Two spruce beams of the same section, 12 by 24 centimetres, can have very different strengths: one comes from a board with regular grain and no large knots, the other from a piece with knots on the tension edge and sloping grain. For wood, the species is not enough to describe the material. What counts is how the individual piece was selected and which class it was assigned to.
Why wood is graded
Wood is a natural material, with defects that vary from board to board: knots, grain deviation, shakes, density. Every structural element must therefore be graded, visually or by machine, and CE marked according to EN 14081-1. In Italy visual grading refers to UNI 11035, while machine grading measures stiffness or density with instruments. Grading yields an EN 338 strength class: the C series, from C14 to C50, for softwoods and poplar, and the D series, from D18 to D80, for hardwoods. The number indicates the characteristic bending strength in N/mm2.
| Class | Bending fm,k | Tension ft,0,k | Compression fc,0,k | Mean modulus E0,mean (GPa) | Characteristic density (kg/m3) |
|---|---|---|---|---|---|
| C16 | 16 | 8.5 | 17 | 8.0 | 310 |
| C18 | 18 | 10 | 18 | 9.0 | 320 |
| C20 | 20 | 11.5 | 19 | 9.5 | 330 |
| C24 | 24 | 14.5 | 21 | 11.0 | 350 |
| C27 | 27 | 16.5 | 22 | 11.5 | 360 |
| C30 | 30 | 19 | 24 | 12.0 | 380 |
| C35 | 35 | 22.5 | 25 | 13.0 | 390 |
| C40 | 40 | 26 | 27 | 14.0 | 400 |
Glued laminated timber
For glued laminated timber EN 14080 defines its own classes, with the code GL followed by the bending strength. The letter h indicates a homogeneous section, with all laminations of the same quality, the letter c a combined section, with outer laminations stronger than the inner ones. The laminations are thinner than solid timber and the defects are spread and dispersed through the section, which is why, for the same species, glulam is stronger and less variable.
| Class | Bending fm,g,k | Tension ft,0,g,k | Compression fc,0,g,k | Mean modulus E0,g,mean (GPa) | Characteristic density (kg/m3) |
|---|---|---|---|---|---|
| GL24h | 24 | 19.2 | 24 | 11.5 | 385 |
| GL28h | 28 | 22.3 | 28 | 12.6 | 425 |
| GL30h | 30 | 24 | 30 | 13.6 | 430 |
| GL32h | 32 | 25.6 | 32 | 14.2 | 440 |
Moisture and load duration: the kmod coefficient
Wood loses strength when it is wet and when the load lasts a long time. Eurocode 5 (EN 1995-1-1) and the NTC 2018 take both effects into account with the coefficient kmod, which depends on the service class and on load duration. Service class 1 corresponds to a wood moisture content not exceeding about 12 per cent (heated, enclosed spaces), class 2 to a moisture content below about 20 per cent (covered and ventilated, unheated), class 3 to wetter conditions, for example structures exposed outdoors.
| Load duration | Example | Class 1 | Class 2 | Class 3 |
|---|---|---|---|---|
| Permanent (over 10 years) | self-weight | 0.60 | 0.60 | 0.50 |
| Long-term (6 months - 10 years) | storage loads | 0.70 | 0.70 | 0.55 |
| Medium-term (1 week - 6 months) | imposed loads, snow above 1,000 m | 0.80 | 0.80 | 0.65 |
| Short-term (less than 1 week) | snow at altitudes up to 1,000 m | 0.90 | 0.90 | 0.70 |
| Instantaneous | wind, accidental actions | 1.10 | 1.10 | 0.90 |
From characteristic value to design value
The design strength is Xd = kmod Xk / γM. The NTC 2018 set γM at 1.50 for solid timber and 1.45 for glued laminated timber. For a beam in service class 1 under a medium-term load, a C24 has fm,d = 0.80 x 24 / 1.50 = 12.8 N/mm2, and a GL24h has fm,d = 0.80 x 24 / 1.45 = 13.2 N/mm2. Under permanent load, at equal class, the strength falls to 9.6 and 9.9 N/mm2: load duration decides much more than the class label.
Deformations also grow over time. The final deflection is calculated by multiplying the instantaneous one by (1 + kdef), with kdef equal to 0.60 in service class 1, 0.80 in class 2 and 2.00 in class 3. In a damp environment the long-term deflection can therefore be triple the initial one, and it is often this, not strength, that sizes the floor.
Anisotropy
The values in the tables apply parallel to the grain. Perpendicular to it, wood is much weaker: the characteristic tensile strength is 0.4 N/mm2 and the compressive strength, for a C24, 2.5 N/mm2. The perpendicular modulus of elasticity is about one thirtieth of the parallel one. Transverse tension is the critical point of details: a notch on the tension edge at a support, an unplanned hole or a connection that loads the grain crosswise are the cause of many cracks in service. The design of a timber structure is largely the design of its connections and its supports.