Structural grading

Structural grading is the process by which timber is sorted into groups - or stress grades - with ideally, similar structural properties in each group. Inevitably there is a very substantial range of properties within a group and significant overlap in properties between the groups. Structural grading can be performed in a number of ways including the following:

• Visual stress-grading 

• Machine stress-grading 

• Machine proof-grading

The two most commonly used methods in Australia are visual stress-grading and machine stress-grading, with use of machine proof-grading being limited.

In theory, any method that sorts timber into groups of material with similar structural properties could be used. However, in order to be valid, the sorting methods must produce results, which are both consistent and repeatable. To achieve this level of reliability, rules for using acceptable sorting methods have been developed. In most countries, including Australia, these rules are generally either industry standards or national standards.

The relevant Australian Standards are:

• AS/NZS 1748 Timber Stress-graded Product requirements for mechanically stress-graded timber

• AS 2082 Visually stress-graded hardwood for structural purposes

• AS 2858 Timber softwood - visually stress-graded for structural purposes

• AS 2878 Timbers classification into strength groups

• AS 3519 Timber Machine Proof Grading

These Standards are used in production as the basis of fixing a grade stamp to each piece of timber. Design professionals do not normally have to be expert in the use of these Standards, but should be aware of their existence as they are often referred to, or used in materials specifications. A basic knowledge of the principles of visual stress-grading is certainly useful for engineers and architects undertaking site inspections and BCA certification.

Stress grades
Stress grades are only used for structural timber products. Appearance grades are not stress grades. The end product of any stress grading system is packages of timber that have been assigned grade properties that can be used for design. While in the past, F-grades have been the common timber grade designations, over the past 15 years or so other grade designations have been derived for in-grade tested pine (MGP grades), Australian ash (A-grades) and engineered products such as glulam (GL grades), plywood and LVL (which both have their own independent grades).

Two different methods have been used to derive design properties for grades of timber:

• F-grades found in AS 1720.1 Table H2.1. This information is derived from strengths on the results of tests on small clear specimens and discounted for characteristics such as knots. The properties for lesser grade structural timber (lower than F17) graded in this way have a lower reliability and attract a lower capacity reduction factor AS1720.1 Table 2.1.

• In-grade testing and/or verification - deriving the respective grade properties using in-grade test data.

An overview of the structural grading systems is provided in the presentation Structural Grading which is available for download below.

The F-grade system
An F-grade is a name for the grouping of the timber. The F-grade system gives a key to characteristic design strengths for graded structural timber without having to determine different properties for each of the thousands of timbers milled for structural purposes worldwide. The F-grades are a series of categories into which different grades of different species can be placed. Once a species has been commercialised, its basic engineering properties can be determined on the basis of a few tests on small clear specimens of that species. The results will determine which series of F-grades will suit that species.

The actual properties associated with the F-grades can be obtained from the Timber Structures Code. For example F8 timbers have the following properties (from AS 1720.1 Table H2.1):

f 'b = 22 MPa the characteristic bending strength
f 'c = 18 MPa the characteristic compression strength (parallel to the grain)
f 't = 13 MPa the characteristic tensile strength (parallel to the grain) - hardwoods
f 's = 2.2 MPa the characteristic shear strength
E = 9100 MPa the characteristic modulus of elasticity parallel to grain

However, almost all species have some properties that do not fit the F grade system perfectly. In nearly all cases where there is not a good match with all properties, conservative design properties are used.

TABLE H2.1

CHARACTERISTIC VALUES FOR DESIGN—F-GRADES—BENDING AND SHEAR FOR BEAMS, TENSION, COMPRESSION AND ELASTIC MODULI PARALLEL TO GRAIN

Significance of an F-grade classification

An F-grade is the designation attached to a graded parcel of timber. It can be verified by testing a large number of pieces from the group. However, natural variation in the types of characteristic found in commercially available timber, and the loose correlation between grading parameters and the actual strength of the timber, means that there is a wide range in the strength of single lengths of timber in any grade. Coefficient of Variation for the flexural strength of timber from a single species, single size and single grade are often more than 20 per cent.

For example, it is impossible to say that any one piece of F8 timber will have a flexural strength of 22 MPa. It has a very high probability of exceeding 22 MPa, and there is even a finite, but very low probability that it may not reach 22 MPa. The 22 MPa is an estimate of the fifth percentile strength of the whole population of F8 timber even though it may have been derived from the test results on a small sample with corrections to allow for sampling errors.

