Fire Design of Concrete and Masonry Structures

Introduction

Concrete and masonry are generally regarded as materials with good fire resistance because they are non-combustible and have relatively low thermal conductivity. Nevertheless, fire can cause serious loss of strength, stiffness, bond and durability. Reinforced concrete members may suffer explosive or moisture-induced spalling, while masonry walls may lose stability, insulation and integrity. Prestressed members are particularly sensitive because elevated temperature rapidly reduces prestressing steel strength and may destroy prestress.

Fire design therefore has two objectives: first, to prevent collapse during the required fire-resistance period; and second, to maintain compartmentation by limiting the passage of flames, hot gases and excessive heat. Design must consider the fire scenario, member function, load level, restraint, detailing, construction quality and the governing building regulations or structural fire standard. Common design systems include the tabulated-data method, simplified calculation methods and advanced thermal–structural analysis.

Learning Objectives

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

• explain the behaviour of concrete, reinforcement, prestressing steel and masonry during fire;
• distinguish fire resistance requirements for load-bearing, separating and compartment elements;
• determine nominal cover and assess members using tabulated data;
• apply simplified temperature and capacity checks within their limitations;
• identify risks of spalling, restraint-induced damage and inadequate detailing; and
• recommend appropriate inspection, protection and repair measures.

1. Fire Behaviour of Concrete and Masonry

1.1 Concrete

Concrete is a composite of cement paste, aggregate, water and, commonly, reinforcement. Its fire performance depends on moisture content, permeability, aggregate type, member size, stress level, heating rate and restraint.

At approximately 100 °C, free and physically bound water begins to evaporate. Between about 100 and 200 °C, pore pressure can increase substantially. Around 300–500 °C, chemically bound water is released and cement paste progressively loses cohesion. Calcium hydroxide decomposes at approximately 400–550 °C. Above about 500–600 °C, substantial loss of mechanical properties occurs. Exact temperatures vary with mixture, heating history and test method; they must not be treated as universal failure thresholds.

Aggregate type is important. Siliceous aggregates may undergo expansion and, in some cases, abrupt behaviour near the quartz transformation temperature of approximately 573 °C. Carbonate aggregates generally have different thermal expansion and decomposition characteristics. Lightweight aggregates often provide favourable thermal performance because of their lower conductivity.

1.2 Reinforcing steel

Reinforcing steel retains much of its ambient-temperature strength at moderate temperatures, but its yield strength and elastic modulus decline progressively as temperature rises. The reduction is affected by steel grade, heating and cooling history, and the relevant design standard. Steel temperature is usually lower than the exposed concrete surface temperature because concrete provides thermal insulation.

1.3 Prestressing steel

Prestressing wire and strand are more vulnerable than ordinary reinforcement because their initial stress is high and their performance depends strongly on elastic modulus and relaxation. A prestressed member can lose a significant portion of its resistance before visible concrete failure. Anchorage zones, ducts and end regions require particular attention.

1.4 Masonry

Masonry consists of units, mortar, grout and sometimes reinforcement. Clay units may perform well because they are already manufactured at high temperature, although cracking, moisture movement and loss of mortar bond can occur. Concrete masonry may experience dehydration, cracking and spalling. The fire performance of a wall depends on unit type, thickness, density, workmanship, joints, openings, load level and whether the wall is load-bearing or separating.

A masonry wall must satisfy:

• R—load-bearing capacity: continued resistance to applied load;
• E—integrity: prevention of flames and hot gases passing through; and
• I—insulation: limitation of temperature rise on the unexposed face.

These criteria are commonly expressed together, for example, as REI 120, meaning resistance, integrity and insulation for 120 minutes, subject to the applicable classification system.

2. Fire Design Basis

2.1 Fire-resistance period

The required period—such as 30, 60, 90 or 120 minutes—is selected from the building code according to occupancy, height, compartment size, escape strategy, fire-fighting provisions and structural importance. It is not a prediction of the actual duration of every fire. The standard fire exposure is a reproducible classification test or design curve and may differ considerably from a natural fire.

The commonly used standard fire temperature relationship is:

where T_g is gas temperature in °C and t is time in minutes. This relationship is an idealized nominal fire exposure. It does not directly provide the temperature of concrete, reinforcement or masonry.

