially improved fire performance, but only when the interaction between steel, concrete, reinforcement and connections is properly understood.

Fire design therefore differs from ordinary structural design. The engineer must consider the complete fire scenario, the temperature history of each member, material degradation, restraint, load redistribution and the duration for which structural stability is required. Fire resistance is not simply a property of a material; it is a performance of a structural system under a specified thermal and loading condition.

This chapter presents the practical principles used to assess protected and unprotected steel members, connections, composite beams and slabs, critical temperatures, buckling, membrane action and fire-protection systems. Detailed design must follow the applicable national code, such as the relevant parts of Eurocode 3 and Eurocode 4, AISC provisions, or local fire and building regulations.

Learning objectives

After studying this chapter, the reader should be able to:

1. Explain how elevated temperature changes the mechanical and thermal properties of steel and concrete.
2. Distinguish protected from unprotected steel members and select appropriate protection systems.
3. Estimate steel temperatures using simplified heat-transfer concepts.
4. Explain critical temperature, fire-induced buckling and connection behaviour.
5. Describe the fire performance of composite beams and slabs, including membrane action.
6. Apply a rational assessment procedure to a practical structural-fire problem.
7. Identify limitations, inspection requirements and regulatory responsibilities in fire design.

1. Fire exposure and structural response

A standard compartment fire is commonly represented by a nominal time–temperature curve. The ISO standard fire is often expressed as:

where θ_g is the gas temperature in °C and t is time in minutes. This curve is a testing exposure, not a complete prediction of every real fire. Natural fires depend on fuel load, ventilation, compartment geometry, fire growth, suppression and firefighting actions.

A structural fire assessment normally has four linked stages:

1. Fire scenario: establish the fire exposure and required fire-resistance period.
2. Thermal analysis: calculate or estimate member temperatures.
3. Mechanical analysis: determine reduced resistance and deformation under fire loads.
4. System assessment: consider restraint, load redistribution, connections and possible collapse mechanisms.

Figure: Sequential but interdependent stages of structural fire design.

2. Material behaviour at elevated temperature

2.1 Steel

The principal changes in carbon and structural steel are:

• yield strength decreases as temperature increases;
• Young’s modulus decreases, often more rapidly than yield strength;
• thermal expansion increases member length and curvature;
• creep and transient strain become significant at high temperature;
• oxidation and local thinning may occur during long or severe fires.

Design standards express these changes using reduction factors. If k_y,θ is the reduction factor for yield strength and k_E,θ is the reduction factor for elastic modulus, then:

Here f_y and E are the room-temperature yield strength and elastic modulus, while f_y,θ and E_θ are values at steel temperature θ. The factors depend on the adopted standard and should not be extrapolated beyond its tabulated range.

Thermal expansion may be represented by:

where ∆L is free expansion, α is the coefficient of thermal expansion in °C⁻¹, L is the original length and ∆θ is the temperature increase. This equation describes free expansion only. In a restrained frame, part of the expansion is converted into compression, bending or connection force.

2.2 Concrete and reinforcement

Concrete has low thermal conductivity compared with steel and provides thermal mass. However, it loses stiffness and strength with temperature, may crack or spall, and can suffer explosive spalling when moisture pressure is high. Reinforcement remains comparatively protected inside concrete, although its strength also reduces.

The behaviour depends on aggregate type, moisture content, heating rate, member dimensions, restraint and load level. Lightweight concrete generally provides favourable thermal performance, whereas dense concrete may be more vulnerable to spalling under certain conditions.

3. Thermal assessment of steel members

An unprotected steel section heats rapidly because steel has high thermal conductivity and a relatively small heat capacity. A useful geometric parameter is the section factor:

where A_m is the heated steel perimeter per unit length and V is the steel volume per unit length. For a prismatic member, A_m/V has units of m⁻¹. A larger section factor generally produces faster heating because more exposed surface supplies heat to a given volume of steel.

For a protected member, the effective heated perimeter and protection thickness must be defined according to the protection product and design standard. Encased or boxed protection can reduce the exposed perimeter substantially.

A simplified incremental heat-balance concept for unprotected steel is:

where:

• ∆θ_a = steel-temperature increment, °C;
• k_sh = shadow factor, where applicable;
• A_m/V = section factor, m⁻¹;
• h_c = convective heat-transfer coefficient, W/(m²·K);
• θ_g and θ_a = gas and steel temperatures, °C;
• ε = effective emissivity;
• σ = Stefan–Boltzmann constant, 5.67 × 10⁻⁸ W/(m²·K⁴);
• ρ_a = steel density, kg/m³;
• c_a = temperature-dependent specific heat, J/(kg·K);
• ∆t = time increment, s.