The F-grade properties

The F-grade properties published in the Standard represent a system of structural properties on a logarithmic scale, where the increase in properties between grades increases with the grade. Ratios between compressive, bending and tensile strengths allow most commercially available species in Australia to be accommodated in the system. Timbers can be classified by a producer in an F-grade by three methods as follows:

• For a visual grading operation where testing of small clear specimens has been performed in the past, a strength group will have been assigned to the species and can be found in either AS 2878, AS 1720.2 or AS 1720.1. The progression from a structural grouping to an F-grade can be made by using the data given in the relevant visual grading rules AS 2082 or AS 2858.

• For visual grading of a species where the timber species does not appear in AS 1720.2, a conservative F-grade may be awarded by some simple testing. An appropriate small clear testing program can be initiated and F-grades awarded using the factors given in AS 2878 to assign a provisional strength grouping. The data in the visual grading rules can then be used to assign an F-grade to the timber.

• For production grading by any method, where in-grade testing has been performed on commercial sized timber pieces, the characteristic properties can be used to assign an F-grade for which the F-grade properties exceed all of the characteristic properties found in the testing program.

For a guide as to the performance of timber species within this grading species additional information can be found within the document Timber Species and Properties, available for download below.

MGP grades
MGP was introduced into the market place in 1996 by Pine Australia (now Plantation Timber Association Australia), following an extensive, nation wide in-grade testing program of Australian Pine (radiata pine, pinaster pine, slash pine and Caribbean pine), undertaken by CSIRO and State Forests of NSW. The MGP grades are the result of a substantial research and development program by the pine industry to ensure that accurate and reliable design properties are available for structural pine timber in Australia.

As a result of this testing program, some MGP structural properties reflect the performance of pine better than the properties derived using the traditional F-grade system. These properties are appropriate for higher levels of confidence in engineering design.

The table below shows the properties for MGP12 compared with those tabulated in AS 1720.1 Table H2.1 for F8 material (its closest F-grade equivalent). The discrepancy in properties is minimal for tensile strength but significant for compressive and shear strengths and stiffness.

Table - In-grade properties for MGP12 compared with those for F8 material

The benefits of increased reliability for MGP products are not only based on MGP having more accurate information on the grade properties, but also on the fact that all mills producing MGP are subject to stringent third party auditing. This assesses appropriate machine grading, and monitors property and other quality control procedures to ensure the validity and consistency of the MGP design properties.

GL grades
Glulam products are engineered wood products, made in accordance with AS/NZS 1328.

AS/NZS 1328 defines methods for grading glulam products into GL grades and products complying with them have been included in Section 7 of AS 1720.1. These grades are performance targets which are independent of the species, milling procedure and manufacturing process and are based on a 'standard' beam depth of 300 mm. Design procedure using GL grade properties is similar in principle to that for solid timber.

Any producer of glulam products can manufacture to meet a specified GL grade, with the onus of meeting the performance requirement (characteristic properties) resting with the producer. Glulam must be produced in accordance with AS/NZS 1328 and GL grades can be established and/or verified using in-grade testing or calculations from the properties of the laminates. GLTAA (Glued Laminated Timber Association of Australia) member producers have established an industry regulated quality assurance system, which permits the use of the GLTAA Quality Endorsed Mark.

A-grades

A-grades apply to visually graded mixture of seasoned alpine ash and mountain ash. Design properties were awarded and published after extensive in-grade testing at CSIRO in 2000. The timber is produced in Victoria, but marketed elsewhere in Australia. Each piece is marked with a grade stamp and the properties for the A-grades can be found in AS1720.1 Appendix H3.

Other grades
Other grade designations or suites of characteristic properties are available for specific timber products such as plywood and LVL.

The characteristic strength properties for many plywood products presented in AS 1720.1 Table 5.1 have been validated by in-grade testing and whilst they use an F-grade designation, they are different properties to F-graded sawn timber and have a high level of reliability. Australian produced plywood is linked to an audited, quality control and product certification system, when designated with an EWPAA (Engineered Wood Products Association of Australasia) Grade Mark.

The characteristic properties of LVL are determined by in-grade testing by each manufacturer. They also have responsibility for verifying the properties of their production by regular testing and associated quality control. Each manufacturer publishes their own set of design properties which means that each LVL product has its own individual grade.