| Density (spruce C24) | 420–500 kg/m³ |
|---|---|
| Elastic modulus (C24) | 11.000 N/mm² |
| Characteristic bending strength (C24) | 24 N/mm² |
| Compressive strength, parallel to grain (C24) | 21 N/mm² |
| Tensile strength, perpendicular to grain (C24) | 0.4 N/mm² (critical) |
| Service moisture content | SC1: ≤12% / SC2: ≤20% |
| Fire behaviour | Charring rate: 0.7 mm/min (spruce) |
| EU reference standard | EN 338, EN 14080 · Eurocode 5 (EN 1995) |
In 2017, at the University of British Columbia in Vancouver, the Brock Commons Tallwood House opened its doors: 18 storeys, 53 metres, 400 resident students. The load-bearing structure is in CLT and GLULAM, with just two reinforced-concrete cores for lateral stiffness. It was the tallest timber skyscraper in the world at the time of completion. The construction required 70 days of structural assembly, with crews of 4 workers per floor. An equivalent concrete building would have taken twice as long.
What made Brock Commons possible is not wood itself — wood has been used for millennia. It is the combination of engineered products (CLT, GLULAM, LVL) with advanced calculation systems and up-to-date regulations. Solid wood alone could not have done that job. Understanding the difference between the variants is the first duty of anyone designing in timber.
Composition and anatomy of the material
Wood is an anisotropic cellular material: its mechanical properties depend on orientation relative to grain direction. Parallel to the grain, wood is excellent in compression and tension (strengths of 20–30 N/mm² for spruce C24). Perpendicular to the grain, it is weak in tension (0.4 N/mm²) — more than fifty times less. This anisotropy is the central design parameter: in joints, bearing details, and nodes, stress perpendicular to the grain must always be checked first.
Moisture is the other critical variable. Wood is hygroscopic: it absorbs and releases water vapour, changing its own dimensions. A 1% increase in moisture content causes swelling of about 0.2–0.3% in the radial/tangential direction, almost nil along the axial direction. In exposed structures, dimensional changes must be designed for, not simply endured: connections allowing controlled movement, expansion joints, protection from differential moisture.
The structural variants
Solid wood: grades C14–C40
Structural solid wood is classified by mechanical strength according to EN 338: C14, C16, C18, C20, C22, C24, C27, C30, C35, C40. The letter C stands for "coniferous" (softwood — spruce, pine, larch); the letter D for deciduous (hardwood — oak, beech). The number is the characteristic bending strength in N/mm². Grade C24 is the minimum normally accepted for load-bearing structures; C30 and C35 are used for elements subject to significant loads or spans of a certain size.
The limitation of solid wood is size: elements are constrained by the dimensions of the trunk, with natural defects (knots, grain curvature, shrinkage cracking) reducing mechanical properties compared with "theoretical" defect-free wood. This is why engineered products exist: recombining lamellae to eliminate or distribute the defects.
GLULAM: GL24h, GL28h, GL32h
Glued Laminated Timber (GLULAM) is produced by gluing dried wood lamellae 20–45 mm thick with structural adhesives (resorcinol, polyurethane, melamine-urea). The result is an element with mechanical properties more homogeneous and predictable than solid wood, because defects distribute randomly and statistically cancel out. The GL24h, GL28h, GL32h grades (h = homogeneous, lamellae of the same grade; c = combined, differentiated lamellae) indicate the characteristic bending strength.
GLULAM makes possible large-span elements (trusses, arches, portal frames) impossible with solid wood. The record span without intermediate supports for GLULAM structures exceeds 100 metres (arena roofs, airports). GLULAM allows curved geometries: arch beams are produced by gluing the lamellae over shaped formwork.
