Engineered timber as a structural material

Wood is a natural fibre composite: cellulose microfibrils wound through a matrix of lignin and hemicelluloses. That construction yields a material strong along the grain and weak across it, one that swells and shrinks with humidity and keeps deforming under a load that never changes. Engineered timber — glulam, cross-laminated timber, laminated veneer lumber — is not upgraded wood; it is wood with each of these properties measured and routed around. Understanding why it works, and where it stops, means taking the properties in turn.

What cross-lamination buys#

Nearly all the axial stiffness of the cell wall sits in cellulose microfibrils aligned almost parallel to the grain. The consequence: the longitudinal stiffness of softwood is more than an order of magnitude higher than the transverse, and shrinkage from green to dry is a few tenths of a percent along the grain but several percent across it, with tangential roughly twice radial. A solid-sawn board therefore cups and splits as it dries. Cross-lamination interleaves layers at 90°, so every layer restrains the movement of its neighbours: panel movement in either direction approaches the small longitudinal value, and the differential shrinkage that would tear a solid panel apart is spread through the glue lines. Laminating does a second thing: defect averaging. Knots and grain deviation set the strength ceiling; slicing wood into lamellas or millimetre-thin veneers disperses them so no single defect governs, and graded layers put the stiffest material where bending stress peaks.

Moisture as the governing variable#

The cell wall is hygroscopic: hydroxyl-rich hemicelluloses and amorphous cellulose bind water vapour until the wood equilibrates with the air. Up to roughly 30% moisture content — the fibre saturation point — water sits inside the wall itself, and every property shifts: stiffness falls, creep accelerates, dimensions move. Above it, water is free in the cell cavities and nothing structural changes. Hence lumber is kiln-dried to the moisture content of its future service environment, mass-timber plants check every board before pressing, and design codes specify service classes rather than one allowable stress. Most timber failures in service are moisture events: a detail that traps water creates a local gradient, the wet zone swells and shrinks against dry wood, and checks open that admit more water.

Fire as predictable charring#

Wood does not melt and has no single failure temperature. In a fire the surface layer pyrolyses, ignites and burns out to charcoal, and the char — porous, low-density, a poor conductor — insulates the wood behind it while pyrolysis itself absorbs heat. The result is the strangest fact in structural fire engineering: loss of section proceeds at a nearly constant rate, about 0.65 mm per minute for softwood in the standard test, a figure written directly into Eurocode 5. The designer does not rate the material; the designer sacrifices a calculated outer layer and counts on the cool core retaining its room-temperature strength. Unprotected steel, by contrast, has lost roughly half its yield strength by 600 °C, well within a developed fire. The real weak points of a timber frame are its steel connections, which conduct heat past the char front and need separate protection.

Time under load#

Wood creeps. Under a constant load, deformation continues for years as the matrix slowly yields between load-bearing microfibrils, and long-term deflection grows to match or exceed the instantaneous elastic value. Worse is mechano-sorptive creep: moisture cycling under load multiplies deformation several-fold, because each wetting softens the matrix and molecular slip accumulates. Codes answer with duration-of-load factors that cut allowable stress for permanent loads; the physics is not negotiable, only priced in.