Timber, Engineered Wood and Bio-Based Materials

Introduction and Context

Timber is one of the oldest structural materials, yet it remains a technologically advanced option for low-carbon and prefabricated construction. Its contemporary relevance arises from renewable feedstock, comparatively low density, high specific strength, off-site manufacturability and the ability of forests and harvested products to store biogenic carbon. These benefits are not automatic: they depend on responsible forestry, durable detailing, moisture control, fire engineering, responsible adhesives and transparent life-cycle assessment.

Timber is a biological, heterogeneous and anisotropic material. Its properties vary with species, density, growth conditions, defects, moisture content, loading direction and duration. Engineered wood products reduce some sources of variability by selecting, drying, grading and reconstituting wood, but they do not eliminate moisture sensitivity, biological decay or fire exposure.

This chapter examines wood anatomy and behaviour, then evaluates glulam, cross-laminated timber (CLT), laminated veneer lumber (LVL), bamboo and other bio-based products. Emphasis is placed on structural application, resilience, durability, fire safety, sustainability and professional decision-making.

Learning Objectives

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

1. Explain the anatomical structure and anisotropic behaviour of wood.

2. Relate moisture content and environmental exposure to dimensional stability, strength and durability.

3. Compare sawn timber, glulam, CLT, LVL, bamboo and selected bio-based materials.

4. Apply basic equations for moisture content, density, bending stress and fire charring.

5. Develop material-selection and detailing strategies for resilient construction.

6. Critically evaluate carbon, durability, fire, toxicity, supply-chain and regulatory issues.

1. Wood as a Structural Material

1.1 Biological origin and hierarchy

A tree transports water and nutrients through sapwood and gains mechanical support from its woody tissues. Heartwood is older inner wood that is generally less active physiologically and may contain extractives that improve natural durability. Sapwood is often more permeable and, in many species, more vulnerable to fungal attack. These are tendencies rather than universal rules; species-specific data and grading standards govern design decisions.

Wood is organized at several scales:

• Cellular scale: cells such as tracheids, vessels, fibres and parenchyma provide transport, storage and support.
• Growth-ring scale: earlywood is commonly less dense and more porous than latewood. Seasonal variation produces growth rings.
• Member scale: knots, slope of grain, checks, splits, reaction wood and decay interrupt an otherwise continuous fibre structure.
• Structural-system scale: connections, diaphragms, panels and moisture barriers determine the performance of the building, not merely the nominal material strength.

Softwood and hardwood are botanical classifications, not direct rankings of engineering performance. Some hardwoods are dense and strong, while selected softwoods provide excellent structural efficiency and are widely used in engineered products.

1.2 Orthotropy and anisotropy

Wood is approximately orthotropic: it has different properties in three mutually perpendicular directions—longitudinal (parallel to grain), radial (normal to growth rings) and tangential (tangent to growth rings). Longitudinal stiffness and strength are generally much greater than transverse values. Radial and tangential shrinkage are also unequal, which can cause warping, cupping and checking during drying.

For a simple bending member, the nominal extreme-fibre bending stress is

\[

\sigma_b = \frac{M}{Z}

\]

where \(\sigma_b\) is bending stress, \(M\) is bending moment and \(Z=I/c\) is the section modulus; \(I\) is the second moment of area and \(c\) is the distance from the neutral axis to the extreme fibre. The equation assumes elastic beam behaviour, a known effective section and a stress distribution consistent with elementary beam theory. Design standards additionally require adjustment for duration of load, moisture, stability, defects, fire and connection effects.

For a rectangular section of breadth \(b\) and depth \(h\), bending about the strong axis gives

\[

Z=\frac{bh^2}{6}.

\]

The square dependence on depth explains why increasing depth is usually more efficient than increasing breadth for bending resistance. In real timber, however, grain deviation, knots, local crushing, lateral instability and connection detailing can control performance.

1.3 Density and specific performance

Oven-dry density is commonly expressed as

\[

\rho_0=\frac{m_0}{V_0},

\]

where \(m_0\) is oven-dry mass and \(V_0\) is the corresponding volume. Density is not constant: it changes with moisture content, and volume changes during drying. Specific strength or stiffness—property divided by density—is often more informative than absolute strength when comparing timber with concrete or steel. Nevertheless, comparisons must use consistent design states, safety factors, fire conditions and functional requirements.