LVL: Laminated Veneer Lumber
LVL (Kerto, Metsä Wood, Ultralam) is produced by gluing 3 mm-thick wood veneers with the layers parallel to each other. Parallel alignment maximizes mechanical properties along the axis: bending strength up to 44 N/mm², elastic modulus up to 14,000 N/mm² — the highest values among structural wood products. It is the material of choice for long-span beams, floor panels and stiffening elements. It is supplied continuously from the manufacturer: widths up to 2.5 m, lengths up to 23 m.
CLT / X-Lam
CLT (Cross Laminated Timber), also known as X-Lam, is the real structural revolution in wood of the last twenty years. It is produced by gluing alternating layers of lamellae at 90°: 3, 5, 7 or more layers (always an odd number). The orthogonal alternation reduces anisotropy, makes the panel rigid in two directions and limits differential shrinkage. The result is a large-format flat panel (up to 3 × 16 m) that works as a two-dimensional slab — it behaves like a concrete slab, but weighs a fifth as much.
CLT has made feasible the construction of multi-storey buildings entirely in wood. Seismic behaviour is surprisingly good: the connection between panels dissipates energy through friction and plastic deformation of the metal connectors, with behaviour factors q = 2–3 in European standards. In taller buildings, CLT floor slabs are combined with concrete cores or steel bracing for resistance to horizontal forces.
Heat-treated wood
Heat treatment (200–230°C in the absence of oxygen) chemically modifies the wood's hemicelluloses, reducing its hygroscopicity. Heat-treated wood is more dimensionally stable, more resistant to decay fungi and insects — but has mechanical strength reduced by 10–30% compared with untreated wood. It is suited to outdoor uses (façade cladding, deck flooring, window frames) where dimensional stability matters more than strength. The characteristic dark brown colour is an indicator of the process, not a surface treatment: it is stable over time.
Accoya — acetylated wood
Accoya (produced by Accsys Technologies from radiata pine) is obtained by acetylation: the wood's hemicelluloses react with acetic anhydride, permanently reducing the binding sites for water molecules. The result is wood with moisture absorption reduced by 80%, exceptional dimensional stability, and durability class 1 (over 50 years outdoors with no treatment). Mechanical strength is preserved. The cost is about 3–5 times comparable solid wood, but the life-cycle makes it competitive for high-quality outdoor uses: windows, ventilated façades, pedestrian bridges.
OSB: Oriented Strand Board
OSB is produced by hot-pressing oriented wood strands in crossed layers with resins (MDI or UF). It is not a top-tier structural product like CLT or GLULAM, but in lightweight frame structures (platform frame, balloon frame) OSB panels on solid-wood studs form horizontal diaphragms and vertical bracing. Grades OSB/3 and OSB/4 (EN 300) are certified for structural use in environments with variable humidity. Its low cost and availability make it the most widespread structural panel in Northern Europe and the American market.
Structural plywood
Structural plywood (EN 636) is produced by gluing veneers with alternating grain at 90°. Compared with OSB it has smoother surfaces and more uniform mechanical properties, but costs more. Used for formwork, floor panels, and diaphragms in frame structures where the surface finish is visible. Marine plywood (BS 1088), with okoumé or beech veneers and phenolic glue, is specific to very humid environments.
Behaviour over time: fire, moisture, biology
Wood burns, but it burns predictably. Spruce chars at 0.7 mm/min: the char layer formed at the surface has low thermal conductivity and protects the still-intact core. In massive CLT or GLULAM structures, the section is sized with a sacrificial layer that is consumed over the required fire duration (REI 60 = 42 mm of charring at 0.7 mm/min), leaving an "effective residual section" still able to carry the load. In many European countries, unprotected steel does not exceed REI 30 without intumescent coating, while a GLULAM beam oversized by 40 mm can reach REI 60 passively. It is a real technical advantage, not a marketing reassurance.