Temperatures in the fourth-power radiation term must be in kelvin. The equation is an incremental approximation and assumes a reasonably uniform steel temperature over the time step. It is not a substitute for a validated code method when temperature gradients, protection interfaces or complex geometry are important.

Protected steel receives heat through the protection layer. Increasing protection thickness generally delays heating, but performance also depends on conductivity, density, moisture, adhesion, cracking, joints, fixings and installation quality. Product-specific fire-test evidence is therefore essential.

4. Member resistance and critical temperature

At a specified fire time, the design action effect is compared with the reduced resistance at the calculated temperature. A simplified utilization ratio is:

where E_d,fi is the design action effect in the fire situation and R_d,fi,θ is the design resistance at temperature θ. Adequacy requires η_fi ≤ 1.0. The exact resistance model depends on whether the member is in tension, compression, bending, shear or combined action.

The critical temperature is the steel temperature at which the available resistance equals the applied fire design action, considering the selected stability model. It is not a universal temperature for a steel grade. It changes with load ratio, restraint, slenderness, moment gradient, axial force, connection behaviour and protection assumptions.

For a tension member, a simplified resistance may be expressed as:

where A is the relevant cross-sectional area and γ_M,fi is the fire design material factor specified by the governing code. For bending or compression, local and global instability must also be checked; simply replacing room-temperature yield strength with f_y,θ is insufficient.

The fire load combination is normally less severe than the accidental combination used for normal-temperature design. A generic form is:

where G_k is the characteristic permanent action, Q_k is the leading variable action and ψ_2 is the appropriate accidental-combination factor. The governing code must define the complete combination and factors.

5. Buckling and second-order effects

Buckling is often the controlling failure mode of heated compression members. As temperature rises, both yield strength and elastic modulus decline, reducing resistance and increasing sensitivity to imperfections. Thermal bowing may introduce bending even when the member carries nominally concentric load.

A useful stability indicator for a pin-ended elastic column is the Euler critical load:

where N_cr is the elastic critical load, I is the second moment of area, and L_cr is the effective buckling length. This equation assumes an ideal straight member, elastic behaviour, idealized end conditions and a uniform temperature. Real fire design must include imperfections, temperature-dependent properties, residual stresses and the appropriate buckling curve.

For a member with high slenderness, loss of stiffness may dominate before the reduced yield strength is reached. Bracing, continuity and composite interaction can improve performance, but restraint can also generate substantial secondary forces. A frame that appears stable when members are checked individually may still be vulnerable to redistribution-induced connection failure.

6. Connections in fire

Connections are part of the load path and must not be treated as automatically safe because they are small. Bolts, welds, plates, angles and end-plate components experience temperature-dependent strength and stiffness. Thin connection components heat rapidly; bolts may be shielded by surrounding steel or concrete, producing temperature differences within the joint.

Important connection issues include:

• prying action and bolt tension;
• loss of bolt pretension or slip resistance;
• weld metal and heat-affected-zone behaviour;
• tearing of thin plates;
• rotational capacity and ductility;
• forces caused by restrained thermal expansion;
• catenary or membrane forces transferred through the connection.

A connection designed only for the initial gravity load may be inadequate when the beam sags and changes from flexural action to tensile catenary action. Fire-resistant connection design therefore requires compatibility with the deformation capacity of the connected members and the surrounding frame.

7. Protected and unprotected steel members

7.1 Unprotected steel

Unprotected steel is exposed directly to the compartment fire. It is attractive where the required fire period is short, the section factor is low, the member is shielded by slabs or walls, or system behaviour provides substantial reserve capacity. It requires careful evaluation of temperature, stability, connections and local exposure.

Advantages include low initial cost, easy inspection and visible architectural steelwork. Limitations include rapid heating, vulnerability of slender sections, coating or corrosion considerations, and uncertain performance where fire exposure differs from the assumed scenario.

7.2 Applied protection systems

Common systems include:

A protection specification should state the required fire period, exposure faces, steel section factor, protection thickness, substrate preparation, fixing method, inspection procedure, repair procedure and evidence of tested or assessed performance. Field thickness measurements and records are important for quality assurance, especially for intumescent coatings.

Protection must continue across joints, stiffeners, connection zones, penetrations and changes in section. A protected beam with an unprotected connection may not provide a protected load path.

