# Mass Timber Construction ## Table of Contents - [Introduction](#introduction) - [Mass Timber Product Types](#mass-timber-product-types) - [Cross-Laminated Timber (CLT)](#cross-laminated-timber-clt) - [Glue-Laminated Timber (Glulam)](#glue-laminated-timber-glulam) - [Laminated Veneer Lumber (LVL)](#laminated-veneer-lumber-lvl) - [Nail-Laminated Timber (NLT)](#nail-laminated-timber-nlt) - [Dowel-Laminated Timber (DLT)](#dowel-laminated-timber-dlt) - [Structural Properties](#structural-properties) - [Strength Classes](#strength-classes) - [Load Duration and Moisture Effects](#load-duration-and-moisture-effects) - [Span Capabilities](#span-capabilities) - [Fire Performance](#fire-performance) - [Charring Rate and Residual Section](#charring-rate-and-residual-section) - [Fire Design Methodology](#fire-design-methodology) - [Encapsulation Strategy](#encapsulation-strategy) - [Exposed Timber and Code Requirements](#exposed-timber-and-code-requirements) - [Connection Systems](#connection-systems) - [Self-Tapping Screws](#self-tapping-screws) - [Steel Plate Connections](#steel-plate-connections) - [Concealed Connections](#concealed-connections) - [Hold-Down and Angle Bracket Systems](#hold-down-and-angle-bracket-systems) - [Tall Timber Buildings](#tall-timber-buildings) - [Notable Projects](#notable-projects) - [Structural Systems for Height](#structural-systems-for-height) - [Regulatory Frameworks](#regulatory-frameworks) - [Hybrid Systems](#hybrid-systems) - [Timber-Concrete Composite](#timber-concrete-composite) - [Timber-Steel Hybrid](#timber-steel-hybrid) - [Timber with Concrete Core](#timber-with-concrete-core) - [Acoustic Performance](#acoustic-performance) - [Airborne Sound Insulation](#airborne-sound-insulation) - [Impact Sound Insulation](#impact-sound-insulation) - [Design Solutions](#design-solutions) - [Moisture Management](#moisture-management) - [Moisture Content Targets](#moisture-content-targets) - [Construction Phase Protection](#construction-phase-protection) - [In-Service Conditions](#in-service-conditions) - [Practical Notes for Architects](#practical-notes-for-architects) - [Related Topics](#related-topics) - [References](#references) --- ## Introduction Mass timber construction refers to the use of large-section, engineered timber products as the primary structural system in buildings. Unlike traditional light-frame timber construction (studs and joists), mass timber employs solid or laminated panels and members with sufficient cross-sectional area to achieve significant load-bearing capacity, fire resistance, and acoustic performance. The resurgence of mass timber since the early 2000s has been driven by sustainability imperatives (carbon sequestration, renewable material, low embodied energy), advances in manufacturing technology, and a growing body of research demonstrating its structural viability for mid-rise and tall buildings. Mass timber represents a paradigm shift for the construction industry — combining the environmental benefits of wood with the precision of modern manufacturing and the structural performance needed for contemporary buildings. ## Mass Timber Product Types ### Cross-Laminated Timber (CLT) CLT consists of layers (typically 3, 5, or 7) of dimensional lumber boards arranged in alternating perpendicular directions and bonded with structural adhesive. **Key characteristics:** - Panel thickness: 60-300mm (typical), up to 500mm available - Panel width: up to 3.5m (standard), up to 4.8m (some manufacturers) - Panel length: up to 16-18m (transport limited) - Bidirectional load-carrying capacity (due to cross-lamination) - Used for walls, floors, and roofs - Strength class: typically C24 laminations, panel properties per EN 16351 CLT is manufactured from spruce, pine, or fir lumber, dried to 12 ± 2% moisture content. The cross-lamination provides dimensional stability, reduces the effect of defects, and enables two-way spanning. ### Glue-Laminated Timber (Glulam) Glulam is manufactured by bonding layers (laminations) of stress-graded timber parallel to the grain direction, creating beams and columns of virtually any size and shape. - **Standard widths:** 90, 115, 140, 160, 185, 200, 240mm - **Depths:** up to 2,000mm or more - **Lengths:** up to 40m+ (transport limited, not manufacturing) - **Curved members:** Can be manufactured with curved profiles (minimum radius depends on lamination thickness) - **Strength classes:** GL24h, GL28h, GL32h, GL36h (h = homogeneous); c variants for combined layups See [[Glulam and Cross Laminated Timber]] for detailed