Fire Design of Timber and Emerging Structural Systems

Introduction

Timber is a combustible structural material, but combustible does not mean incapable of providing fire resistance. Large timber members can retain load-bearing capacity for significant periods because fire exposure produces a relatively insulating char layer. The uncharred inner timber, often called the residual section, continues to carry load. Fire design therefore depends on predicting charring, evaluating the reduced section and strength, protecting connections, and controlling fire spread through concealed spaces and interfaces.

Modern construction increasingly uses engineered timber—cross-laminated timber (CLT), glued-laminated timber (glulam), laminated veneer lumber (LVL), nail-laminated timber (NLT), and composite timber systems. Modular construction and hybrid systems combine timber with concrete, steel, gypsum board, insulation and mechanical services. These systems offer speed, low embodied carbon and prefabrication benefits, but they also create fire-safety issues that are not fully represented by the behaviour of an isolated timber beam.

This chapter presents the principles used by fire-safety professionals and structural designers. Numerical values are indicative of commonly used Eurocode-based methods; the adopted national standard, product assessment and authority requirements must always be checked before design.

Learning Objectives

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

1. Explain the stages of timber pyrolysis, ignition and charring.
2. Distinguish the nominal charring rate, notional char depth and residual section.
3. Apply a simplified reduced-section method to a timber member.
4. Identify the influence of load, moisture, density, geometry and protection on timber fire resistance.
5. Explain why connections, joints, penetrations and interfaces frequently govern performance.
6. Recognize fire-safety risks in mass timber, modular and hybrid construction.
7. Apply a systematic approach to inspection, detailing and fire-risk management.

1. Timber Combustion and Fire Behaviour

Timber is a porous, organic material. When heated, it does not immediately burn throughout its volume. Its behaviour usually develops through the following stages:

• Heating and drying: free water evaporates and the temperature rises.
• Pyrolysis: at elevated temperature, timber decomposes into gases, tar and char.
• Ignition: combustible gases may ignite when their concentration and temperature are sufficient.
• Flaming combustion: gases burn above the timber surface.
• Charring: the exposed surface converts to a carbon-rich layer with low thermal conductivity.
• Smouldering or glowing combustion: after flaming decreases, char may continue to oxidise if oxygen and heat are available.

The char layer is not structurally equivalent to sound timber. It has low strength and must normally be removed from the effective structural section. However, it reduces heat flow into the residual section. The temperature at the boundary between char and uncharred timber is often approximated as about 300 °C for simplified design, although actual thermal gradients are continuous.

Timber fire behaviour is affected by:

• species, density and moisture content;
• member dimensions and exposed perimeter;
• grain direction and end-grain exposure;
• surface protection and gaps;
• ventilation and fire severity;
• applied load and restraint;
• adhesives, coatings, connectors and concealed cavities.

A large member generally performs better than a small member of the same material because its rate of temperature rise is lower relative to its volume. This is a geometric effect, not an indication that large timber is non-combustible.

2. Charring-Rate Design Model

2.1 Basic relationship

For a one-dimensional, uniform exposure, the idealised char depth is calculated from:

where:

• d_char,0 is the idealised char depth, in mm;
• β₀ is the one-dimensional charring rate, in mm/min;
• t is the fire exposure time, in min.

The equation assumes a specified standard-fire exposure and a sufficiently large, approximately uniform timber surface. It is not a universal prediction for natural fires or irregular local burning.

Indicative nominal values used in Eurocode-based design include approximately 0.65 mm/min for many softwood and glulam products and approximately 0.8 mm/min for certain hardwood products. The correct value depends on the timber product, density, exposure and governing standard.

The notional char depth includes a nominal zero-strength layer beyond the idealised char:

Here β_n is the notional charring rate and d₀ is commonly taken as 7 mm in simplified Eurocode applications. The zero-strength layer represents heated timber that may retain little or no design strength even though it has not visibly charred.

The notional rate may be expressed in a general form as:

The factors account, depending on the design situation, for the development of charring, the influence of adjacent surfaces and the geometry or configuration of the member. Their use must follow the governing standard; they are not arbitrary correction factors.

2.2 Protected timber

A board, gypsum lining, insulation layer or other approved protection can delay heating and charring. A simplified protected-member calculation may include a protection time t_prot:

For t ≤ t_prot, the expression does not mean that the timber is automatically undamaged. The protection system must remain attached, continuous and capable of performing its assessed function. Joints, fasteners, edges, service penetrations and board discontinuities are critical.