2. Moisture, Hygrothermal Behaviour and Durability

2.1 Moisture content

Moisture content on a dry-mass basis is

\[

w=\frac{m_w-m_0}{m_0}\times100\%,

\]

where \(m_w\) is the mass at the measured condition and \(m_0\) is oven-dry mass. This definition is essential because the denominator is dry mass, not wet mass. For example, a specimen weighing 12.0 kg at test condition and 10.0 kg after oven drying has \(w=(12-10)/10\times100=20\%\).

Below the fibre saturation point, commonly approximately 25–30% but species-dependent, loss or gain of bound water causes substantial shrinkage or swelling. Above this range, free-water changes generally have less effect on dimensions, although mass and transport behaviour continue to change. Equilibrium moisture content depends on temperature, relative humidity, exposure, coating and ventilation.

2.2 Consequences of moisture variation

Moisture affects:

• dimensional movement and joint opening;
• modulus of elasticity and strength;
• creep and long-term deformation;
• adhesive performance and delamination risk;
• electrical and thermal properties;
• fungal decay and insect activity;
• fire ignition and heat transfer.

Moisture movement is anisotropic. Tangential shrinkage is commonly greater than radial shrinkage, while longitudinal shrinkage is comparatively small unless reaction wood or juvenile wood is present. In mass-timber buildings, moisture management must address transport, storage, erection, façade interfaces, plumbing, roofs, balconies and the end of temporary weather protection.

2.3 Biological durability

Wood-destroying fungi require moisture, oxygen, suitable temperature and a food source. Preventing persistent wetting is usually more reliable than relying solely on preservative treatment. Principal strategies include:

1. keep wood above ground or separated from wet substrates;

2. provide drainage, flashing and drip edges;

3. avoid end-grain water traps;

4. permit inspection and drying;

5. specify naturally durable or treated species where exposure demands it;

6. seal or protect vulnerable cut ends according to the treatment system;

7. control concealed condensation and plumbing leaks.

Insects such as termites and beetles may attack timber depending on climate, species and building conditions. Fire-retardant treatment is not a substitute for biological durability, and preservative treatment does not automatically provide fire resistance.

2.4 Creep and duration of load

Timber is viscoelastic. Under sustained load it deforms over time, with creep increased by moisture cycling and elevated moisture content. A simplified serviceability representation is

\[

\delta_{final}=\delta_{initial}(1+k_{def}),

\]

where \(\delta_{initial}\) is immediate deformation and \(k_{def}\) is a code- or test-based creep factor appropriate to the product, service class and loading duration. This is not a universal material constant. It must be selected from the governing design standard and exposure category. Long-span floors may be governed by vibration and long-term deflection rather than ultimate strength.

3. Fire Behaviour and Fire Engineering

Wood is combustible, but combustible does not mean incapable of fire-resisting construction. Fire performance depends on ignition, heat release, member size, surface protection, ventilation, connections, cavities and fire-fighting conditions.

When exposed to fire, wood heats, dries and undergoes pyrolysis. A char layer forms and may provide insulation to the remaining section. A simplified residual-section method is

\[

a_{eff}=\beta t+a_0,

\]

where \(a_{eff}\) is the effective sacrificial depth, \(\beta\) is an adopted charring rate, \(t\) is fire exposure time and \(a_0\) represents an additional zero-strength or heated layer used by the applicable design method. The residual depth of a member exposed from one side is approximately its original depth minus \(a_{eff}\); for exposure from two opposite sides, loss occurs from both sides. Values of \(\beta\) and \(a_0\) must come from the relevant code, product assessment or fire test; they vary with species, density, product, orientation, moisture and exposure.

Fire design must also consider:

• protective gypsum layers and their failure time;
• concealed cavities and fire stopping;
• delamination of cross-laminated panels;
• connection charring and steel heat conduction;
• fall-off of charred material;
• compartmentation and evacuation;
• sprinklers, detection and firefighter access;
• post-fire structural stability.

A nominal fire-resistance rating is a system property. It cannot be inferred solely from a material’s density or from an unprotected member’s char depth.