properties. ### Laminated Veneer Lumber (LVL) LVL is manufactured by laminating thin wood veneers (typically 3mm) with structural adhesive, with grain predominantly parallel (some products include cross-veneers for stability). - **Higher strength than equivalent glulam** (fewer defects due to thin veneers) - **Consistent properties** (defects are distributed and minimised) - Available as panels (Kerto-Q with cross-veneers) or beams (Kerto-S, all parallel) - Excellent for long-span beams and high-load columns - Typical use: beams, headers, truss chords, rim boards ### Nail-Laminated Timber (NLT) NLT consists of dimensional lumber (typically 38mm or 89mm wide) stacked on edge and fastened together with nails. - **Traditional system** experiencing renewed interest - Can be fabricated on-site or in factory - One-way spanning (perpendicular to board direction) - Good acoustic mass for floor assemblies - Lower cost than CLT but requires more on-site labour - Suitable for floor and roof panels (typically 3-8m spans) ### Dowel-Laminated Timber (DLT) DLT uses friction-fit hardwood dowels (rather than nails or adhesive) to connect the lumber laminations. - **Adhesive-free** — fully reversible, excellent end-of-life recyclability - Similar performance to NLT but with better dimensional precision - CNC-manufactured for high accuracy - Growing market, particularly in sustainability-focused projects - Structural performance comparable to NLT; acoustic properties similar ## Structural Properties ### Strength Classes | Product | Strength Class | Bending Strength fm,k (MPa) | E₀,mean (MPa) | |---|---|---|---| | CLT (C24 lams) | Per EN 16351 | 24 (lamellar) | 11,600 | | Glulam GL24h | EN 14080 | 24 | 11,500 | | Glulam GL28h | EN 14080 | 28 | 12,600 | | Glulam GL32h | EN 14080 | 32 | 13,700 | | LVL (Kerto-S) | EN 14374 | 44 | 13,800 | | NLT (C24) | Based on component grade | 24 | 11,000 | ### Load Duration and Moisture Effects Timber strength is affected by load duration and moisture content. Eurocode 5 (EN 1995-1-1) applies modification factor kmod: | Load Duration Class | Example | kmod (Service Class 1) | |---|---|---| | Permanent | Self-weight | 0.60 | | Long-term | Storage | 0.70 | | Medium-term | Live load | 0.80 | | Short-term | Snow, wind | 0.90 | | Instantaneous | Accidental, seismic | 1.10 | **Service classes:** - Service Class 1: Indoor, heated (moisture content ≤ 12%) - Service Class 2: Sheltered outdoor (moisture content ≤ 20%) - Service Class 3: Exposed outdoor (moisture content > 20%) — mass timber generally not suitable ### Span Capabilities | Product | Typical Floor Span | Maximum Practical Span | |---|---|---| | CLT (5-layer, 160mm) | 4-6m | 7-8m | | CLT (7-layer, 240mm) | 5-8m | 9-10m | | Glulam beam (400mm deep) | 6-10m | 12m | | Glulam beam (800mm deep) | 10-15m | 20m+ | | LVL beam | 6-12m | 18m+ | | Glulam truss | 12-25m | 40m+ | ## Fire Performance ### Charring Rate and Residual Section Mass timber has predictable and favourable fire behaviour. When exposed to fire, timber chars at a known rate, and the char layer insulates the uncharred core, which retains its full structural properties. **Design charring rates (EN 1995-1-2):** | Product | One-Dimensional Charring Rate β₀ (mm/min) | |---|---| | Softwood (≥290 kg/m³) | 0.65 | | Glulam (softwood) | 0.65 | | CLT (with non-falling-off layers) | 0.65 | | CLT (with potential delamination) | 0.65 initially, then increased rate | | LVL | 0.65 | | Hardwood (≥450 kg/m³) | 0.50 | **Notional charring rate βn** includes the effect of corner rounding: βn = 0.70 mm/min for softwood (commonly used in simplified design). ### Fire Design Methodology **Reduced cross-section method (simplified):** 1. Calculate the charring depth: dchar = βn × t (where t is the fire exposure time in minutes) 2. Add zero-strength layer: d₀ = 7mm 3. Determine the residual cross-section: original dimensions minus (dchar + d₀) on each exposed face 4. Verify the residual section for the accidental (fire) load combination using ambient-temperature strength properties with modified partial factors (kfi × fk / γM,fi) **Example:** A 200mm thick CLT wall panel exposed to fire on one side for 60 minutes: - Charring depth = 0.70 × 60 = 42mm - Zero-strength layer = 7mm - Effective residual thickness = 200 - 42 - 7 = 151mm ### Encapsulation Strategy For higher fire resistance ratings or to avoid exposed timber debate, mass timber can be fully encapsulated with non-combustible materials: - Type X gypsum board: 15.9mm provides approximately 30 minutes additional protection - Two layers of Type X: approximately 60 minutes additional protection - Encapsulation allows mass timber to be treated as non-combustible for regulatory purposes in many jurisdictions ### Exposed Timber and Code Requirements The debate on exposed mass timber in tall buildings continues to evolve: - Most codes allow exposed timber for buildings up to 6-8 storeys (with sprinklers) - Some jurisdictions (e.g., IBC 2021 Types IV-A, IV-B, IV-C in the US) permit mass timber buildings up to 18 storeys with varying levels of encapsulation - The UK regulatory framework post-Grenfell restricts combustible materials in external walls above 11m but does not prohibit internal structural mass timber - Fire testing of full compartments (e.g., Zelinka et al., 2018) has demonstrated that CLT rooms with sprinklers can achieve self-extinguishment ## Connection Systems ### Self-Tapping Screws Self-tapping screws (STS) are the most common fastener for mass timber connections: - Diameters: 6-14mm; lengths: up to 600mm+ - Can be installed at angles (typically 30-45°) for improved withdrawal and shear capacity - Proprietary systems (e.g., Rothoblaas, SWG, Heco) with published design values - Full-thread screws (for withdrawal loads) and partial-thread screws (for shear and combined loads) - Installed without pre-drilling in softwood (pilot holes for hardwood) ### Steel Plate Connections - **Concealed steel plates (knife plates):** Slotted into the timber member and pinned with dowels — clean aesthetic - **External steel plates:** Bolted or screwed to the timber face — visible but easier to inspect - **Gusset plates:** For truss and bracing connections - **Steel brackets (angle brackets):** Standard CLT-to-CLT floor-to-wall connections ### Concealed Connections Architecturally desirable for exposed timber: - **Slotted-in steel plates with dowels:** Most common concealed moment connection - **Glued-in rods:** Threaded steel rods bonded into drilled holes — high stiffness and strength - **Timber-to-timber joinery with hidden fasteners:** Inspired by traditional joints but CNC-cut for precision ### Hold-Down and Angle Bracket Systems Standard connection hardware for platform-frame CLT construction: - **Hold-downs:** Resist uplift forces at shear wall ends (wind and seismic overturning) - **Angle brackets:** Transfer shear forces from walls to floors or foundations - **Proprietary systems:** Simpson Strong-Tie, Rothoblaas, etc. — with tested and published capacities ## Tall Timber Buildings ### Notable Projects | Project | Location | Height / Storeys | System | Year | |---|---|---|---|---| | Mjostaarnet | Brumunddal, Norway | 85.4m / 18 storeys | Glulam + CLT | 2019 | | HoHo Vienna | Vienna, Austria | 84m / 24 storeys | Timber-concrete hybrid | 2019 | | Ascent MKE | Milwaukee, USA | 86.6m / 25 storeys | Mass timber + concrete core | 2022 | | Sara Kulturhus | Skelleftea, Sweden | 75m / 20 storeys | CLT + glulam | 2021 | | Brock Commons | Vancouver, Canada | 53m / 18 storeys | CLT + glulam + concrete | 2017 | ### Structural Systems for Height - **Platform construction (up to ~10 storeys):** CLT walls and floors stacked floor-by-floor. Compression perpendicular to grain accumulates; limited by crushing - **Post-and-beam with CLT infill (10-20 storeys):** Glulam or LVL columns and beams carry gravity; CLT panels provide diaphragm and shear wall action - **Hybrid with concrete core (15-25+ storeys):** Concrete core resists all lateral loads; timber frame carries gravity — current approach for the tallest timber buildings - **Tube systems and diagrid:** Emerging concepts for very tall timber buildings ### Regulatory Frameworks - **IBC 2021 (USA):** Three new construction types for mass timber: IV-A (up to 18 storeys), IV-B (up to 12 storeys), IV-C (up to 9 storeys) - **National Building Code of Canada:** Allows mass timber to 12 storeys (as of 2020 revision) - **Eurocode 5 + National Annexes:** No explicit height limit in many European countries — performance-based approach - **UK Building Regulations:** No combustible material ban for internal structure; external wall restrictions per Regulation 7 ## Hybrid Systems ### Timber-Concrete Composite Timber beams or CLT panels with a concrete topping connected by shear connectors: - **Benefits:** Increased stiffness and span, improved acoustic performance (mass), fire resistance - **Shear connectors:** Notch-and-screw, proprietary connectors (e.g., HBV mesh), inclined screws - **Typical concrete topping:** 60-80mm non-structural screed or structural composite topping - **Span increase:** Typically 20-40% improvement