Strength groups and stress grades

This guide describes timber strength groups and their relationship with the stress grades specified for structural applications.

Use the quick reference tool to determine the minimum strength group required to meet a specified stress grade. Stress grades table

In this guide:


Strength groups

A timber’s strength represents the ability of that timber to maintain stress without failure.

A strength group represents the inherent strength of all the timber species in that group. There are seven strength groups for unseasoned timber (S1, strongest, to S7, weakest) and eight for seasoned timber (SD1, strongest, to SD8, weakest).

S = unseasoned (green) timber strength group
SD = seasoned (dry) timber strength group

The strength group classification is defined in the standard: AS/NZS 2878-2000 Timber—classification into strength groups6. Seasoned and unseasoned timber have different strength groupings due to differences in the mechanical properties of defect-free timber of a given species in each condition. Seasoned (dry) timber has better stiffness properties than unseasoned (green) timber of the same species and the same dimensions and quality (except for white cypress, which has similar properties in both conditions).

S and SD group classifications (without brackets) were determined from mechanical test data using small, clear specimens.

(S) and (SD) group classifications (in brackets) are provisional, based on density and/or limited mechanical test data. Provisional classifications are conservative, so they can be used with confidence.

Individual timber species: strength values are determined from mechanical test data for small, clear specimens; timbers are grouped accordingly.

Species groups or broader categories (e.g. ‘mixed hardwoods’, ‘pine’, ‘spruce-pine-fir’): these are grouped according to the lowest species-strength rating. Rare exceptions occur where it is known that the lowest strength-rated species has only a small influence on the mechanical properties of the species mixture as marketed.

Timber strength groups are related to timber stress grades specified for building applications.


Stress grades

Structural timber is generally sold as a (stress) graded product. A stress grade is the classification of a timber when used in structural applications1.

Stress grades are derived from either visual- or machine-grading, which specify the stress limits that apply to timbers used for structural applications.

Stress grades are known by either:

  • F’ grades F4 to F34: E.g. F14 indicates that the basic working stress (in bending) for that timber is around 14 MPa. Most native forest structural timbers such as white cypress and eucalypt hardwoods are graded using the F grade system; or
  • Machine-graded pine MGP, MGP10 to MGP15: E.g. MGP10 indicates a minimum threshold for stiffness properties of 10,000 MPa. Most exotic plantation softwoods (Pinus species) are graded using this system.

Timber to be used in a specific structural application must meet the required stress grade specified by the architect/designer.

Grading methods

Grading methods used to predict the strength of a timber, are either machine-based or visually-based, and each one described in an Australian Standard. The system of stress grading is fundamental to the structural use of timber1,2,8.

  • Visual grading: stress grades determined by the inherent strength (strength group) of the species and the visual quality (grade) of the timber piece. The structural properties of individual stress grades are described in Australian Standard specifications1,7. These identify the limits on the size or extent of characters such as knots or sloping grain that reduce strength.
  • Machine grading: stress grades determined by mechanical grading, using a technique that correlates strength directly with stiffness.

Stress grades are allocated to timbers using the strength group classification in conjunction with the grade-quality descriptions from relevant Australian Standards for structural timbers.

Some definitions

‘F’ grades: A stress grade traditionally allocated to a timber based on its strength group and a visual or machine stress grading system. Higher F grade numbers are given to higher strength groups.

Visual stress grading system:  Stress grades determined by the visual properties of small, clear pieces.

Machine stress grading system:

  • MGP: Machine-graded pine. MGP10 can be used where F5 is specified, MGP12 can replace F8 and MGP12 can replace F11.
  • SP: Machine graded pine for structural application.

Structural class/visual grade: Structural classes/visual grades 1 to 4 (1 to 5 for softwoods) to which stress grades F4 to F34 apply.


Note 3#: Stress grades

#Notes are numbered as they are included in the publication Construction Timbers in Queensland

Stress grades (and hence structural properties) for a species or species group are allocated using several methods. In order of precedence, these are:

  1. Determination by full-size, in-grade testing in accordance with AS 4063—2010. Timber—stress graded—in-grade strength and stiffness evaluation5 and associated parts, amendments and supplements.
  2. Confirmation of traditional ‘F’ grades (based on strength group or visual grade relationships) using full-size in-grade testing in accordance with AS 4063—2010. Timber—stress graded—in-grade strength and stiffness evaluation5 and associated parts, amendments and supplements.
  3. Allocating traditional ‘F’ grades based solely on strength group or visual grade relationships determined from small, clear properties testing9.

