Structural Timber for Multi-Storey Buildings: Case Studies & Limits

Multi-storey timber construction represents the most significant shift in structural engineering practice since the introduction of reinforced concrete. After a century of relegation to low-rise domestic buildings, timber is reclaiming its position as a primary structural material for urban buildings — driven by carbon reduction imperatives, prefabrication advantages, and a growing body of engineering knowledge demonstrating that timber can safely and efficiently reach heights previously reserved for concrete and steel.

This guide examines the structural systems, design challenges, regulatory framework, and real-world performance of multi-storey timber buildings, providing architects and engineers with the technical foundation for specifying timber at height.

The Case for Timber at Height

Carbon Imperative

The built environment accounts for approximately 40% of global carbon emissions, with structural materials (concrete, steel) responsible for a significant proportion of embodied carbon. Multi-storey timber offers:

• Biogenic carbon storage: 1 m³ of CLT stores approximately 700–800 kg CO₂e
• Low processing energy: timber manufacturing requires 2–5× less energy than steel or concrete
• Substitution effect: each m³ of timber replacing concrete avoids 0.8–1.2 tonnes CO₂e
• End-of-life options: reuse, recycling, or energy recovery

A typical 8-storey CLT building stores 1,500–2,500 tonnes of CO₂ in its structure — equivalent to removing 500–800 cars from the road for one year. For detailed carbon accounting, see embodied carbon in timber and net zero timber buildings.

Construction Speed

Multi-storey timber construction is significantly faster than concrete:

The speed advantage comes from prefabrication: CLT panels and glulam frames arrive on site pre-cut, pre-drilled, and ready for assembly. A typical floor plate can be erected in 2–3 days with a small crew and single crane.

Weight Advantage

The 40–60% weight reduction translates directly to smaller foundations, reduced material in substructure, and the ability to build on sites with limited bearing capacity.

Structural Systems

System 1: CLT Platform Construction

Principle: Load-bearing CLT wall panels support CLT floor panels, stacked floor-by-floor in a platform arrangement.

Height range: 3–10 storeys (limited by cumulative compression in lower walls and lateral stability)

Advantages:

• Simple, repetitive construction
• All elements are planar — easy to manufacture and transport
• Good acoustic separation (mass of CLT)
• Inherent fire resistance (charring behaviour)
• No exposed steel connections

Limitations:

• Limited plan flexibility (load-bearing walls constrain layout)
• Cumulative compression requires thick walls at lower levels
• Lateral stiffness limited by panel-to-panel connections
• Vertical movement (compression perpendicular to grain at floor-wall junctions)

Typical specifications:

• Walls: 100–200 mm CLT (3 or 5 layer)
• Floors: 140–240 mm CLT (5 or 7 layer)
• Connections: self-tapping screws, angle brackets, hold-downs

System 2: Post-and-Beam with CLT Cores

Principle: Glulam columns and beams form the primary frame, with CLT shear walls providing lateral stability. Floors may be CLT panels, timber-concrete composite, or other systems.

Height range: 5–15 storeys

Advantages:

• Open floor plans (columns at 4–8 m grid)
• Architectural expression of frame
• CLT cores provide stiffness and fire compartmentation
• Efficient use of materials (glulam in bending, CLT in shear)

Limitations:

• More complex connections than platform construction
• Requires careful coordination of tolerances
• Column-beam connections may require steel components
• Fire protection of connections needs attention

Typical specifications:

• Columns: GL28h to GL32h glulam, 240 × 240 mm to 400 × 400 mm
• Beams: GL24h to GL28h glulam, 200 × 400 mm to 240 × 600 mm
• Floors: 160–200 mm CLT or timber-concrete composite
• Cores: 120–200 mm CLT panels with steel angle connections

System 3: Hybrid Timber-Concrete

Principle: Concrete core(s) provide lateral stability and house services/stairs, while timber columns and floor plates form the occupied space.