over timber alone ### Timber-Steel Hybrid - Steel beams with CLT floor panels — common for commercial buildings - Steel moment frames or braced frames for lateral resistance with CLT floors and walls - Allows longer spans than pure timber systems - Steel columns with glulam beams — mixed aesthetic expression ### Timber with Concrete Core The dominant system for tall mass timber buildings: - Concrete core provides lateral stability, stair/lift shafts, and fire-protected egress - Mass timber frame (glulam columns, CLT or TCC floors) carries gravity - Clear separation of structural functions - Concrete core constructed first; timber frame erected rapidly around it ## Acoustic Performance ### Airborne Sound Insulation CLT panels alone have limited airborne sound insulation due to relatively low mass: - 100mm CLT: approximately Rw 33-35 dB - 160mm CLT: approximately Rw 36-38 dB Building regulations typically require Rw ≥ 55 dB (or DnT,w ≥ 53 dB) for separating walls and floors. ### Impact Sound Insulation Mass timber floors are particularly susceptible to impact sound transmission. Bare CLT performs poorly for impact insulation (Ln,w of 70-80+ dB); building regulations typically require Ln,w ≤ 55-62 dB. ### Design Solutions - **Floating floor:** Resilient layer (e.g., mineral wool, rubber mat) under a screed or raised floor — the single most effective measure - **Suspended ceiling:** Decoupled ceiling on resilient hangers below the CLT soffit - **Added mass:** Concrete topping or sand-filled cavity — improves low-frequency performance - **Complete build-up:** CLT + resilient layer + screed + floor finish + decoupled ceiling = Rw 56-62 dB, Ln,w 45-55 dB ## Moisture Management ### Moisture Content Targets - CLT is manufactured at 12 ± 2% moisture content - Target in-service moisture content: 8-14% (Service Class 1) - Prolonged exposure above 20% risks fungal growth and decay - Equilibrium moisture content in heated buildings: typically 8-10% ### Construction Phase Protection Moisture during construction is the greatest risk to mass timber: - **Just-in-time delivery:** Minimise on-site storage time - **Temporary weather protection:** Tenting, wrapping, or progressive roofing - **Site drainage:** Prevent standing water near timber elements - **Moisture monitoring:** Embedded sensors in critical locations - **Drying protocol:** Allow wet elements to dry before encapsulation (below 19% MC) ### In-Service Conditions - Vapour barriers and membranes positioned correctly (warm side of insulation in cold climates) - Detailing to prevent moisture traps at interfaces (timber-to-concrete, timber-to-steel) - Proper flashings and drip edges at external exposures - Monitoring systems in high-risk locations (bathrooms, kitchens, plant rooms, roof connections) ## Practical Notes for Architects 1. **Design for manufacture:** CLT and glulam are CNC-cut to exact dimensions — coordinate openings, service routes, and connections in the digital model before manufacturing 2. **Erection speed:** Mass timber buildings can be erected at 1-2 floors per week — programme advantage is significant 3. **Tolerances:** Tighter than concrete (±2-3mm manufacturing), requiring precision in supporting structures 4. **Carbon benefit:** 1 m³ of CLT sequesters approximately 0.7-0.8 tonnes of CO₂ and substitutes for higher-carbon materials 5. **Coordinate acoustics early:** Acoustic performance requires careful build-up design from concept stage 6. **Engage the manufacturer at Stage 2** to optimise panel sizes, connection details, and manufacturing efficiency 7. **Exposed timber is beautiful but demanding** — protect surfaces during construction and specify appropriate finishes 8. **Insurance and warranty:** Engage insurers and warranty providers early, as some have limited experience with mass timber ## Related Topics - [[Glulam and Cross Laminated Timber]] - [[Timber Engineering Fundamentals]] - [[Fire Safety in Timber Buildings]] - [[Engineered Wood Products]] - [[Modular and Prefabricated Construction]] ## References - EN 1995-1-1: Eurocode 5 — Design of Timber Structures - EN 1995-1-2: Eurocode 5 — Structural Fire Design - EN 16351: Timber Structures — Cross Laminated Timber - EN 14080: Timber Structures — Glued Laminated Timber - Karacabeyli, E. and Gagnon, S., *CLT Handbook* (Canadian Edition), FPInnovations - Swedish Wood, *Design of Timber Structures*, Volumes 1-3 - Structural Timber Association (STA), *Mass Timber Structures Guidance Document* --- #structures #timber #mass-timber #CLT #glulam #fire #tall-buildings #sustainability