A2: Strength

Wood is an anisotropic material with different strength properties in different directions. Its strength is directly dependent on the grain direction; axial, radial or tangential. Its properties also change with environmental conditions. The properties not only vary from species to species but even within the same species. To be able to design timber structures successfully, the practising engineer needs to be aware of the particular properties of the timber being specified.

 

Timber properties

To assist the designer, species with similar strength properties are grouped together in the same strength class and are thus inter-changeable in the design process. This simplifies the selection process and extends the range of materials available to the designer specifier.

European Standard EN 338 Structural Timber Strength Classes outlines the characteristic strength and stiffness properties for softwoods and hardwoods. The strength classes for softwoods range from C14 to C40 and for hardwoods they range from D30 to D70. The timber visual strength grades and strength classes for some commonly used softwood species are shown below. The most commonly available strength class for general carcassing work is C16.


Table A 2.1.1 Softwood grades and Strength Classes

Origin	Timber Species	Strength Grade IS127/BS4978	Strength Class (IS444/EN338)
Ireland/U.K.	Douglas fir	GS SS	C14 C18
	Larch	GS SS	C16 C22
	Lodgepole pine	GS SS	C14 C18
	Norway spruce	GS SS	C14 C18
	Scots pine	GS SS	C16 C22
	Sitka spruce	GS SS	C14 C18
Europe	Redwood	GS SS	C16 C24
	Whitewood	GS SS	C16 C24
Canada/USA	Douglas fir/Larch	GS SS	C16 C24
	Hemlock /Fir	GS SS	C16 C24
	Spruce - Pine - Fir	GS SS	C16 C24

Note: Timber can be machine graded directly into any strength class

Table A 2.1.2 Hardwood grades and strength classes

Origin	Timber Species	Strength Grade BS 5756	Strength Class
Ireland/U.K.	Oak	TH 1	D 30
	Oak	TH A	D 40
	Oak	TH B	D 30
USA	Am. White oak	TH 1	D 50
	Am. Red oak	TH 1	D 40
	Am. Ash	TH 1	D 35
	Am. Tulipwood	TH 1	(D 40)*
Tropical	Balau	HS	D 70
	Greenheart	HS	D 70
	Ekki	HS	D 60
	Keruing	HS	D 50
	Iroko	HS	D 40

Notes.

1. Strength Class allocation for Irish/ UK oak from values given in Table 7 of BS 5268 Pt2.  
2. Strength Class allocation for USA species from tests carried out at BRE in accordance with EN 408 & EN 384. * Tulipwood meets the requirements for strength & stiffness of D 40 but not in density.  
3. Strength Class allocation for tropical timber from Table 6 of BS 5368 Pt 2 /Table 2 of EN 1912 (Proposed EN 338 to follow)

Strength grading

Timber should be visually strength graded in accordance with IS127/BS4978 or mechanically strength graded in accordance with EN 519.

Note: BS 4978 can be considered synonymous with IS127.

American species listed in Table A2.1.2 must be visually strength graded to BS 5756 (to be replaced by EN 14081-1). Tropical species must be graded to Grade HS of BS 5756.

Strength grading and strength classification shall be carried out by an approved person and/or machine, operating under an independent quality control scheme.

 

Strength classification

The classification of timber into a particular strength class is undertaken on the basis of:

 

a combination of the species and visual strength grade as shown in table A 2.1.1 or

 

 

directly by use of a machine.

 

 

Permissible stresses

The permissible stresses and moduli of elasticity values, for the strength classes shown in table A 2.1.1 are given in table A 2.2.1. The permissible stress values in design are determined by applying appropriate modification factors to the stress values shown. Refer to IS444 and/or BS 5268 Part 2 for further information.

Table A 2.2.1 Softwood strength classes (permissible stress and moduli values)

Properties	Strength classes
	C14	C16	C18	C22	C24	C27
Stress Bending Tension Compression parallel to grain Compression perpendicular to grain Shear parallel to grain	MPa 4.1 2.5 5.2 1.6 0.6	MPa 5.3 3.2 6.8 1.7 0.6	MPa 5.8 3.5 7.1 1.7 0.6	MPa 6.8 4.1 7.5 1.7 0.7	MPa 7.5 4.5 7.9 1.9 0.7	MPa 10.0 6.0 8.2 2.0 1.1
Modulus of elasticity Emean Emean 1MPa=1N/mm2	MPa 6800 4600	MPa 8800 5800	MPa 9100 6000	MPa 9700 6500	MPa 10800 7200	MPa 12300 8200
NOTE: The characteristic strength, stiffness and density values for each of the strength classes is given in EN 228.