2.2 Fire design actions

Fire combinations normally use reduced variable actions because the probability that all imposed loads are simultaneously at their characteristic maximum is low. A generic accidental fire combination may be written as:

where G_k is the characteristic permanent action, Q_k,1 is the leading variable action, Q_k,i are accompanying variable actions, and ψ₂,fi and ψ₂,i are code-defined combination factors. The exact combination and safety factors must be taken from the governing national standard.

Fire design is an accidental design situation, but it is not an excuse to ignore stability, load paths or robust detailing. Reduced actions must not be selected informally.

3. Heat Transfer and Section Behaviour

Heat enters an exposed member mainly by convection from hot gases and radiation from the fire environment. It then travels through the section by conduction. The one-dimensional heat-conduction relationship is:

where ρ_c is concrete density in kg/m³, c_c is specific heat in J/(kg·K), λ_c is thermal conductivity in W/(m·K), T is temperature in °C or K, t is time, and x is depth. In practical design, these properties vary with temperature and moisture; therefore, hand calculations are approximate unless code-specified properties are used.

Concrete cover creates a thermal delay. The relevant dimension is the distance from the exposed surface to the centre of the reinforcement, often called axis distance:

where a is axis distance, c_nom is nominal concrete cover to the reinforcement surface, and φ is bar diameter. For bundled bars, tendons or unusual arrangements, the standard may specify a different geometric definition.

Greater axis distance generally lowers steel temperature, but it does not automatically solve spalling, anchorage, congestion or detailing problems.

4. Main Fire-Design Methods

4.1 Tabulated-data method

Tabulated design gives minimum dimensions, axis distances, wall thicknesses and reinforcement conditions for specified fire ratings. It is efficient and appropriate for regular members within the range of the table.

A designer must verify:

1. the correct fire exposure—three-sided, four-sided, internal or external;
2. member type—beam, slab, column, wall or masonry element;
3. load-bearing and/or separating function;
4. required fire period;
5. concrete strength, aggregate and reinforcement type;
6. minimum dimensions and axis distance; and
7. reinforcement arrangement, spacing and detailing rules.

Tabulated values are not universal. They depend on the standard, test evidence and assumptions behind the table. A table for a simply supported beam should not be used uncritically for a restrained continuous frame.

4.2 Simplified calculation method

A simplified method normally involves three stages:

• calculate or obtain the temperature distribution after the required exposure;
• reduce material strengths and stiffnesses according to temperature; and
• check the residual cross-section and reduced structural resistance against fire design actions.

For flexure, a simplified resistance concept is:

where M_Rd,fi is approximate fire resistance, A_s,i is the area of reinforcing bar group i, f_y,θ,i is its temperature-dependent design yield strength, and z_i is its lever arm. This expression is only a simplified tensile-force model. It assumes a valid compression-block model, adequate bond, appropriate strain compatibility and a recognised temperature-dependent design procedure. It must not replace the complete code method where those assumptions are not satisfied.

For a rectangular beam under a simplified singly reinforced model, the ambient-style equilibrium may be represented by:

where A_s is tensile reinforcement area, f_yd is design steel strength, f_cd is design concrete strength, b is width and x is neutral-axis depth. At fire temperature, material strengths and the permitted compression zone must be modified using the relevant code. Applying ambient strengths directly to a fire situation is unsafe.

4.3 Advanced analysis

Advanced finite-element or thermo-mechanical analysis can model temperature-dependent properties, cracking, spalling, restraint, second-order effects and changing load paths. It requires reliable material data, mesh control, boundary conditions and validation. A sophisticated model with poorly justified inputs may be less reliable than a conservative tabulated design.

5. Reinforced Concrete Members

5.1 Beams and slabs

Beams and slabs are commonly designed using axis distance, member width or thickness and reinforcement arrangement. In a simply supported beam, sagging reinforcement is exposed mainly from below and the sides. In a continuous beam, hogging reinforcement over supports may be critical because the top face can be heated in some situations and restraint changes the moment pattern.

A slab exposed from below develops a temperature gradient: the lower concrete is hot while the upper region remains cooler. This can produce thermal curvature and membrane forces. Reinforcement continuity may enable tensile membrane action, which can improve overall survival, but it must not be assumed without checking anchorage, edge restraint, connection capacity and robustness.