4. Engineered Wood Products

Engineered wood products use controlled processing to improve dimensional stability, structural reliability, material efficiency and prefabrication. Product-specific testing and standards remain essential.

4.1 Glued laminated timber

Glulam consists of timber laminations bonded with their grain generally parallel. Defective zones can be removed or distributed, and higher-grade laminations may be placed in zones of higher stress. Glulam is suitable for beams, columns, arches, portal frames and long-span roof systems.

Advantages include large cross-sections, architectural form, efficient use of smaller boards and good prefabrication potential. Limitations include adhesive-line sensitivity, transport dimensions, moisture exposure, connection design and the possibility of strength-reducing checks or delamination. Curved members require careful control of grain deviation and lamination geometry.

4.2 Cross-laminated timber

CLT comprises layers of boards arranged in alternating directions and bonded into thick panels. It acts as a two-way plate or wall panel, although its orthotropic stiffness and rolling shear must be recognized. CLT supports rapid erection, reduced wet trades and combined structural and architectural functions.

Important design issues include:

• rolling shear in cross layers, especially near supports and openings;
• panel-to-panel and panel-to-foundation connections;
• diaphragm action and torsional load paths;
• concentrated loads and punching-type local effects;
• vibration and acoustic separation;
• moisture at panel edges and construction joints;
• delamination and char fall-off during fire.

CLT should be designed as part of a load path, not as isolated panels. Openings, lifting inserts and tolerances must be coordinated before fabrication.

4.3 Laminated veneer lumber

LVL is manufactured by bonding thin veneers, usually with grain predominantly parallel to the member axis. Veneer selection and pressing produce high uniformity and efficient use of logs. LVL is used in beams, joists, headers, columns, rim boards and flanges of composite systems.

LVL has relatively consistent longitudinal properties, but transverse and connection behaviour remains important. Veneer joints, end grain, moisture exposure and fastener splitting require specific detailing. LVL should not be treated as interchangeable with solid timber solely because the nominal dimensions are similar.

4.4 Other composite wood products

Parallel strand lumber and laminated strand lumber use aligned strands; oriented strand board uses oriented flakes; plywood uses cross-bonded veneers. These products differ in stiffness, moisture response, surface durability, fastener behaviour, adhesive chemistry and allowable applications. Product certification and declared performance should govern specification.

5. Bamboo and Other Bio-Based Materials

5.1 Bamboo

Bamboo is a grass rather than a tree. Its hollow culm has nodes, a variable wall thickness and strong fibres concentrated toward the outer wall. It grows rapidly and can provide high strength-to-mass performance. Structural bamboo may be used as whole culms, laminated bamboo, strand-woven bamboo or scrimber-type products.

Challenges include geometric irregularity, natural durability, splitting, connection design, moisture movement, treatment quality, fire behaviour and supply-chain consistency. Whole-culm joints often govern capacity. Bolts can cause splitting and local crushing; steel sleeves, wrapped joints, dowels, clamps and carefully detailed gussets may improve performance. Laminated bamboo offers greater dimensional regularity but uses adhesives and energy-intensive processing.

5.2 Agricultural fibres and bio-composites

Straw panels, hemp-lime, flax or jute composites, cork, rice-husk boards, mycelium composites and wood-fibre insulation illustrate a broad bio-based materials field. They are not interchangeable:

• Hemp-lime is generally a lightweight infill or insulation material rather than a primary structural frame.
• Straw panels may provide insulation and enclosure, with structural performance dependent on the panel system and connections.
• Cork and wood fibre are valuable insulation materials with moisture-buffering capacity.
• Mycelium composites are promising for low-load insulation and packaging applications but require further evidence for structural, moisture and fire-critical uses.
• Bio-based polymers and natural-fibre composites can reduce fossil feedstock but may have durability, fire and end-of-life challenges.

A bio-based label does not establish structural adequacy, low toxicity or low life-cycle impact. Functional performance and verified environmental data are required.

6. Selection for Resilient Construction

A resilient design maintains safety and recoverability under hazards such as moisture intrusion, fire, wind, earthquake, flood and supply disruption. Material selection should use a multi-criteria framework:

1. Structural efficiency: strength, stiffness, stability, fatigue and connection capacity.

2. Exposure resistance: wetting, drying, ultraviolet radiation, insects, fungi, salts and freeze–thaw effects.

3. Fire safety: ignition, heat release, compartmentation, protection and residual capacity.

4. Constructability: tolerances, lifting, sequencing, weather protection and repair access.

5. Environmental performance: forest management, carbon storage, transport, adhesives, replacements and end-of-life scenarios.

