Strength classes: what C14 to C50 mean, and what to check

The class system

An EN 338 strength class is a label such as C18, C24 or C30. The softwood classes run from C14 to C50. Hardwood classes carry the letter D and, according to the Polish papers in the portal, run from D30 to D70; the portal found no open table of D-class values, so none are taught here. In both families the number is the characteristic bending strength in MPa.

Each class also states a mean modulus of elasticity parallel to grain (E0,mean) and a 5th percentile density (rho_k). The structure is the point to learn: strength, stiffness and density travel together in one label, so a customer who asks for C24 gets a bundle of properties.

An indicative table, with a warning

The table below gives selected softwood classes. It comes from a secondary source (an Italian design-office page reproducing the EN 338:2016 table), not from the standard itself. Treat it as an illustration of the pattern, not as a reference.

The bending strengths and mean moduli agree between the two sources the portal compared; the C24 and C30 values (11 000 MPa and 350 kg/m3; 12 000 MPa and 380 kg/m3) also appear in Stapel's doctoral thesis.

Conflicts exist. A Silesian paper prints an EN 338:2016 extract that differs from the design-office page in the C50 density (460 versus 430 kg/m3 characteristic, 550 versus 520 kg/m3 mean), in C24 tension parallel to grain (14 versus 14.5 MPa) and in C24 shear (2.5 versus 4.0 MPa). The portal's authors suspect an older edition but could not confirm it. Also, the 2016 edition supersedes EN 338:2009, and an older paper in the portal's sources used the 2000 edition. Therefore:

• Take any number you quote to a customer or put in a design from the current, purchased edition of the standard.
• Always state the standard edition together with the class.

Reading the pattern

Moving up one class step raises all three properties, but not in proportion. From C24 to C30 in the table above:

• bending strength rises from 24 to 30 MPa, a factor of 30 / 24 = 1.25 (25% higher);
• mean MOE rises from 11.0 to 12.0 GPa, only about 9% higher;
• the characteristic density rises from 350 to 380 kg/m3, about 9% higher.

Strength is therefore the most demanding part of a step up; stiffness and density change less. This is why machines that measure stiffness and density still need a correction for strength-reducing features.

Which method can reach which class

According to the Finnish wood information site (Europe), classes C14 to C50 exist and the most common in Finland are C18, C24, C30, C35 and C40. Classes C14 to C30 can be sorted visually or mechanically, while C35 to C50 are machine-sorted only. A visual rule can therefore not give you the highest classes, however careful the grader. In the Silesian pine study, machine grading placed 32.4% of its batch in C40 and C35 combined.

Worked example: choosing a route

A customer wants C35 floor joists.

1. Is C35 reachable visually? No: C35 to C50 are machine-sorted only (Finnish source).
2. So the mill needs an approved machine setting for its species, size range and class combination.
3. The class declared must be accompanied by the standard edition.

Checklist for a class declaration

• Class and edition of EN 338.
• Method: visual rule (listed in EN 1912) or machine setting.
• Species or species group and source area.
• Size range, because settings and results depend on cross-section.

Key points

• The class number is the characteristic bending strength in MPa; stiffness and density are attached to the class.
• The class table used in this course is a secondary reproduction. Sources conflict on several cells (C50 density, C24 tension and shear), so use the current standard for real values.
• Between C24 and C30 in the indicative table, strength rises 25% but stiffness and density only about 9%.
• C35 to C50 are machine-graded only (Finnish source); C14 to C30 can be visual or machine.