5.2 Columns

Columns are often exposed on four sides and are highly sensitive to axial load, slenderness, eccentricity, spalling and second-order effects. Fire resistance improves with larger dimensions, greater axis distance, lower load ratio and effective confinement, but confinement reinforcement must itself remain adequately anchored.

An approximate fire load ratio can be expressed as:

where N_Ed,fi is the design axial force in fire, and N_Rd,20 is the ambient design axial resistance. This ratio is a screening parameter, not a complete fire calculation. It does not account by itself for slenderness, thermal bowing, eccentricity, restraint or degradation of concrete and steel.

5.3 Detailing

Good detailing includes adequate cover, closed links, appropriate bar spacing, sound compaction, continuity and anchorage. Sudden changes in section, congested reinforcement, poorly compacted zones and large embedded ducts can create local weaknesses.

6. Spalling

Spalling is the loss of concrete from the heated surface. It may be:

• surface spalling, involving thin flakes or local delamination;
• explosive spalling, involving rapid ejection caused by pore pressure and thermal stress; or
• corner spalling, which exposes reinforcement on edges and corners.

Risk increases with high-strength, low-permeability concrete; high moisture; rapid heating; high stress; dense reinforcement; sharp corners; and restraint. High-strength concrete is not automatically fire-safe. Polypropylene fibres can reduce pore pressure by melting and creating pathways for vapour, but dosage, distribution, mixing and code acceptance must be verified.

Practical controls include rounded or chamfered corners, suitable concrete permeability, controlled moisture, polypropylene fibres where specified, adequate cover, fire protection systems and avoiding premature heating of wet members. Fire protection must be compatible with the substrate and must remain bonded during heating.

After a fire, loose or delaminated concrete must be removed safely, reinforcement temperature history estimated, steel loss assessed and residual capacity checked. A superficial visual inspection is insufficient.

7. Restraint and Structural Continuity

Heating causes expansion. If expansion is restrained, compression develops during heating; subsequent cooling can produce tensile forces, cracking and connection damage. Restraint may arise from adjacent beams, columns, slabs, walls, supports, facades and non-structural partitions.

A restrained member may therefore experience actions not present in the original gravity-load model. Thermal bowing, differential heating and stiffness loss can redistribute load to cooler members. Connections must tolerate rotation and force transfer without premature brittle failure.

Design responses include:

• identifying realistic thermal expansion paths;
• checking beams, slabs, columns and connections as a system;
• providing continuity and ductility where beneficial;
• preventing unintended restraint by vulnerable finishes or partitions; and
• considering cooling-phase damage, not only the heating phase.

8. Masonry Fire Design

Masonry walls are assessed using thickness, unit type, density, mortar, load level, slenderness, restraint and openings. A separating wall requires integrity and insulation; a load-bearing wall additionally requires resistance. Fire-stopping must continue at slab edges, service penetrations, joints and interfaces.

Openings are frequent failure locations. Doors, ducts, cable trays and pipes must have tested fire-stopping systems. A wall that is adequate in an uninterrupted furnace test may fail in practice if joints are discontinuous or penetrations are poorly sealed.

Masonry design should verify:

• wall thickness and construction category;
• load eccentricity and slenderness during fire;
• support and edge restraint;
• crack control and movement joints;
• stability of parapets and free edges; and
• compatibility of finishes, insulation and fire-stopping.

Do not substitute a generic wall thickness for a tested or code-recognised assembly.

9. Worked Example: Preliminary Beam Assessment

Illustrative example. A 300 mm wide reinforced-concrete beam has 20 mm diameter bottom bars. The nominal cover to the bar surface is 40 mm. Determine the approximate axis distance to the bar centre and explain its significance. This is not a complete fire-resistance verification.

Given:

• c_nom = 40 mm;
• φ = 20 mm.

Using:

The approximate axis distance is therefore 50 mm. This distance is used when comparing the beam with applicable fire tables or when estimating reinforcement temperature. The result does not establish a fire rating because the designer must still check exposure sides, beam width, fire period, reinforcement arrangement, concrete type, spalling risk, shear, anchorage, restraint and the governing standard.

10. Applied Case: Illustrative Parking-Structure Fire

Background

A three-storey reinforced-concrete car park uses flat slabs supported by columns. A 90-minute fire resistance period is required. The slabs contain high-strength concrete and several columns have congested reinforcement.

