What is a strength class?
A strength class is a standardised grading of a structural material by the strength it reliably achieves (its characteristic value), written as a letter and a number. The structural engineer designs with that class, and drawings and orders state which class is required. Each material has its own system, and the number means something different in each.
The main European systems
- Concrete (EN 206): for example C25/30. The first figure is the characteristic cylinder compressive strength, the second the cube strength, both in N/mm² at 28 days.
- Timber (EN 338): C for softwood (for example C16, C24), D for hardwood (for example D30). The number is the characteristic bending strength in N/mm². Glulam has GL classes under EN 14080, such as GL24h (homogeneous) or GL24c (combined).
- Structural steel (EN 10025-2): S235, S275, S355. The number is the minimum yield strength in N/mm² for thicknesses up to 16 mm; thicker sections have slightly lower values. Reinforcing bar is graded B500, with a yield strength of 500 N/mm².
- Masonry: mortar to EN 998-2 has classes such as M5 or M10, where the number is the compressive strength in N/mm². The manufacturer declares the compressive strength of the units; the strength of the masonry follows from units and mortar together (Eurocode 6).
These are the European (EN) systems used in the UK and the EU. North America uses other grading systems, such as concrete specified in psi and lumber grades.
| Concrete (EN 206), e.g. C25/30 | First figure: characteristic cylinder compressive strength; second figure: cube strength; both in N/mm² at 28 days |
|---|---|
| Timber (EN 338), e.g. C16, C24, D30 | C = softwood, D = hardwood; the number is the characteristic bending strength in N/mm² |
| Glulam (EN 14080), e.g. GL24h, GL24c | Suffix only: h = homogeneous, c = combined |
| Structural steel (EN 10025-2), S235, S275, S355 | The number is the minimum yield strength in N/mm² for thicknesses up to 16 mm; thicker sections have slightly lower values |
| Reinforcing bar, B500 | Yield strength of 500 N/mm² |
| Mortar (EN 998-2), e.g. M5, M10 | The number is the compressive strength in N/mm²; masonry strength follows from units and mortar together (Eurocode 6) |
Application
- Copy the class exactly. Order concrete, timber and steel in precisely the class the engineer specifies. Never go lower without a recalculation.
- Check the marking. Structural timber should carry a grade stamp or documentation showing its strength class. Ungraded timber is not suitable for load-bearing members.
- Strength is not everything. For concrete, the exposure class also governs durability; for timber, the use class and species matter too.
- Have old designations converted. Drawings of older buildings often use superseded grades. Let the engineer translate them.
Common mistake: comparing numbers across materials. C24 (softwood) and C25/30 (concrete) both start with C, but one is the bending strength of timber and the other the compressive strength of concrete.
Related terms
- Compressive strength
- Flexural strength
- Concrete strength class
- Exposure class
- Mortar
- Eurocode
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