3. Residual-Section Method

3.1 General principle

The residual-section method removes the charred and zero-strength material from the original cross-section and checks the remaining timber at the required fire time. If a rectangular beam has original width b and depth h, with three-sided exposure and equal charring from the exposed sides, the residual dimensions may be approximated as:

where b_fi and h_fi are the effective fire-design dimensions. The top face is not reduced in this example because it is assumed to be protected by a floor or otherwise unexposed. For four-sided exposure, the depth is also reduced from both faces.

The residual area is:

and the residual second moment of area for bending about the major horizontal axis is:

The elastic section modulus is:

These equations apply to a rectangular residual section and assume that the residual timber can be represented by a uniform effective material. They do not automatically account for local defects, delamination, temperature-dependent material properties or connection failure.

3.2 Fire resistance checks

A simplified bending check compares the fire design moment with the reduced fire resistance:

A common form for the resistance is:

where:

• M_Ed,fi is the design bending moment during fire;
• k_mod,fi is a fire modification factor specified by the design method;
• f_m,fi is the relevant fire design bending strength or adjusted strength basis;
• W_fi is the residual section modulus;
• γ_M,fi is the fire design partial factor.

The exact strength model varies by standard. Some methods use temperature-dependent strength, while simplified timber procedures use fire modification factors and reduced sections. The designer must not combine factors from incompatible standards.

For axial compression, a simplified check may be written as:

where k_c,fi represents fire effects on compression stability, f_c,fi is the applicable compression strength basis and A_fi is the residual area. Buckling, eccentricity, restraint and member slenderness remain important. A residual area check alone is inadequate for a slender column.

3.3 Worked example: glulam beam

Illustrative example. A glulam beam is 160 mm wide and 360 mm deep. It is exposed on three sides for 30 minutes. Use β_n = 0.8 mm/min and d₀ = 7 mm for this illustrative calculation. Ignore protection and assume uniform charring.

The notional char depth is:

For three-sided exposure:

Thus, the residual area is:

The residual section modulus is:

This calculation does not prove a 30-minute fire rating. A complete design must establish the fire load effect, material properties, lateral stability, shear resistance, connection resistance, protection continuity and applicable national requirements.

4. Connections and Joints

Connections often govern the fire performance of timber structures. A beam may retain a substantial residual section while a steel bracket, dowel group, bolt assembly or concealed hanger loses capacity rapidly.

4.1 Heat transfer and reduced fastener resistance

Steel conducts heat efficiently. Exposed steel plates and fasteners can heat faster than timber and may reduce the capacity of the surrounding timber through splitting, charring and loss of embedment strength. Connection failure modes include:

• withdrawal of screws or nails;
• loss of dowel bearing capacity;
• splitting along the grain;
• tear-out at timber edges;
• local charring around a metal plate;
• weakening of adhesive or concealed connection components;
• premature loss of a hanger or seat.

Connections should therefore be designed using tested or codified fire details. Protection must cover the entire heat path, including plate edges, fastener heads, gaps and interfaces. A small exposed metal area can become a local ignition or heat-transfer point.

4.2 Detailing principles

Good practice includes:

1. Place connections within the timber where feasible, rather than exposing them on the surface.
2. Use sacrificial timber cover or tested fire-protection boards.
3. Maintain sufficient edge and end distances after fire-induced section loss.
4. Avoid unsealed gaps that allow flames or hot gases to reach concealed steel.
5. Provide compatible protection around bolts, screws and proprietary connectors.
6. Verify the fire resistance of the connection, not merely that of the connected members.
7. Prevent water ingress and corrosion, which can reduce the original connection capacity before a fire occurs.

5. Engineered Timber and Mass-Timber Systems

5.1 Cross-laminated timber

CLT consists of layers of boards arranged in alternating directions and bonded into a panel. Its fire behaviour depends on layer thickness, adhesive performance, exposed face, panel restraint and whether layers remain attached after charring.

A major concern is delamination, in which adhesive or interlayer behaviour allows a charred lamella to detach. The newly exposed timber can then burn rapidly, causing increased heat release and renewed charring. This is sometimes described as a falling-char or second-charring phenomenon. The designer must use a product-specific assessment or a standard method that addresses adhesive performance; a simple calculation based only on the original panel thickness may be unsafe.