Problem

A preliminary review finds that nominal cover is adequate according to an ordinary ambient durability schedule, but no fire-specific check has been made. Several service penetrations pass through masonry compartment walls, and the building has rigid façade connections.

Analysis

The ambient cover schedule cannot be assumed to provide the required axis distance. High-strength concrete and moisture retention increase spalling risk. Flat slabs require attention to punching shear and column-slab integrity after heating. Rigid façade connections may restrain thermal movement. Penetrations may compromise wall integrity and insulation.

Approach and solution

The design team should:

1. identify the required fire exposure and design actions;
2. check slab thickness, reinforcement axis distance and punching resistance using the applicable fire method;
3. assess columns for four-sided exposure, load ratio, slenderness and spalling;
4. specify concrete and fibre measures supported by the governing standard;
5. detail movement-tolerant façade interfaces;
6. install tested fire-stopping around every penetration; and
7. prepare an inspection and post-fire assessment plan.

Outcome

The review may show that most members satisfy the requirement by tabulated data, while selected columns require additional protection or a refined calculation. The compartment walls require tested penetration seals rather than merely thicker masonry.

Learning points

Fire safety is a system property. Adequate cover alone cannot compensate for high load ratio, explosive spalling, weak connections or unsealed penetrations.

11. Limitations, Regulation and Professional Practice

Fire design must follow the adopted national building regulations and structural standard. Relevant documents may include a national concrete design code, a masonry design code, a structural fire design standard, tested fire-stopping standards and guidance from the authority having jurisdiction. Edition, national annex and product certification must be confirmed.

Engineers should document assumptions, exposure faces, fire combination, material properties, cover, axis distance, calculation method, protection products and inspection requirements. Site verification is essential: misplaced reinforcement, inadequate cover, poor compaction or unapproved substitutions can invalidate the design.

Computer software and AI tools can assist with temperature calculations, drawing review, document checking and sensitivity studies. They cannot replace engineering judgement, code interpretation, validation or responsibility for life safety. AI-generated calculations must be independently checked, particularly for units, fire curves, boundary conditions and material reduction factors.

Best-Practice Checklist

• Define the required R, E and I performance before selecting a method.
• Use the correct accidental fire combination and code-defined reduction factors.
• Check axis distance, not cover alone.
• Consider all exposed faces and realistic restraint.
• Assess spalling risk, especially for high-strength or wet concrete.
• Treat prestressed members and anchorage zones as sensitive elements.
• Verify masonry joints, interfaces and service penetrations.
• Use tabulated data only within its stated scope.
• Record construction tolerances and inspect completed work.
• Plan post-fire assessment, repair and reoccupation decisions.

Chapter Summary

Concrete and masonry provide valuable inherent fire resistance, but their performance depends on temperature, moisture, material properties, load level, geometry, restraint and construction quality. Fire design requires a defined resistance period and appropriate fire actions. Tabulated methods are efficient for standard cases; simplified methods combine temperature estimation with reduced material properties; advanced analysis is justified for unusual or highly consequential structures.

Concrete cover and axis distance delay reinforcement heating, but they do not alone prevent failure. Spalling can rapidly expose reinforcement, while restraint can create additional forces and damage during heating and cooling. Prestressed concrete requires special caution because prestress loss and anchorage vulnerability may control. Masonry must be assessed for resistance, integrity and insulation, with particular attention to joints and penetrations. Sound detailing, tested protection, site inspection and professional code compliance are central to reliable fire safety.

Key Terms

Axis distance: Distance from the exposed concrete surface to the centre of reinforcement.

Concrete spalling: Loss or ejection of concrete from a heated surface.

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

Cover: Concrete thickness between the surface and reinforcement.

Fire combination: Load combination used for the accidental fire design situation.

Fire resistance: Ability to perform specified structural or separating functions during fire.

Integrity: Ability to prevent flames and hot gases from passing through an element.

Insulation: Ability to limit temperature rise on the unexposed face.

Load ratio: Relationship between fire-design action and a reference structural resistance.

Masonry unit: Brick, block or stone forming part of a masonry assembly.

Nominal fire: Idealized standard fire exposure used for classification or design.

Prestressed concrete: Concrete containing tendons intentionally stressed before or after casting.