Stress grade allocation to timber species and species groups

1. Stress grades determined by in-grade testing

The following species or species groups have had stress grades established by full-size, in-grade testing programs:

  • seasoned and unseasoned white cypress, designated F4, F5 and F7
  • seasoned hoop pine, designated F5, F7 and F8
  • seasoned plantation Pinus species (maritime, slash, radiata and Caribbean pines) designated *MGP10, MGP12 and MGP15, as well as *SP1, SP2, SP3 and SP4
  • seasoned Victorian ash, designated A17.

*MGP: machine-graded pine; SP: machine-graded pine for structural application.

The stress grades and structural properties allocated to these species follow AS 1720.1 & AS 1720.21,2.

2. Traditional stress grades confirmed or adjusted by in-grade testing

The following species or species groups have had traditional stress grades confirmed by full-size, in-grade testing:

  • north-eastern Australian hardwoods, including spotted gum, blackbutt, ironbarks
  • north American Douglas fir
  • spruce-pine-fir
  • Australian-grown plantation Pinus species, including slash, maritime, radiata and Caribbean pines.

The stress grades and structural properties allocated to these species follow AS 1720.1 & AS 1720.21,2.

3. Traditional stress grades (‘F’ grades) based on strength grouping

For species or species groups not covered by in-grade testing, stress grades can be determined from the table and their respective strength groups. For structural properties, refer to AS 1720.11.

Stress grades in relation to strength group and visual grade for unseasoned (S) and seasoned (SD) timber*

Unseasoned strength group→

 

 

S1

S2

S3

S4

S5

S6

S7

Seasoned strength group→

SDI

SD2

SD3

SD4

SD5

SD6

SD7

SD8

 

Visual grades

Stress grades

 

Structural 1

F34

F34

F27

F22

F17

F14

F11

F8

F7

Structural 2

F34

F27

F22

F17

F14

F11

F8

F7

F5

Structural 3

F27

F22

F17

F14

F11

F8

F7

F5

F4

Structural 4

F22

F17

F14

F11

F8

F7

F5

F4

Structural 5** - Softwoods only

F7

F5

F4

Source: AS 2878-2000 Timber—classification into strength groups6

* This table does not apply to white cypress (Callitris glaucophylla), which has equivalent strength properties in both seasoned and unseasoned conditions, with available stress grades of F7, F5 and F4.

** The visual grade ‘Structural 5’ only applies to softwoods (eg, Pinus, Picea, Larix, Araucaria and Abies species).

Branding and marking graded timber

Stress grades applicable to timber graded by a mechanical process are recorded on the piece by branding or colour-coded markings in accordance with the relevant Australian Standard. Applicable structural properties and other engineering design data (including joint design) are published in AS 1720.11.

Moisture content affects F grade allocation to structural timber that has been seasoned or partially seasoned.

The F grades applicable to seasoned structural timber apply only when the average moisture content in the piece does not exceed 15% at the time the main gravity loads are applied. Timber that is partially seasoned to not more than 20%, for use in certain applications, must be allocated an F grade appropriate to the unseasoned timber strength or joint group, or be specially designed in accordance with the requirements of AS 1720.11.

Note 4:   Manufactured products

Most engineered and panel products such as glued, laminated timber (glulam), laminated veneer lumber (LVL) and I-beams have specific structural properties for each proprietary product or grade. Characteristic properties of these materials can be obtained from the appropriate Australian Standard or the product manufacturer or relevant industry association (e.g. Glued Laminated Timber Association of Australia GLTAA, for glulam and Engineered Wood Products Association of Australasia, EWPAA, for plywood and LVL).

Note 5: Framing sizes

Framing sizes, spans, spacings and other design information for timber species of a particular stress grade may be obtained from the Australian Standard AS 1684 series3,4, or may be derived by calculation, based on the requirements of the Australian Standard AS 1720.11 or from manufacturers or industry association information.

Australian Standards referred to in this guide.

1AS 1720.1-2010. Timber structures—design methods.

2AS 1720.2-2006. Timber structures—timber properties.

3AS 1684.2-2010. Residential timber-framed construction – Non-cyclonic areas.

4AS 1684.3-2010. Residential timber-framed construction – Cyclonic areas.

5AS 4063-2010 Timber—stress graded—in-grade strength and stiffness evaluation

5aAS/NZ 4063.1-2010. Characterisation of structural timber. Part 1: Test methods

5bAS/NZ 4063.2-2010. Characterisation of structural timber. Part 2: Determination of characteristic values.

6AS 2878-2000 Timber—classification into strength groups.

7AS 2082-2007 Timber – hardwood – visually stress-graded for structural purposes

Other

8Building Code of Australia (Queensland Building Act 1975)

9Mack, J.J. (1979). Australian methods for mechanically testing small clear specimens of timber. CSIRO Division of Building Research technical paper. Second series; no. 31. p 19.

Last modified
05/21/2017 - 23:20