Open CLT surfaces may contribute fuel to a compartment fire. Encapsulation with gypsum board can reduce exposed fuel, but the enclosure must remain intact during the required fire period. Board joints, penetrations and impact damage must be controlled.

5.2 Glulam and LVL

Glulam beams and columns are manufactured from bonded laminations. Their predictable geometry and large dimensions can support a charring-based design. However, adhesive performance, finger joints, lamination orientation and local stress concentrations require product-specific information. LVL has thin veneer layers and may show different failure and charring behaviour from solid-sawn timber; values must not be transferred without justification.

5.3 Timber-concrete and timber-steel hybrids

Hybrid construction can improve stiffness, vibration control, acoustic performance and fire resistance. Concrete slabs may shield timber floor panels, while steel can provide long-span capacity. At the same time, differential thermal expansion, interface slip, hidden voids and connection heating become important.

A concrete topping does not automatically protect every timber surface. Fire can enter through edge details, service zones and joints. Steel beams protected by concrete or boards need continuity at supports and connections. The whole assembly should be assessed, rather than assigning the fire rating of one component to the complete system.

6. Modular Construction and Fire Safety

Modular buildings are assembled from three-dimensional units manufactured partly or fully in a factory. Their advantages include quality control and rapid erection. Fire-safety challenges arise at the boundaries between modules:

• compartment walls and floors may be interrupted at lifting or joining points;
• seals may be damaged during transport or installation;
• services may pass through multiple fire compartments;
• cavities can create hidden horizontal or vertical fire paths;
• combustible finishes and insulation may be continuous between units;
• tolerances can create gaps larger than those used in laboratory testing.

A robust modular strategy includes a fire-stopping schedule, documented interface details, inspection hold points and photographic records before concealment. The installed condition must match the tested or assessed construction. Substitution of sealants, boards or insulation without technical approval can invalidate the fire strategy.

7. Fire Spread, Encapsulation and Compartmentation

Structural fire design is only one part of timber fire safety. A building must also limit:

• fire growth within the compartment;
• flame spread across exposed timber surfaces;
• fire spread through cavities and service risers;
• external fire spread through façades and balconies;
• fire spread between modules or adjoining compartments;
• falling burning materials;
• smoke movement and tenability loss.

A fire strategy should coordinate structural resistance, active systems and passive protection. Sprinklers can reduce fire severity and improve life safety, but they do not justify ignoring structural or compartmentation requirements unless the applicable regulations explicitly permit such an approach. Detection and suppression systems require water supply reliability, inspection, maintenance and suitable coverage.

8. Applied Case: Illustrative Mass-Timber Educational Building

Background

An illustrative three-storey educational building uses CLT floor panels, glulam beams and columns, gypsum-lined service cores and a sprinkler system. Several architectural areas propose exposed timber ceilings.

Problem

The design team initially assumes that the CLT panel rating can be calculated by subtracting a uniform char depth. Fire engineers identify three additional risks: delamination of the exposed panel, combustible contents increasing compartment fire severity, and unprotected steel knife plates at beam-column connections.

Analysis

The fire strategy separates the building into compartments and identifies all concealed cavities. The structural engineer checks the CLT using an approved product-specific method, including adhesive performance and layer fall-off. Glulam members are checked using a residual-section method. Connections are assessed separately, with protected steel plates and verified fastener details. The architect revises the exposed timber area to remain within the permitted fire-engineering design assumptions.

Approach and solution

• Encapsulate selected CLT surfaces with tested gypsum systems.
• Retain exposed timber only where flame-spread and fire-load assumptions are satisfied.
• Protect beam-column connections with continuous, impact-resistant covers.
• Install tested fire stops at panel joints, façade edges and service penetrations.
• Coordinate sprinkler heads with beams, soffits and concealed cavities.
• Inspect all seals and protection before closing walls and ceilings.
• Prepare a maintenance plan for sprinklers, fire doors, smoke control and damaged protection.

Outcome

The revised design treats the building as an interacting system rather than as separate timber members. The result is a defensible fire strategy with traceable assumptions, inspected interfaces and a clear distinction between tested performance and engineering judgement.