6. Economic performance: material, labour, programme, maintenance, insurance and risk costs.

7. Social and governance factors: legal sourcing, worker safety, community impacts and equitable procurement.

Life-cycle assessment should distinguish biogenic carbon storage from permanent emissions reduction. A harvested product may store carbon during service, but the result depends on forest regrowth, land-use change, product lifespan, substitution effects and end-of-life treatment. Carbon claims should state system boundaries, reference service life and assumptions.

7. Worked Applications

7.1 Bending check for a rectangular timber beam

An illustrative simply supported beam has a rectangular section 140 mm wide by 360 mm deep and carries a maximum design bending moment of 18 kN·m. Ignoring lateral instability and code modification factors for this preliminary calculation,

\[

Z=\frac{bh^2}{6}

=\frac{140(360)^2}{6}=3.024\times10^6\;\text{mm}^3.

\]

Since 18 kN·m equals 18,000,000 N·mm,

\[

\sigma_b=\frac{18,000,000}{3.024\times10^6}=5.95\;\text{N/mm}^2.

\]

The calculated stress must be compared with a design bending resistance, not an unadjusted catalogue strength. The designer must still check shear, deflection, vibration, lateral torsional stability, notches, supports, connections, moisture class and fire exposure.

7.2 Illustrative residual-depth calculation

Suppose a protected design method adopts, for a particular product and exposure, an effective sacrificial depth of 0.7 mm/min and an additional layer of 7 mm. After 60 minutes, the effective loss on one exposed face is

\[

a_{eff}=0.7(60)+7=49\;\text{mm}.

\]

For a 200 mm deep member exposed on one face, the idealized residual depth is 151 mm before structural checks. This example is illustrative only: the adopted values must be justified by the governing fire standard or assessment, and protection failure, connection heating and multiple-face exposure may alter the result.

8. Applied Case: Illustrative Mid-Rise Mass-Timber Building

Background

A developer proposes a six-storey educational building using glulam columns and beams, CLT floors and walls, and wood-fibre insulation. The site has high seasonal rainfall, a constrained urban plot and a requirement for low embodied carbon.

Problem

The project team must balance rapid construction against fire resistance, acoustic performance, moisture risk, procurement reliability and future adaptability. The architectural concept includes exposed timber ceilings and several balconies.

Analysis

Early analysis identifies four dominant risks: rain during erection, balcony and façade water ingress, acoustic transmission through continuous panels, and fire spread through concealed cavities. Structural analysis also identifies vibration sensitivity in long-span teaching spaces and high connection forces at the lateral-load-resisting core.

Approach and solution

The team adopts a moisture-management plan requiring factory moisture documentation, covered transport, raised storage, daily inspection, temporary roofs and defined moisture acceptance criteria before enclosure. CLT panels are detailed with protected edges and drained interfaces. Balconies are separated thermally and structurally from the primary timber frame where practicable.

Fire engineering combines compartmentation, protected connections, tested wall and floor assemblies, cavity barriers, automatic detection and sprinklers. Exposed timber is limited to areas where the fire strategy and smoke-development assessment support it. Acoustic build-ups use floating floors and resilient interfaces rather than relying on panel mass alone. Glulam and CLT are digitally coordinated with mechanical openings, lifting points and connection hardware before fabrication.

Outcome

The solution preserves much of the programme and carbon benefit while reducing uncontrolled exposure and fire uncertainty. The project cost is higher at design stage because of testing, coordination and temporary weather protection, but construction risk and rework are reduced.

Learning points

The case demonstrates that mass timber is a building system rather than a material substitution. Moisture, fire, acoustics, vibration and connection design must be integrated from concept design. Exposed timber should be selected deliberately, not assumed to be the default expression of sustainability.

9. Professional Practice, Regulation and Emerging Technologies

Design must comply with the applicable building code, timber design standard, fire regulations, product standards and environmental declarations. Traceability is important: chain-of-custody certification can support legal and responsible sourcing, but it does not replace structural grading or a project-specific sustainability assessment.