8. Composite beams and slabs

Composite construction usually consists of a steel beam connected to a concrete slab by shear connectors. At ambient temperature, the steel beam resists tension and the concrete slab commonly resists compression in positive bending. In fire, the slab acts as a thermal shield and may develop alternative load-carrying mechanisms.

8.1 Composite beams

A composite beam may benefit from:

• protection of the upper flange by the slab;
• concrete heat capacity;
• reinforcement acting as a tensile tie;
• continuity and redistribution to adjacent members;
• catenary action at large deflection.

The lower flange and web may still heat rapidly if exposed. Differential temperature can cause curvature and local stresses. Shear connectors must retain enough strength and ductility to transfer force while the beam deforms.

8.2 Composite slabs

A composite slab can initially act by one-way bending. At large deflection, tensile membrane action may develop in the central region, while compressive membrane action may occur near supports. This mechanism can substantially increase resistance, but it depends on reinforcement continuity, anchorage, edge restraint, integrity of surrounding beams and the ability of the slab to sustain large deformation without rupture.

Membrane action is not a free additional capacity. It transfers forces to supports and surrounding members. Edge beams, columns and connections must be checked for these forces. Openings, discontinuous reinforcement, weak ties and premature connection failure can prevent membrane action from developing.

A simplified conceptual equilibrium for a membrane strip is:

where T is an idealized tensile membrane force, q is the uniformly distributed load per unit length, L is the span and δ is the compatible central deflection. This expression is only a conceptual relation for an idealized strip and must not be used as a complete slab design equation. Real membrane capacity depends on geometry, reinforcement, compatibility, yield-line mechanisms, anchorage and boundary restraint.

8.3 Composite columns

Concrete-filled or encased steel columns benefit from thermal mass and load sharing. However, differential expansion, concrete cracking, spalling, local buckling of steel, reinforcement behaviour and load introduction must be checked. The apparent protection of concrete does not eliminate the need to assess the steel temperature and composite interaction.

9. Illustrative worked assessment

Illustrative example—not a substitute for code design. A simply supported unprotected steel beam has a steel temperature of 550 °C after a specified fire exposure. Its room-temperature yield strength is 355 N/mm². The adopted material model gives k_y,θ = 0.60 at 550 °C. Assume, for this preliminary check only, that the relevant bending resistance is proportional to yield strength and that the room-temperature plastic resistance is 180 kN·m. Take γ_M,fi = 1.0.

The reduced yield strength is:

Under the stated simplified proportional-resistance assumption:

If the fire design bending action is 85 kN·m:

The simplified utilization is below 1.0. Nevertheless, a complete assessment must check lateral-torsional buckling, temperature gradients, shear, web slenderness, restraint, connections and the validity of the adopted resistance model. The result demonstrates the procedure, not a general rule that every beam at 550 °C is adequate.

10. Applied case: composite office floor

Background

This illustrative case concerns a four-storey office building with composite steel beams, profiled decking and reinforced concrete slabs. The design requires 60 minutes of fire resistance. Primary columns are protected; secondary beams are proposed without applied protection to reduce cost and preserve the architectural appearance.

Problem

The unprotected secondary beams have high section factors and would reach temperatures that reduce their flexural and buckling resistance. A conventional member-by-member check indicates failure before 60 minutes. However, the floor slab has continuous reinforcement and beams are connected to a stable protected frame.

Analysis and approach

The design team carries out a compartment fire assessment, thermal analysis of the exposed beam surfaces and a structural analysis including beam–slab interaction. The following issues are examined:

• temperature of the lower flange, web and protected upper region;
• reduced beam bending and shear resistance;
• shear-connector capacity;
• slab reinforcement continuity and anchorage;
• forces delivered to edge beams and connections;
• column stability and robustness of the protected frame;
• effects of openings and service penetrations.

The analysis indicates that slab shielding delays heating of the upper flange. At large deflection, the slab develops tensile membrane action, while the surrounding protected frame provides adequate restraint and anchorage. Detailing is modified by adding reinforcement continuity at slab edges, improving connection ductility and protecting selected perimeter beams where membrane forces are greatest.

Outcome

The revised system satisfies the specified performance criterion in the adopted assessment, subject to construction quality, inspection and maintenance. The solution uses limited protection rather than applying protection to every beam. Its safety depends on system-level behaviour, not merely on the calculated temperature of an isolated member.

Learning points

1. A failed isolated-member check may identify the need for system analysis, not immediate rejection of the structure.
2. Membrane action requires reliable reinforcement, anchorage and compatible connections.
3. Unprotected members still require explicit thermal and stability checks.
4. Fire engineering conclusions are valid only for the stated geometry, loading, fire scenario and detailing.