Learning points

1. A nominal charring rate is a design tool, not a complete fire-safety assessment.
2. Connections and interfaces can fail before the main timber member.
3. Encapsulation must be continuous and durable.
4. Engineered timber products require product-specific fire information.
5. Construction inspection is part of fire engineering, not merely a quality-control activity.

9. Design Workflow and Professional Practice

A practical workflow is:

1. Define the required fire-resistance period and performance criteria: load-bearing, integrity and insulation.
2. Identify the fire exposure, compartment geometry, ventilation and combustible contents.
3. Classify each timber product and obtain verified material and adhesive data.
4. Determine exposed surfaces, protection systems and likely charring directions.
5. Calculate the notional char depth or use an approved advanced model.
6. Determine residual dimensions and check bending, shear, compression, tension, stability and vibration-related restraints where relevant.
7. Design and check connections, joints, supports, penetrations and interfaces.
8. Evaluate fire spread, smoke, cavities, façade details and modular boundaries.
9. Coordinate sprinklers, detection, evacuation, firefighting access and structural stability.
10. Produce inspection, commissioning and maintenance requirements.

Engineering judgement is required when the actual building differs from a standard test. Any deviation should be recorded, technically justified and reviewed by the responsible fire and structural professionals.

10. Limitations, Regulation and Emerging Technology

Simplified charring methods are most reliable for standard-fire design of regular members with known products and stable exposure. They are less reliable for:

• travelling or natural fires;
• severe ventilation changes;
• thin or small members;
• complicated three-dimensional joints;
• delaminating panels;
• unusual adhesives or coatings;
• members exposed on irregular surfaces;
• fire after mechanical damage or construction alteration.

Advanced finite-element thermal analysis, furnace testing and large-scale demonstration tests may be needed for unusual systems. Computational models require validated thermal properties, boundary conditions and failure criteria. Artificial intelligence may assist with inspection-image classification, digital records and clash detection, but it cannot replace validated fire tests, professional responsibility or regulatory approval. AI-generated design recommendations must be traceable to approved data and independently checked.

Regulatory compliance includes the building code, structural timber standard, fire-test classification, product certification, passive-fire protection requirements and authority approval. Ethical practice requires transparent assumptions, competent review, protection of occupants and firefighters, and refusal to conceal uncertainties or unverified substitutions.

Chapter Summary

Timber can achieve significant fire resistance because charring creates an insulating layer and leaves a residual section capable of carrying load. The principal simplified design relationships are the charring-depth equation, the zero-strength-layer adjustment and residual-section calculations. Their use requires correct exposure assumptions, product data and compatible fire-design factors.

Engineered timber introduces additional issues, particularly adhesive performance, CLT delamination, exposed fuel load and layer fall-off. Connections, joints, penetrations, cavities and modular interfaces frequently govern fire performance. Encapsulation, compartmentation, fire stopping, sprinklers, inspection and maintenance must be coordinated with structural calculations. Hybrid and modular buildings should be assessed as complete assemblies, not as isolated components. Where simplified rules do not represent the system, tested or validated advanced methods are necessary.

Key Terms

Charring: Thermal decomposition of timber that forms a carbon-rich residue.

Charring rate: Rate at which the char boundary advances through timber, usually expressed in mm/min.

Zero-strength layer: Heated timber outside the visible char assumed to have negligible structural strength.

Residual section: Effective uncharred structural section remaining after fire exposure.

Nominal charring depth: Char depth including the notional zero-strength layer.

Pyrolysis: Thermal decomposition of timber into gases, vapours, tar and char.

Glulam: Structural timber made by bonding layers of timber laminations.

CLT: Cross-laminated timber panel made from bonded layers arranged in alternating directions.

LVL: Laminated veneer lumber manufactured from thin bonded wood veneers.

Delamination: Separation of layers in an engineered timber product.

Encapsulation: Protecting timber with a continuous fire-resistant lining.

Compartmentation: Dividing a building to restrict fire and smoke spread.

Fire stopping: Tested construction used to close gaps and prevent fire or smoke passage.

Connection fire resistance: Capacity of a joint or connector to perform during fire exposure.

Hybrid construction: Construction combining timber with materials such as concrete or steel.

Modular construction: Building assembly from prefabricated three-dimensional units.

Integrity: Ability of a separating element to prevent flames and hot gases from passing through.