Height range: 8–25 storeys

Advantages:

• Concrete core provides proven lateral stability system
• Timber floor plates reduce weight and carbon
• Familiar design approach for structural engineers
• Fire strategy simplified (concrete core for escape)
• Acoustic separation at core walls

Limitations:

• Requires two structural trades (concrete + timber)
• Interface details between concrete and timber are critical
• Programme advantage reduced by concrete core construction
• Not a "pure timber" solution (marketing/sustainability narrative)

Typical specifications:

• Core: reinforced concrete, 200–300 mm walls
• Columns: GL28h–GL32h glulam, 300 × 300 mm to 500 × 500 mm
• Floors: CLT panels 160–240 mm, or timber-concrete composite
• Connections: steel brackets cast into concrete, bolted to glulam

System 4: Timber-Steel Hybrid

Principle: Steel moment frames or braced frames provide lateral stability, with timber columns, beams, and floor plates carrying gravity loads.

Height range: 5–12 storeys

Advantages:

• Steel provides high lateral stiffness in minimal space
• Timber carries majority of gravity load (carbon benefit)
• Familiar connection details for steel elements
• Flexible plan arrangements

Limitations:

• Steel elements increase embodied carbon
• Thermal bridging at steel-timber interfaces
• Fire protection required for steel elements
• Mixed material procurement and coordination

Design Challenges

Vertical Movement

Multi-storey timber buildings experience vertical shortening from:

1. Elastic compression: immediate shortening under load (predictable, calculable)
2. Creep: long-term deformation under sustained load (30–60% of elastic for timber)
3. Shrinkage: moisture loss after installation (0.1–0.3% for CLT, more for solid timber)
4. Compression perpendicular to grain: at floor-wall junctions in platform construction

This movement must be accommodated in:

• Service connections (flexible joints in risers)
• Facade attachments (slotted fixings)
• Interface with rigid elements (concrete cores, lift shafts)
• Floor-to-floor height tolerances

Lateral Stability

Wind loading on multi-storey timber buildings creates challenges due to timber's relatively low stiffness:

The serviceability limit (lateral drift) typically governs before the ultimate limit state (strength), meaning the building feels uncomfortable in wind before it is structurally unsafe.

Disproportionate Collapse

Buildings above 4 storeys (or meeting other criteria) must be designed to resist disproportionate collapse — the progressive failure of the structure following localised damage. Strategies for timber include:

• Tie forces: continuous horizontal ties at each floor level connecting all elements
• Alternative load paths: redundancy allowing load redistribution if one element is removed
• Key element design: critical elements designed for accidental loading (34 kN/m²)
• Compartmentation: limiting the extent of collapse to a defined area

CLT platform construction inherently provides good robustness through the continuous floor diaphragms and multiple load paths in wall panels.

Fire Engineering

Regulatory Framework

Encapsulation Strategy

For buildings above 4–6 storeys, full or partial encapsulation of timber is typically required:

Full encapsulation (K260 per EN 13501-2):

• 2 layers of 15 mm fire-rated plasterboard (Type F)
• All timber surfaces protected
• Timber does not contribute to fire load
• Simplest approval route

Partial encapsulation:

• Selected timber surfaces exposed (typically ceilings)
• Exposed timber has additional sacrificial layer (25–40 mm)
• Detailed fire engineering calculation required
• Balances aesthetics with fire safety

Charring Design for Multi-Storey

Mass timber elements (CLT, glulam) char at predictable rates:

CLT delamination is a critical consideration: when the char layer reaches a glue line, the charred layer may fall off, exposing fresh timber to the fire at an increased charring rate. This must be accounted for in fire design using the approach in EN 1995-1-2 Annex C or manufacturer-specific test data.