11. Design procedure and professional practice

A practical workflow is:

1. Define occupancy, compartmentation, fire load, ventilation and required resistance period.
2. Identify the governing code and whether a nominal or natural-fire design is required.
3. Classify members as protected, unprotected, partially protected or shielded.
4. Determine exposed perimeters and section factors.
5. Calculate temperatures using validated simplified or advanced methods.
6. Apply temperature-dependent material properties.
7. Check tension, compression, bending, shear, combined actions and stability.
8. Check connections, restraints, load paths and possible redistribution.
9. For composite floors, assess slab integrity, connectors, reinforcement and membrane action.
10. Specify protection, installation tolerances, inspection and repair procedures.
11. Document assumptions, fire scenarios, uncertainties and acceptance criteria.
12. Coordinate the design with the architect, fire engineer, contractor, inspector and building-control authority.

Computer models can assist thermal and structural analysis, but their output is only as reliable as the geometry, boundary conditions, material data and failure criteria. Artificial intelligence may support detection of missing data, drawing review or sensitivity studies, but it must not replace code compliance, engineering judgement, traceable calculations or competent approval.

12. Limitations, regulation and safety governance

Simplified methods are valuable for routine cases but may be unreliable when there are severe temperature gradients, unusual restraints, large openings, transfer structures, long-span slabs, composite columns, progressive collapse concerns or nonstandard protection products. Test evidence may be limited to a particular configuration and cannot automatically be generalized.

The responsible engineer should maintain a design basis identifying assumptions and interfaces. Protection installers should provide certificates, batch records and inspection results. Damaged protection should be repaired using approved materials and methods. Unauthorized penetrations for services can create local heating and must be controlled.

Fire design also has an ethical dimension. An apparently economical unprotected solution is unacceptable if it depends on undocumented assumptions or ignores evacuation, firefighting access and structural robustness. The objective is to preserve life safety, support firefighting and prevent disproportionate collapse for the required performance period.

Contemporary developments and best practices

Current practice is moving toward performance-based fire engineering, natural-fire simulation, probabilistic treatment of uncertain fire loads, digital inspection records and integrated building information models. Research continues on ultra-high-strength steel, high-performance concrete, intumescent coatings, engineered timber–steel systems, advanced composite floors and robustness under travelling fires.

Best practice is to:

• use tested or code-recognized material data;
• avoid assigning a single universal critical temperature;
• design connections and interfaces explicitly;
• treat protection as a quality-controlled construction system;
• verify membrane-action assumptions through compatibility and anchorage checks;
• perform sensitivity checks for fire duration, load ratio and restraint;
• retain clear calculation records for inspection and future alteration.

Chapter Summary

Steel loses strength and stiffness as temperature rises, while thermal expansion, creep, bowing and buckling alter the structural response. Fire design links the fire scenario, thermal analysis, reduced material properties, member stability and system behaviour. Section factor strongly influences the heating rate of unprotected steel, whereas protection thickness and installation quality control the temperature of protected members.

Critical temperature is load- and system-dependent, not a universal material constant. Connections must be checked for reduced resistance, deformation compatibility and forces caused by redistribution. Composite beams and slabs benefit from concrete shielding, reinforcement and alternative load paths. Tensile membrane action can provide important reserve capacity, but only when reinforcement, anchorage, edge restraint and connections are adequate. A reliable design combines appropriate analysis with code compliance, tested protection systems, competent construction inspection and documented engineering judgement.

Key Terms

Accidental design situation: A design condition involving an exceptional event such as fire.

Buckling: Loss of stability of a member under compression or combined action.

Catenary action: Tensile load-carrying behaviour developed by a beam or slab after large deflection.

Composite beam: A beam in which steel and concrete act together through shear connection.

Critical temperature: Temperature at which the available resistance equals the relevant fire design demand.

Emissivity: A surface property governing thermal radiation exchange.

Fire resistance: Ability of a structural element or system to maintain specified performance during fire.

Fire load: Potential heat released by combustible contents and construction materials.

Intumescent coating: Coating that expands and forms an insulating char when heated.

Membrane action: In-plane tensile or compressive action developed in a slab or plate.

Nominal fire: Standardized fire exposure used for design or testing.

Protection system: Material or construction applied to delay heating of a structural member.

Section factor: Heated perimeter divided by steel volume, commonly A_m/V.

Shear connector: Device transferring longitudinal force between steel and concrete.