Insulation: Ability of an element to limit temperature rise on the unexposed side.

Load-bearing resistance: Ability to sustain applied loads during fire.

Natural fire: Fire represented by fuel, ventilation and compartment conditions rather than a prescribed standard-fire curve.

Review Questions

Short Answer Questions

1. Why can a large timber beam retain load-bearing capacity after its surface has ignited?
2. Define the zero-strength layer and explain its purpose.
3. State two reasons why timber connections may fail before the main member.
4. What is delamination in a CLT panel, and why is it significant?
5. List four fire-safety concerns specific to modular construction.

Long Answer Questions

1. Explain the stages of timber heating, pyrolysis, ignition and charring, and relate them to structural fire design.
2. Describe the residual-section method for a three-sided exposed rectangular timber beam, including its assumptions and limitations.
3. Compare the fire-design issues of glulam, CLT and hybrid timber-concrete construction.
4. Explain how encapsulation, compartmentation, fire stopping and sprinklers work together in a mass-timber building.
5. Develop a professional workflow for checking a timber structure for a specified fire-resistance period.

Analytical / Application Questions

1. A rectangular timber beam is 180 mm wide and 400 mm deep, exposed on three sides for 45 minutes. Using β_n = 0.8 mm/min and d₀ = 7 mm, calculate the notional char depth and residual dimensions. State whether the calculation alone proves fire resistance.
2. A modular timber building has compliant fire-rated wall panels, but inspection finds unsealed service penetrations between modules. Analyse the likely fire paths and propose a verification and remediation plan.
3. A CLT floor passes a member-level calculation but contains an adhesive system with uncertain high-temperature delamination behaviour. Decide what additional evidence is required before approval and justify the decision.

Multiple Choice Questions with Answers

Question 1

What is the main structural benefit of the char layer on a large timber member?

A. It increases the original timber strength

B. It insulates the uncharred residual section

C. It prevents all heat from entering the member

D. It converts timber into non-combustible material

Correct Answer: B. It insulates the uncharred residual section

Question 2

Using β_n = 0.8 mm/min, t = 30 min and d₀ = 7 mm, what is the notional char depth?

A. 24 mm

B. 31 mm

C. 37 mm

D. 45 mm

Correct Answer: C. 37 mm

Question 3

For a rectangular beam exposed on three sides, which dimension is normally reduced from both sides in the simplified residual-section model?

A. Width

B. Unexposed top cover

C. Support length

D. Span

Correct Answer: A. Width

Question 4

Which phenomenon can expose fresh timber and cause renewed rapid burning in CLT?

A. Carbonation

B. Delamination

C. Condensation

D. Elastic buckling

Correct Answer: B. Delamination

Question 5

Why are exposed steel connection plates a particular fire concern?

A. Steel is always weaker than timber at ambient temperature

B. Steel conducts heat rapidly into the connection region

C. Steel prevents timber from charring

D. Steel automatically increases timber moisture

Correct Answer: B. Steel conducts heat rapidly into the connection region

Question 6

Which statement best describes encapsulation?

A. Increasing the applied structural load

B. Covering timber with a tested continuous protective lining

C. Removing all fire stops from service zones

D. Replacing sprinklers with ventilation

Correct Answer: B. Covering timber with a tested continuous protective lining

Question 7

What is the purpose of fire stopping at a modular interface?

A. To improve architectural appearance only

B. To provide a path for smoke movement

C. To restrict fire and smoke passage through gaps and joints

D. To increase the charring rate

Correct Answer: C. To restrict fire and smoke passage through gaps and joints

Question 8

A beam is 160 mm wide and has a notional char depth of 31 mm on two side faces. What is its simplified residual width?

A. 98 mm

B. 129 mm

C. 160 mm

D. 222 mm

Correct Answer: A. 98 mm

Question 9

Which item must be checked separately from the timber member in a complete fire design?

A. Connection resistance

B. Room colour

C. Furniture style

D. Daylight factor only

Correct Answer: A. Connection resistance

Question 10

Why is a member-level charring calculation insufficient for many mass-timber buildings?

A. Timber never chars uniformly

B. Fire spread, connections, cavities and interfaces also affect performance

C. Structural calculations are prohibited for timber

D. Only concrete can be assessed in fire

Correct Answer: B. Fire spread, connections, cavities and interfaces also affect performance