Adhesives require scrutiny for emissions, worker exposure, recyclability and formaldehyde or other regulated substances. Designers should request product declarations, test evidence, durability classifications and installation limits. Substitution of an adhesive, preservative or panel supplier without technical review can invalidate performance assumptions.

Digital manufacturing and building information modelling improve precision, clash detection and material optimization. Machine vision and artificial intelligence can assist grading by identifying knots, grain deviation and surface defects, but automated classification requires validated datasets, calibration, uncertainty management and human oversight. AI-generated design decisions must remain auditable and must not override code responsibilities or professional judgement.

Contemporary development is moving toward bio-based insulation, low-emission adhesives, reversible mechanical connections, hybrid timber–concrete floors, robotic fabrication, design for disassembly and improved fire-performance testing. Research challenges include standardizing bamboo and agricultural-fibre products, predicting moisture in complex assemblies, quantifying long-term adhesive durability and comparing carbon outcomes across regional supply chains.

Challenges and Limitations

Timber and bio-based materials have important limitations:

• property variability and dependence on grading;
• vulnerability to prolonged wetting and concealed moisture;
• combustible surfaces and complex fire interfaces;
• creep, vibration and dimensional movement;
• connection behaviour that may govern the structure;
• limited standardization for emerging products;
• adhesive emissions and difficult recycling of bonded products;
• transport and lifting constraints for large panels;
• possible biodiversity loss or land-use conflict from poor sourcing;
• uncertainty in carbon accounting and future climate exposure.

These limitations do not disqualify timber. They define the conditions under which it should be designed and managed.

Best-Practice Checklist

• Specify a recognized structural product with declared properties.
• Define moisture conditions during manufacture, transport, erection and service.
• Design drainage, flashing, ventilation and inspection access before façade approval.
• Treat connections, openings and interfaces as critical design zones.
• Use tested or code-recognized fire assemblies and cavity barriers.
• Check serviceability, vibration and creep as well as ultimate capacity.
• Verify legal and responsible sourcing and document carbon assumptions.
• Coordinate digital fabrication data with surveyed site conditions.
• Plan maintenance, repair, replacement and end-of-life pathways.
• Review emerging bio-based products through evidence, not marketing claims.

Chapter Summary

Wood is a naturally anisotropic and variable material whose structural behaviour depends on anatomy, grain, density, defects, moisture and loading duration. Moisture below the fibre saturation point produces significant dimensional movement and influences stiffness, strength, creep and biological durability. Effective timber construction therefore depends on drainage, drying potential, inspection and exposure-appropriate specification.

Engineered products such as glulam, CLT and LVL improve uniformity, dimensional capacity and prefabrication, but each has distinctive failure modes and connection requirements. Bamboo and other bio-based products offer promising renewable alternatives, although standardization, fire, moisture, durability and supply-chain evidence remain essential.

Fire engineering must address charring, protection, compartmentation, cavities, connections and delamination as a system. Resilient material selection combines structural, environmental, economic, regulatory and social criteria. Life-cycle carbon claims require explicit boundaries and assumptions. The central professional lesson is that mass timber and bio-based construction succeed through integrated design, disciplined moisture control, verified fire performance and responsible governance.

Key Terms

Anisotropy: Variation of a material property with direction.

Bamboo: A grass with fibrous hollow culms used in whole or engineered forms.

Biogenic carbon: Carbon originating from biological material.

Char layer: Thermally degraded wood layer formed during fire exposure.

CLT: Cross-laminated timber made from bonded, alternating board layers.

Creep: Time-dependent deformation under sustained load.

Equilibrium moisture content: Moisture condition approached by wood in a specified environment.

Fibre saturation point: Approximate moisture condition below which bound-water loss causes shrinkage.

Glulam: Structural timber made by bonding laminations, generally parallel to grain.

Heartwood: Inner wood that is generally older and physiologically inactive.

LVL: Laminated veneer lumber made from bonded veneers, predominantly parallel to grain.

Orthotropic: Having different properties along three perpendicular material directions.

Pyrolysis: Thermal decomposition of wood producing gases, liquids and char.

Rolling shear: Shear deformation across the grain, important in cross layers of CLT.

