Structural hardwoods are strength graded in accordance with BS 5756: 1997 Specification for visual strength grading of hardwood and are allocated into strength classes defined in BS EN 338: 1995 Structural timber. Strength classes.
The density of many of the species used for structural purposes provides greater fire resistance than that of the lower density softwoods. This is acknowledged by the slower charring rates given in BS 5268-4 Section 4.1 Recommendations for calculating fire resistance of timber members.
Strength grading
The strength of individual pieces of timber is assessed by strength grading. For hardwoods, this is undertaken to the rules laid down for visual grading in BS 5756. Five strength grades are defined at two moisture content levels; dry and wet:
- HS Tropical hardwoods
- TH1 and TH2 General structural temperate hardwood – for timber of a cross-sectional area less than 20,000mm² and a thickness less than 100mm. TH1 is a higher grade than TH2.
- THA and THB Heavy structural temperate hardwood – for timber of a cross sectional area of 20,000mm² or more and a thickness of 100mm or more. THA is a higher grade than THB.
Dry graded timber is assessed when the batch of timber has an average moisture content of 20% or less, with no reading exceeding 24%. It is marked DRY or, if kiln dried may be marked KD.
Dry graded timber should be used in Service Classes 1 and 2 (see below). The exception to this rule is timber with a target thickness of 100mm or more which is difficult or slow to dry and is therefore graded and marked WET.
Wet graded timber is assessed at a moisture content above 20% and is marked WET. Wet graded timber should be specified for use in Service Class 3 or when the timber has a target thickness of 100m or more.
BS 5268 Part 2 defines three Service classes:
Service Class 1 – characterised by a moisture content in the materials corresponding to a temperature of 20°C and the relative humidity of the surrounding air only exceeding 65% for a few weeks each year. In such conditions most timber will attain an average moisture content not exceeding 12%.
Service Class 2 – characterised by a moisture content in the materials corresponding to a temperature of 20°C and the relative humidity of the surrounding air only exceeding 85% for a few weeks each year. In such conditions most timber will attain an average moisture content not exceeding 20%.
Service Class 3 – due to climatic conditions is characterised by higher moisture contents than Service Class 2.
Marking
Hardwoods strength graded in accordance with BS 5756 should be marked with:
- the species and grade
- the company responsible for the grading
- the certification body and the BS number, BS 5756
- DRY, KD or WET as appropriate.
Strength classes
Timbers of similar strength properties are grouped together into strength classes. These are defined in BS EN 338 which includes six strength classes for hardwoods, D30 – D70, see Table 1.
Strength classes offer a number of advantages both to designers and suppliers. Note that poplar, which is a hardwood is included in the strength classes for softwoods (C14 – C40). The wood information Sheet 4-21 European strength classes and strength grading gives more details
BS EN 338 gives characteristic values for strength and stiffness properties for each strength class. These are used in the limit states design system laid down in Eurocode 5 and are derived directly from test values, taking no account of factors of safety or other significant factors which are applied as part of the design process. Table 2 shows the characteristic values for the hardwood strength classes.
BS 5268-2: 1996 is a permissible stress design code which gives grade stresses for the strength classes defined in BS EN 338 and also for the individual hardwood species included. Table 3 shows the BS 5268-2:1996 grade stresses for the hardwood strength classes.
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Table 1 : Strength graded hardwoods assigned to BS EN 338 strength classes
| Species | Strength Class D30 | Strength Class D40 | Strength Class D50 | Strength Class D60 | Strength Class D70 |
|---|---|---|---|---|---|
| Balau | HS | ||||
| Cumaru | HS | ||||
| Greenheart | HS | ||||
| Iroko | HS | ||||
| Jarrah | HS | ||||
| Kapur | HS | ||||
| Karri | HS | ||||
| Kempas | HS | ||||
| Keruing | HS | ||||
| Merbau | HS | ||||
| Oak* | THI THB | THA | |||
| Opepe | HS | ||||
| Teak | HS | ||||
| *Note that the TH2 grade of oak does not meet the requirements for the D30 strength class. Designs using TH2 grade oak should be based on the grade stresses given in BS 5268-2 for the individual species and grade. | |||||
Table 2 Characteristic values for hardwood strength classes (BS EN 338)
| Strength Properties N/mm² | Strength Class D40 | Strength Class D50 | Strength Class D60 | Strength Class D70 | Strength Class D80 |
|---|---|---|---|---|---|
| Bending | 30 | 40 | 50 | 60 | 70 |
| Tension parallel to grain | 18 | 24 | 30 | 36 | 42 |
| Tension perpendicular to grain | 0.6 | 0.6 | 0.6 | 0.7 | 0.9 |
| Compression parallel to grain | 23 | 26 | 29 | 32 | 34 |
| Compression perpendicular to grain | 8 | 8.8 | 9.7 | 10.5 | 13.5 |
| Shear | 3 | 3.8 | 4.6 | 5.3 | 6 |
| Stiffness properties kN/mm2 | Strength Class D30 | Strength Class D40 | Strength Class D50 | Strength Class D60 | Strength Class D70 |
|---|---|---|---|---|---|
| Mean MOE parallel to grain | 10 | 11 | 14 | 17 | 20 |
| 5th percentile MOE parallel to grain | 8 | 9.4 | 11.8 | 14.3 | 16.8 |
| Mean MOE perpendicular to grain | 0.64 | 0.75 | 0.93 | 1.13 | 1.33 |
| Mean shear modulus | 0.6 | 0.7 | 0.88 | 1.06 | 1.25 |
| Characteristic density kg/m³ | 530 | 590 | 650 | 700 | 900 |
Table 3 : Grade stresses and moduli of elasticity foor Service Classes 1 & 2 (BS 5268-2)
| N/mm² | Strength Class D30 | Strength Class D40 | Strength Class D50 | Strength Class D60 | Strength Class D70 |
|---|---|---|---|---|---|
| Bending parallel to grain | 9 | 12.5 | 16 | 18 | 23 |
| Tension parallel to grain | 5.4 | 7.5 | 9.6 | 10.8 | 13.8 |
| Compression parallel to grain | 8.1 | 12.6 | 15.2 | 18 | 23 |
| Compression perpendicular to grain* | 2.8 / 2.2 | 3.9 / 3.0 | 4.5 / 3.5 | 5.2 / 4.0 | 6.0 / 4.6 |
| Shear parallel to grain | 1.4 | 2 | 2.2 | 2.4 | 2.6 |
| Modulus of elasticity : Mean | 9500 | 10800 | 15000 | 18500 | 21000 |
| Modulus of elasticity : Minimum | 6000 | 7500 | 12600 | 15600 | 18000 |
| Average density kg/m³ at 20°C/65% RH | 640 | 700 | 780 | 840 | 1080 |
| *When specification excludes wane at bearing areas, the higher value of compression perpendicular to grain stress may be used, otherwise the lower values apply. | |||||