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Timber Species

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Explanatory Notes on Timber

Basic Timber Properties

Property	S1	S2	S3	S4	S5	S6	S7
Basic density
K/m3	900	760	640	540	450	375	320
Modulus rupture
Megapascals	103	86	73	62	52	43	36
Modulus of elasticity
Megapascals	16300	14200	12400	10700	9100	7900	6900
Maximum crushing stength
Megapascals	52	43	36	31	26	22	18
Maximum shear stength
Megapascals	13	11	9.1	7.7	6.6	5.5	4.6

Density – the weight ofwood in kilograms that would occupy a volume of one cubic metre at a given moisture content of 12%. Basic density is the weight of wood at 0% moisture content in kilograms that would occupy a volume of one cubic metre when in the green condition.

Strength –it is important to know and understand the strength properties of timber to permit proper design specifications and to ensure the efficient use of each species. The Australian system of strength groups has been adopted and is:

Modulus of Rupture – is the measure of the maximum compression or tensile stress in the fibres at the point of fracture – it is a direct measurement of the strength of wood in bending.

Modulus of Elasticity – is the measure of the stiffness or rigidity of wood e.g. for a beam the modulus of elasticity is a measure of its resistance to deflection.

Maximum crushing strength(compression parallel to grain) - is a measure of the maximum stress sustained by the timber under a load slowly applied parallel to the grain. It indicates the relative suitability of timbers for columns.

Shear – is a measure of timber to resist slipping of one part upon another along the grain.

Natural Durability

The Australian system of classification has been adopted. The general classification is based on the resistance of sound untreated heartwood of a timber species to attack by wood destroying fungi, borer, and insect attack. The sapwood of all species should be regarded as non-durable.

No attempt has been made to include resistance tomarine borers in this classification.

Permeability

Timbers have been classified according to the ease of penetrating the heartwood with preservatives under pressure at 1380 kilopasclas.

Sapwood – Lyctus Susceptibility

The lyctusborer lays eggs in timbers with pore size over 0.09mm, in diameter. In timbers not exposed to lyctus attack, the starch content of sapwood, together with pore size is used to predict susceptibility.

Two ratings are used.

Lyctus susceptible

Not susceptible to lyctus

Shrinkage

Class	Classification for untreated heartwood
Class 1	Very Durable
	Suitable for long use in structures exposed to the weather and in contact with
	the ground
Class 2	Durable
	Suitable for use in the ground and for unprotected exterior use under normal
	conditions
Class 3	Moderately durable
	Suitable for protected exterior work and for interior use. Not suitable for
	use in contact with the ground.
Class 4	Non durable
	Not suitable for exterior use unless treated with preservative

Class
Class 1	Treatable through full section, ideal for treatment @ 1380 kilopascals
Class 2	Wide treatable sapwood ad / or reasonably uniform treatment in heartwood ,
	suitable for treatment at 1380 kilopascals
Class 3	Resistant, erratic non-dependable penetration of heartwood
Class 4	Highly resistant, comparatively narrow sapwood or untreatable heartwood

	Shrinkage from green to oven dry
Shrinkage Classification	% tangential	% radial
Very low	0-3.5	0-2.0
Low	3.5-5.0	2.0-3.0
Medium	5.0-6.5	3.0-4.0
High	6.5-8.0	4.0-5.0
Very high	over 8.0	Over 5.0

Timber shrinks by different amounts in the three cardinal directions

In the longitudinal section i.e along the grain shrinkage is usually insignificant
Shrinkage in the tangential direction for most species is about twice as great as in the radial section (T) is parallel to growth rings and the radial section (r) is at right angles to the growth rings.

Movement

Timber in service will equilibrate with the moisture in the atmosphere. In most instances this will be between 12 to 20%.

In this range the change in dimension, which is known as “movement”, is proportional to the moisture content.

The movement figures are given in the tangential (T) and the radial (R) directions. The figures given are the percentage change in dimension, for each percent change in moisture content.