Acoustic Design

Performance Requirements

Floor Build-Up for Acoustic Performance

A typical high-performance CLT floor assembly achieving DnT,w ≥ 55 dB and L'nT,w ≤ 50 dB:

Total build-up: 340–475 mm floor-to-ceiling

This depth must be accommodated in the floor-to-floor height, which is typically 3.0–3.3 m for multi-storey timber (compared to 2.8–3.0 m for concrete flat slab).

Case Studies

Mjøstårnet, Brumunddal, Norway (2019)

Key engineering features:

• Glulam columns up to 625 × 625 mm at base
• Concrete slabs on upper floors for dynamic performance (mass damping)
• Diagonal glulam bracing in facade provides lateral stability
• All connections concealed within timber members

Dalston Works, London, UK (2017)

Key engineering features:

• Tallest CLT building in the UK at completion
• Platform construction with 120–160 mm CLT walls
• 200 mm CLT floor panels
• Full encapsulation fire strategy
• Acoustic performance exceeding Part E by 5–8 dB

HoHo Wien, Vienna, Austria (2019)

Key engineering features:

• Concrete core with timber-concrete composite floors
• Glulam columns and beams for gravity frame
• Prefabricated facade elements with integrated timber structure
• Mixed-use (hotel, offices, residential, retail)

Procurement and Programme

Design Team Requirements

Multi-storey timber projects require specialist expertise:

• Structural engineer: experienced in timber design to EC5, familiar with connection design and fire engineering
• Fire engineer: essential for buildings above 4 storeys, performance-based fire strategy
• Acoustic consultant: early involvement to define floor build-ups and junction details
• Timber contractor: specialist fabricator with CNC capability and erection experience
• Building control: early engagement to agree fire strategy and structural approach

Programme Considerations

Total programme from design freeze to completion: 12–18 months (compared to 16–24 months for concrete frame).

Supply Chain

The multi-storey timber supply chain in Europe is maturing rapidly:

• CLT manufacturers: 15+ major producers across Austria, Germany, Scandinavia, and UK
• Glulam fabricators: extensive network across Northern Europe
• Specialist contractors: growing number of firms with tall timber experience
• Connection suppliers: proprietary systems from multiple manufacturers

For procurement guidance, see timber lead times in Europe and buy structural timber in bulk.

Regulatory Landscape

The regulatory environment for multi-storey timber is evolving rapidly across Europe, driven by:

1. Carbon reduction targets: governments recognising timber's role in decarbonising construction
2. Fire research: growing evidence base for mass timber fire performance
3. Industry lobbying: timber industry advocating for material-neutral regulations
4. Demonstration projects: successful tall timber buildings proving the concept

Key regulatory developments:

• UK: No prescriptive height limit for timber, but post-Grenfell scrutiny requires robust fire engineering
• Germany: 2023 Model Building Code (MBO) revision allows timber in buildings up to high-rise threshold
• France: RE2020 regulations actively promote bio-based materials including timber
• EU Taxonomy: timber construction recognised as a sustainable economic activity

Future Directions

The multi-storey timber sector is advancing rapidly:

• Taller buildings: 30+ storey timber buildings in design/planning
• Hybrid systems: optimised combinations of timber, concrete, and steel
• Automation: robotic assembly of timber elements on site
• Digital twins: real-time monitoring of timber building performance
• Standardisation: development of standard connection details and floor systems
• Insurance: growing acceptance by insurers as evidence base expands

The trajectory is clear: timber will become a mainstream structural material for urban buildings of 5–15 storeys within the next decade, with specialist applications reaching 20+ storeys.

Related Resources

• Structural Timber Guide — comprehensive timber construction overview
• CLT Modular Homes — CLT construction principles
• Timber Frame vs CLT — comparing structural systems
• Timber Fire Resistance — fire engineering for timber
• Timber Connections Design — connection specification
• Embodied Carbon in Timber — carbon accounting
• Net Zero Timber Buildings — achieving net zero
• Timber Lead Times Europe — procurement planning
• BSH Glued Laminated Beams — glulam products
• C24 Structural Construction Timber — structural softwood