Solid timber has the limits of the tree. Common sections reach 20 by 30 centimetres and commercial lengths rarely exceed 6-8 metres. For longer spans and heavier loads, wood is broken down into boards, veneers or layers and then reassembled with adhesives. The result is products with distributed defects, higher and more regular strengths, and geometries that a trunk would not allow. Glulam, LVL and CLT are three different ways of doing this.
Glued laminated timber
Glued laminated timber (GL, glulam) is made of boards between 6 and 45 mm thick, finger-jointed along their length and glued flat with the grain parallel. Polyurethane, melamine or resorcinol-based adhesives are used, for different service classes and conditions. The result is straight, curved or variable-section beams, up to about two metres deep, able to cover spans of up to 40-50 metres. With arches and portal frames much longer spans are achieved. The product standard is EN 14080 and the homogeneous classes range from GL20h to GL32h. It is produced mainly from Norway spruce.
LVL
LVL (Laminated Veneer Lumber) is made of veneers about 3 mm thick, obtained by peeling the log, glued with the grain mostly parallel. The veneers are thinner than the laminations of glulam and this disperses defects even better, so the strength is higher and more uniform. The product standard is EN 14374. Some producers declare, in their European technical assessment, characteristic bending strengths of about 44 N/mm2 and moduli around 13.8 GPa for products with all veneers parallel. LVL is used for deep, slender beams, floor joists, lintels and studs. A variant with some cross-laid veneers, used for panels, has less extreme stiffness but better stability in two directions.
CLT
CLT (Cross-Laminated Timber, in Italy also X-Lam) is a panel made of layers of boards laid at 90 degrees to one another, in odd numbers (three, five, seven or more). Typical thickness ranges from 60 to over 300 mm, and dimensions depend on the producer and on transport limits, with widths up to about three metres and lengths up to about sixteen. The product standard is EN 16351, and many producers also obtain a European technical assessment for their panels. It works as a plate in two directions, so it is used for load-bearing walls, floors and roofs. Density is around 450-500 kg/m3 and thermal conductivity is about 0.12-0.13 W/(m K).
The behaviour of panels is different from that of a beam. The cross layers have low strength and low stiffness in rolling shear, a phenomenon that weighs on the sizing of floors over medium spans and must be considered in the deflection calculation. Under seismic action ductility does not come from the panels, which remain almost rigid, but from the metal connections (angle brackets and anchors). In 2007 a seven-storey CLT building, built under the SOFIE project of CNR-IVALSA, passed a shaking-table test at Miki, Japan, with the signal of the 1995 Kobe earthquake.
| Product | Constituents | Grain | Product standard | Typical elements |
|---|---|---|---|---|
| Glulam | boards of 6-45 mm | parallel | EN 14080 | beams, columns, arches, portal frames |
| LVL | veneers of about 3 mm | mostly parallel | EN 14374 | deep slender beams, joists, studs |
| CLT | cross-laid layers of boards | crossed at 90 degrees | EN 16351 | panel walls, floors and roofs |
Which to choose
The choice starts from geometry. For linear elements, beams and columns, glulam is the most widespread solution and offers the best ratio of cost to performance. LVL is worthwhile when a slender section with high strength is needed. CLT is the choice for flat elements, and allows very fast construction, because the panels arrive cut with the openings already cut out. It does require care over floor acoustics, fire protection and protection from rain during erection, because panels left wet on site deform and can grow mould.
On the climate side, wood stores carbon, about 0.8 tonnes of CO2 per cubic metre of dry wood (about 450 kg of mass, half of which is carbon). The balance stays positive only if the wood does not burn, does not decompose and is recovered at end of life, and if glues and transport do not cancel the advantage. To go deeper into the origin of the wood and its balance, the article on forests and FSC certification is the next step.