Spalling: Detachment or explosive loss of concrete from a heated surface.

Thermal restraint: Restriction of free expansion or contraction caused by surrounding structure.

Transient strain: Temperature- and load-dependent strain that develops during heating.

Unprotected steel: Steel directly exposed to the fire without applied insulation.

Review Questions

Short Answer Questions

1. Why does a large section factor generally increase the heating rate of an unprotected steel member?
2. Define critical temperature and explain why it is not universal for a steel grade.
3. State two effects of elevated temperature on structural steel.
4. What is the purpose of shear connectors in a composite beam?
5. Identify two quality-control requirements for intumescent coatings.

Long Answer Questions

1. Explain the stages of a structural fire assessment from fire scenario definition to system verification.
2. Compare unprotected steel, intumescent coatings, board encasement and concrete encasement.
3. Discuss the effects of elevated temperature on buckling resistance of a steel compression member.
4. Explain how composite slabs can develop membrane action and identify the conditions required for it.
5. Describe the principal fire-design issues associated with steel connections.

Analytical / Application Questions

1. A beam has a high section factor and is exposed on three sides. Explain how you would decide whether to protect it, including thermal, mechanical, connection and construction considerations.
2. An isolated composite-beam check fails at 45 minutes, but a continuous composite floor has unbroken reinforcement and protected edge members. Develop a reasoned assessment plan to determine whether system behaviour can provide adequate fire performance.
3. A protected column is penetrated by an unsealed service opening. Analyse the likely effects on thermal response, structural resistance and compliance, and propose corrective actions.

Multiple Choice Questions with Answers

Question 1

Which parameter generally causes an unprotected steel member to heat more rapidly?

A. Lower section factor A_m/V

B. Higher section factor A_m/V

C. Lower gas temperature

D. Greater concrete cover

Correct Answer: B. Higher section factor A_m/V

Question 2

In the expression f_y,θ = k_y,θ f_y, what does k_y,θ represent?

A. Steel density at room temperature

B. Thermal conductivity of protection

C. Reduction factor for yield strength at temperature θ

D. Member buckling length

Correct Answer: C. Reduction factor for yield strength at temperature θ

Question 3

Why must temperatures in the radiation term be expressed in kelvin?

A. Because radiation exchange uses absolute temperature to the fourth power

B. Because steel strength is measured in kelvin

C. Because convection coefficients have kelvin units

D. Because section factors are temperature-dependent

Correct Answer: A. Because radiation exchange uses absolute temperature to the fourth power

Question 4

Which statement about critical temperature is correct?

A. It is the same for every steel grade and load case

B. It depends on demand, stability and structural conditions

C. It is always 600 °C

D. It applies only to concrete members

Correct Answer: B. It depends on demand, stability and structural conditions

Question 5

For an ideal pin-ended column, the Euler critical load is proportional to which quantity?

A. L_cr²/(E_θ I)

B. E_θ I/L_cr²

C. E_θ L_cr²/I

D. I/(E_θ L_cr)

Correct Answer: B. E_θ I/L_cr²

Question 6

What is a major fire benefit of a concrete slab over a steel beam?

A. It always prevents all steel deformation

B. It provides thermal shielding and heat capacity

C. It eliminates the need for shear connectors

D. It increases steel yield strength with temperature

Correct Answer: B. It provides thermal shielding and heat capacity

Question 7

Membrane action in a composite slab primarily requires which combination?

A. Discontinuous reinforcement and brittle supports

B. Reinforcement continuity, anchorage and compatible restraint

C. No edge beams and no connections

D. Zero slab deformation

Correct Answer: B. Reinforcement continuity, anchorage and compatible restraint

Question 8

Which is a common limitation of an unprotected steel member?

A. It cannot conduct heat

B. It usually heats rapidly in direct fire exposure

C. Its elastic modulus increases with temperature

D. It is unaffected by restraint

Correct Answer: B. It usually heats rapidly in direct fire exposure

Question 9

A room-temperature yield strength is 355 N/mm² and k_y,θ is 0.60. What is the simplified yield strength at temperature θ?

A. 142 N/mm²

B. 213 N/mm²

C. 355.6 N/mm²

D. 591.7 N/mm²

Correct Answer: B. 213 N/mm²

Question 10

Why must connections be assessed separately in fire design?

A. Connections never carry load during fire

B. Connection components may heat differently and receive redistribution forces

C. Connections always remain at room temperature

D. Only concrete connections can fail

Correct Answer: B. Connection components may heat differently and receive redistribution forces