Sapwood: Outer, younger wood involved in fluid transport.

Section modulus: Geometric quantity \(Z=I/c\) used in elastic bending calculations.

Specific strength: Strength divided by density.

Timber grading: Classification of wood according to strength, stiffness, defects and appearance.

Whole-life carbon: Greenhouse-gas emissions and removals over a defined life-cycle boundary.

Review Questions

Short Answer Questions

1. Why is wood described as an orthotropic material?

2. Define moisture content on a dry-mass basis.

3. State two mechanisms by which moisture damages timber construction.

4. What is the principal structural role of cross-lamination in CLT?

5. Why is a fire-resistance rating regarded as a system property?

Long Answer Questions

1. Explain the relationship between wood anatomy, grain direction and structural anisotropy.

2. Compare glulam, CLT and LVL in terms of manufacture, structural application and limitations.

3. Discuss a comprehensive moisture-management strategy for a mass-timber building.

4. Explain how fire engineering should address both exposed timber members and concealed cavities.

5. Evaluate the sustainability benefits and risks of timber and bio-based construction using a life-cycle perspective.

Analytical / Application Questions

1. A rectangular timber beam is 160 mm wide, 400 mm deep and subjected to a maximum bending moment of 24 kN·m. Calculate its elastic section modulus and nominal bending stress. State the additional checks required before design approval.

2. A CLT floor panel is proposed for a long-span classroom in a humid climate. Develop a decision framework addressing vibration, rolling shear, moisture, acoustics, fire and connections.

3. An architect proposes untreated whole-culm bamboo for an exposed coastal footbridge. Critically assess the proposal and recommend investigations, protective measures and alternative material strategies.

Multiple Choice Questions with Answers

Question 1

Which wood direction generally has the greatest stiffness and strength?

A. Longitudinal, parallel to grain

B. Radial, normal to growth rings

C. Tangential, tangent to growth rings

D. Transverse, independent of grain

Correct Answer: A. Longitudinal, parallel to grain

Question 2

A specimen has a wet mass of 12 kg and an oven-dry mass of 10 kg. What is its dry-basis moisture content?

A. 16.7%

B. 20%

C. C 83.3%

D. 120%

Correct Answer: B. 20%

Question 3

Which engineered wood product is formed from laminations generally parallel to the member axis?

A. CLT

B. LVL

C. Straw board

D. Cork board

Correct Answer: B. LVL

Question 4

What is the principal purpose of alternating layer directions in CLT?

A. To eliminate all moisture movement

B. To create panel action in more than one direction

C. To make the panel non-combustible

D. To remove the need for connections

Correct Answer: B. To create panel action in more than one direction

Question 5

For a rectangular beam, the elastic section modulus about the strong axis is:

A. bh/2

B. b²h/6

C. bh²/6

D. b+h

Correct Answer: C. bh²/6

Question 6

Which condition most directly promotes fungal decay in timber?

A. Persistent wetting with suitable temperature and oxygen

B. Short-term exposure to dry indoor air

C. High longitudinal stiffness

D. Factory grading

Correct Answer: A. Persistent wetting with suitable temperature and oxygen

Question 7

Why can a timber member retain structural capacity during part of a fire exposure?

A. The wood becomes non-combustible

B. The char layer can insulate the residual section

C. Moisture content becomes zero instantly

D. Adhesives always strengthen when heated

Correct Answer: B. The char layer can insulate the residual section

Question 8

Which issue is particularly important for CLT cross layers?

A. Rolling shear

B. Steel corrosion in every panel

C. Concrete carbonation

D. Asphalt bleeding

Correct Answer: A. Rolling shear

Question 9

Which statement about bamboo is most accurate?

A. It is a hardwood tree species

B. Its whole-culm connections may govern structural capacity

C. It is inherently immune to insects

D. Its geometry is always uniform

Correct Answer: B. Its whole-culm connections may govern structural capacity

Question 10

What is the most defensible interpretation of a bio-based material label?

A. It proves the product is structurally adequate in every application

B. It proves zero life-cycle emissions

C. It identifies biological feedstock but does not replace performance evidence

D. It guarantees non-combustibility

Correct Answer: C. It identifies biological feedstock but does not replace performance evidence