 

Loading

Joist sizes for domestic and commercial construction can be selected from span tables listed in IS 444 1998 "The Use of Structural Timber in Building". Tables A 2.3, A 2.4 and A 2.5 list permissible spans for a number of joist sizes and spacing appropriate to six strength classes, C14, C16, C18, C22, C24 and C27. Design loadings can be obtained from BS 6399.

Since all timbers are now specified on the basis of strength class to either IS 444 or BS 5268, the supplier or contractor has the option of supplying any one of several species within the strength system. Refer to the table A 2.2.1 for relative properties of the various strength classes.

Permissible stresses for hardwoods

Table A 2.2.2.1 Permissible stress values for American hardwoods for design in accordance with BS 5268 Pt. 2

 

 

 

PROPERTY	AMERICAN WHITE OAK	AMERICAN RED OAK	AMERICAN ASH	AMERICAN TULIPWOOD
Bending parallel to grain	17.3 N/mm2	17.6 N/mm2	12.5 N/mm2	14.6 N/mm2
Tension parallel to grain	10.3 N/mm2	10.5 N/mm2	7.5 N/mm2	8.8N/mm2
Tension perpendicular to grain	0.7 N/mm2	0.8 N/mm2	0.6 N/mm2	0.7 N/mm2
Compression parallel to grain	12.2 N/mm2	12.2 N/mm2	10.5 N/mm2	11.3 N/mm2
Compression perpendicular to grain	4.6 N/mm2	4.6 N/mm2	3.3 N/mm2	3.9 N/mm2
Shear parallel to grain	2.2 N/mm2	2.3 N/mm2	1.8 N/mm2	2.0 N/mm2
Mean modulus of elasticity parallel to grain	14100 N/mm2	12200 N/mm2	12000 N/mm2	11300 N/mm2
Minimum modulus of elasticity parallel to grain	10700 N/mm2	8400 N/mm2	8000 N/mm2	7800 N/mm2
Mean modulus of elasticity perpendicular to grain	700 N/mm2	610 N/mm2	600 N/mm2	570 N/mm2
Mean shear modulus	880 N/mm2	760 N/mm2	750 N/mm2	700 N/mm2
Characteristic density	709 Kg/m3	633 Kg/m3	634 Kg/m3	470 Kg/m3
Average density	835 Kg/m3	700 Kg/m3	687 Kg/m3	568 Kg/m3

Table A 2.2.2.2 Permissible stress values for European oak (from BS 5268 Part 2.)

PROPERTY	GRADE TH1	GRADE TH2	GRADE THA	GRADE THB
Bending parallel to grain	9.6 N/mm2	7.8 N/mm2	12.6 N/mm2	9.1 N/mm2
Tension parallel to grain	5.8 N/mm2	5.8 N/mm2	7.6 N/mm2	5.5 N/mm2
Compression parallel to grain	9.3 N/mm2	9.3 N/mm2	10.5 N/mm2	9.0 N/mm2
Compression perpendicular to grain	3.0 N/mm2	3.0 N/mm2	3.0 N/mm2	3.0 N/mm2
Shear parallel to grain	2.0 N/mm2	2.0 N/mm2	2.0 N/mm2	2.0 N/mm2
Mean modulus of elasticity	12500 N/mm2	10500 N/mm2	13500 N/mm2	12000 N/mm2
Minimum modulus of elasticity	8500 N/mm2	7700 N/mm2	10500 N/mm2	7500 N/mm2

Tables A 2.3/4/5 Floor joists (domestic/office with/without partition) (pdf 836Kb) 

Bridging

Bridging between joists is used to stiffen the whole floor and is inserted at intervals according to the joist depth to breadth ratio and depending on the span, either one, two or multiple rows of bridging may be required. For joists with spans over 2.7m intermediate bridging shall be provided at 1.35m spacing. Bridging may be achieved by using:

(a) solid timber, or

(b) timber herringbone bridging.

It is important to note that, at the end of the lines of bridging adjacent to a wall, solid packing pieces will be required between the last joist and the wall.

Notching of joists

In normal situations, services in floors will either run parallel to joist spans or, when this is not possible, through the joists themselves. Obviously the reduced depth due to notching or by the provision of holes must be taken into account. Joists may be notched or drilled as shown below; joists must be designed for notching or drilling outside the limits shown.

Fig. A 2.1 Notching and drilling (98Kb)