Against decay fungi and wood-boring insects, the critical threshold is moisture: above 20% moisture content, fungi can proliferate. Detailed design of roofs, joints, and connections to masonry and ground matters more than chemical treatment. A timber element properly protected from water and with sufficient ventilation lasts centuries — as shown by the medieval trusses of Italian cathedrals still in situ.
| Dimension | Score | Assessment |
|---|---|---|
| 🌿 Environmental | 8/10 | Wood from FSC/PEFC certified forests sequesters CO₂ during growth (~1 kg CO₂ per kg of dry wood). Low-energy production compared with steel and concrete: ~0.3 kg CO₂/kg for solid wood. Caveat: illegal logging and opaque supply chains lower the score if not certified. |
| ⚖️ Ethics | 7/10 | Northern European supply chains (Finland, Sweden, Austria) with high transparency. Engineered products (GLULAM, CLT) with certified industrial production. Risks in the tropical supply chain (okoumé, teak) if untraced. EUTR (EU Timber Regulation) certification is mandatory for the EU market. |
| 🏘️ Social | 8/10 | Supply chain strongly rooted in the local economy of Alpine forest areas. Exposed wood indoors creates environments with better perceived well-being (studies on the biophilia effect, cortisol reduction). Quieter, tidier building sites than cast-in-place concrete. |
| 💶 Economic | 7/10 | Higher cost per m³ than cast-in-place concrete for CLT and GLULAM (€600–1,200/m³ vs. €120–180/m³), but faster building sites cut indirect costs. Factory prefabrication reduces waste and rework. The life-cycle (reduced maintenance for a properly protected envelope) is competitive. |
| Average score | 7.5/10 |
Uses through history
Prehistory and vernacular traditions (–1800): Wood is humanity's first building material: pile-dwelling huts, medieval load-bearing structures, Gothic trusses. In Japan, the Horyuji temple (607 AD) is considered the world's oldest timber structure still standing. Alpine vernacular architecture developed solid-timber building systems (blockbau) with dry joints of a technical refinement that anticipates the principles of modern Design for Disassembly.
Industrialization and the balloon frame (1830–1950): The invention of the industrial nail and the mechanical sawmill in 1830s America leads to the balloon frame: lightweight structures in standardized sawn timber (2×4, 2×6 inches) with rapid nailing. Within thirty years, millions of American homes are built with this system. It is the first industrialization of residential building. The system evolves into the platform frame, still dominant in North America and Northern Europe for buildings up to 4–5 storeys.
The modern, engineered era (1990–today): The CLT patent is registered in Austria in 1994. Within twenty years, engineered-timber structures move from a research niche to a mainstream building system. The Mjøsa Tower in Brumunddal (Norway, 2019, 85.4 m, 18 storeys) is today the tallest timber skyscraper in the world. Research is aiming at 100 storeys: feasibility studies exist for "plyscrapers" of 40–80 storeys in LVL and hybrid GLULAM.
Reference figures for design
| C24 solid — fm,k / E₀,mean | 24 N/mm² / 11.000 N/mm² |
|---|---|
| GL28h GLULAM — fm,k / E₀,mean | 28 N/mm² / 12.600 N/mm² |
| GL32h GLULAM — fm,k / E₀,mean | 32 N/mm² / 13.700 N/mm² |
| LVL (Kerto-S) — fm,k / E₀,mean | 44 N/mm² / 13.800 N/mm² |
| CLT — available thicknesses | 60–500 mm (3–15 layers) |
| Charring rate | 0.7 mm/min (spruce/pine) |
| Embodied CO₂, solid wood | ~ –1.5 kg CO₂/kg (net sequestration) |
Research and the contemporary frontier
The most interesting frontier is nano-engineered wood. Liangbing Hu's group at the University of Maryland has produced "super wood" (densified wood) by compressing chemically treated wood: tensile strength of 587 N/mm², higher than S355 steel. It is not yet a commercial product, but it opens up scenarios for ultra-lightweight structures. On the connections front, inclined Self-Tapping Screw (STS) systems are replacing traditional metal joints in CLT structures, simplifying assembly and improving load transfer. In Italy, the CNR's WOODIENSE research project studies the seismic behaviour of CLT buildings over 10 storeys with semi-rigid-joint dissipation systems.