Farm Forestry New Zealand

 NZS 3603 Timber Structures Standard

This Standard specifies characteristic properties and methods for determining design strengths for timber structures.
This standard has been written on the assumption that it will be used for design purposes by qualified professional engineers with some knowledge and experience of the specialised techniques necessary for the design and construction of timber buildings.
*C 1.5
 

Characteristic stresses

In recent years in-grade testing has provided a means of establishing characteristic stresses for building timbers and, where sufficient information is available, stress levels have been set on this basis rather than as previously derived from the testing of small clear specimens.*Foreword
For strength properties, characteristic stress or strength is an estimate of the lower 5-percentile value determined with 75% confidence, from tests on a representative sample of full size test specimens. For stiffness properties, the characteristic value is the mean value.*CHARACTERISTIC STRESS or CHARACTERISTIC STRENGTH, 1.3 Definitions

As an alternative to determining characteristic stresses for timber, Timber buildings or parts of buildings may be test loaded as specified in section 10 of this Standard, and such tests demonstrate that the construction is adequate for its intended purpose it shall be accepted as complying with this Standard.*1.5.3

Tests may be accepted as an alternative to calculation or may become necessary in circumstances which include:
(a) Where a structure or part of a structure is not amenable to sufficiently accurate calculation
(b) Where materials or design methods are used other than those of the relevant specification or code of practice
(c) Where there is doubt or disagreement as to whether the structue or some part of it complies with design rules, or as to whether the quality of the materials used is to the required standard.*C1.5.3

NZS3602:2003 Timber and wood-based products for use in building

The relevant requirements of NZS 3602 shall apply subject to the requirements of this Standard.*1.7 Materials and workmanship

Moisture Content

In the determination of design strengths, timber shall be assumed to be in the dry condition or in the green condition according to its moisture content.*2.1.1
Stresses for the dry condition refer to an annual average moisture content of 16%, which by reference to NZS 3602 implies a maximum of 18%.*C2.1.2
When timber not exceeding 100 mm thick is graded, fabricated, and installed at a definable moisture content between 18% and 25% and will not exceed that moisture content in service, the characteristic stress (see 2.3) may be obtained by linear interpolation between the values for green and for dry timber. For the purposes of interpolation "dry" shall be taken to mean 16% moisture content and "green" to mean 25% moisture content. In such cases dimensions shall be assumed to be dry dimensions (see 3.1.2).*2.1.2
Members exceeding 100 mm thick, unless built up from thinner timbers, shall be assumed to have a moisture content exceeding 18% at the time of installation unless proved otherwise by a special investigation.*2.1.4

NZS 3603:1993 Amendment No. 4 March 2005

Timber species

Tables 2.2 and 2.3 only give characteristic stress values for visually and mechanically graded radiata pine and Douglas fir timber.

The grade stresses for Larch, Rimu, Kahikatea, Silver, Red and Hard Beech have been disestablished because these species are rarely used in new structures today. Larch, if used, can be bracketed with Radiata pine.*EXPLANATORY NOTE, Amendment No. 4

Timber of species and grades not listed in tables 2.2 and 2.3 may be assigned characteristic stresses on the basis of evidence establishing the stresses at the 5% exclusion limit and elastic moduli based on mean values.*2.3 Properties of timber species not listed

For species listed in AS 1720 but not included in table 2.2, the stress values given in AS 1720 for the visual grade concerned may be used. For laminated veneer lumber (LVL) characteristic stresses should be determined as in AS/NZS 4063.*C2.3
(Note, an explanation of Australian structural grading is given here)

Characteristic Stresses and verification

Characteristic stresses and elastic moduli shall be given in tables 2.2 and 2.3 for the appropriate species, grade, and dry or green condition.*2.2.1

Amendment No. 4 March 2005 deleted clauses and replaced them with the following:

For the derivation of characteristic stresses for timber refer to AS/NZS 4063. The characteristic stresses shown in tables 2.2 and 2.3 for radiata pine and Douglas fir are representative of most exotic pine species subject to verification where specified.*C2.2.1

Its interesting to note that for radiata pine:
The characteristic stresses given in table 2.2 are, with the exception of fb  (characteristic extreme fibre stress in bending parallel to the grain) and E (modulus of elasticity parallel to the grain) for radiata pine, obtained from the results of testing of small clear specimens of timber in the standard 20 x 20 mm dimension and in both the green and air-dry condition. For radiata pine in bending, the characteristic stresses are obtained from the results of tests on structural sized beams.*C2.2.2
However,
Amendment No. 4 to NZS 3603 provides for lower design stresses for unverified timber. It recognises deficiencies in sole reliance on visual grading as a means of reliably establishing the characteristic strength and stiffness properties of sawn timber.*EXPLANATORY NOTE, Amendment No. 4

Visually graded timber shall be assigned the design parameters given in table 2.2 depending on whether it is verified or un-verified. Verified timber shall have its bending strength and stiffness (MoE) confirmed, and be identified, in accordance with the requirements of NZS 3622. Timbers not conforming to NZS 3622 shall be considered as un-verified.*2.2.1.1 Visually graded timber

No. 1 framing is not verified and not subject to in-mill monitoring of strength and stiffness properties. No. 1 framing shall be graded to the requirements of NZS 3631.*Note 1 Table 2.2 Characteristic stresses for visually graded timber

Timber Grades provided for in NZS 3603

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