Restraint: Limitation of thermal movement by adjacent structure or components.

Residual capacity: Structural resistance remaining after fire exposure.

Spalling fibre: Usually polypropylene fibre used to reduce vapour-pressure risk.

Tabulated method: Fire-design method based on code tables of dimensions and reinforcement.

Thermal gradient: Variation of temperature through a member’s thickness.

Thermal bowing: Curvature caused by unequal heating through a member.

Thermo-mechanical analysis: Coupled analysis of temperature and structural response.

Review Questions

Short Answer Questions

1. Why does concrete cover improve fire resistance?
2. Define the fire performance terms R, E and I.
3. What is explosive spalling?
4. Why are prestressed members particularly sensitive to fire?
5. State two limitations of tabulated fire-design data.

Long Answer Questions

1. Explain the temperature-dependent behaviour of concrete, reinforcing steel and masonry during fire.
2. Compare tabulated, simplified and advanced fire-design methods.
3. Discuss the causes and prevention of concrete spalling.
4. Explain how restraint and thermal expansion influence reinforced-concrete frames.
5. Describe the fire-design checks required for a load-bearing masonry wall.

Analytical / Application Questions

1. A beam has 35 mm nominal cover and 16 mm bars. Calculate its approximate axis distance and explain why this value is not, by itself, a fire rating.
2. A high-strength concrete column has adequate tabulated cover but is heavily loaded and exposed on four sides. Identify the additional checks required before accepting the design.
3. A compartment wall appears sound after construction, but several cable trays pass through it without tested seals. Evaluate the likely fire-safety consequences and propose corrective action.

Multiple Choice Questions with Answers

Question 1

What does axis distance normally measure?

A. Distance from the unexposed face to the nearest bar

B. Distance from the exposed surface to the centre of reinforcement

C. Total beam width divided by two

D. Distance between two adjacent columns

Correct Answer: B. Distance from the exposed surface to the centre of reinforcement

Question 2

Which fire performance criterion concerns prevention of flames and hot gases passing through an element?

A. R

B. E

C. I

D. S

Correct Answer: B. E

Question 3

A beam has 40 mm nominal cover and a 20 mm bar. What is the approximate axis distance?

A. 30 mm

B. 40 mm

C. 50 mm

D. 60 mm

Correct Answer: C. 50 mm

Question 4

Which condition generally increases the risk of explosive spalling?

A. Low moisture and highly permeable concrete

B. High moisture and low-permeability concrete

C. Very low heating rate only

D. Absence of reinforcement

Correct Answer: B. High moisture and low-permeability concrete

Question 5

Why do prestressed members require particular attention in fire design?

A. Prestressing steel is unaffected by temperature

B. Prestress loss and reduction of tendon properties can occur rapidly

C. They cannot experience thermal expansion

D. They never require anchorage

Correct Answer: B. Prestress loss and reduction of tendon properties can occur rapidly

Question 6

What is the main purpose of a fire-resistance tabulated method?

A. To replace all site inspections

B. To provide dimensions and detailing requirements for defined cases

C. To calculate natural fire duration exactly

D. To determine architectural finishes

Correct Answer: B. To provide dimensions and detailing requirements for defined cases

Question 7

In a fire combination, variable actions are commonly reduced because:

A. Permanent loads disappear during fire

B. All variable actions are always zero

C. Simultaneous occurrence of all characteristic maxima is unlikely

D. Concrete gains strength during heating

Correct Answer: C. Simultaneous occurrence of all characteristic maxima is unlikely

Question 8

Which masonry feature is a frequent weak point for compartmentation?

A. Continuous solid construction without openings

B. Tested and properly installed fire-stopping

C. Unsealed service penetrations

D. Adequately bonded joints

Correct Answer: C. Unsealed service penetrations

Question 9

What can restraint during heating cause?

A. Only permanent cooling

B. Additional thermal forces and cracking

C. Elimination of all deformation

D. Automatic recovery of steel strength

Correct Answer: B. Additional thermal forces and cracking

Question 10

What is the correct professional response when a computer or AI tool produces a fire calculation?

A. Accept it without checking

B. Use it only if it produces a short answer

C. Independently verify assumptions, units, code basis and results

D. Ignore the governing standard

Correct Answer: C. Independently verify assumptions, units, code basis and results