INTERNATIONAL GRADING COMPARISON

South Africa vs. The World

Issue 1

1.0 INTRODUCTION: 1.1 HOW THIS PUBLICATION SHOULD BE USED AND INTERPRETED; 1.2 THE RECOMMENDED PROCEDURE TO FOLLOW WHEN GOING INTO THE EXPORT MARKET; 1.3 GRADES AND GRADING SYSTEMS THAT WILL BE COMPARED; 1.4 DETAILS OF THE STRENGTH COMPARISON FORMULA USED
2.0 MECHANICAL PROPERTIES OF WOOD: 2.1 BENDING STRENGTH (Fb); 2.2 TENSION PARALLEL TO THE GRAIN (Ft); 2.3 COMPRESSION PARALLEL TO THE GRAIN (Fc); 2.4 MODULUS OF ELASTICITY (E)
3.0 FACTORS AFFECTING THE STRENGTH OF TIMBER: 3.1 DENSITY; 3.2 KNOTS; 3.3 SLOPING GRAIN; 3.4 MOISTURE CONTENT; 3.5 DECAY
4.0 METHODS OF STRESS GRADING FOR STRUCTURAL TIMBER: 4.1 VISUAL STRESS GRADING; 4.2 MACHINE STRESS GRADING; 4.3 PROOF GRADING

Issue 2

1.0 SOUTH AFRICA: 1.1 STRUCTURAL GRADES; 1.2 SPECIES USED; 1.3 DETAILS OF DIMENSIONS; 1.4 DETAILS OF MECHANICAL STRENGTH VALUES; 1.5 GRADES SUMMARY AND END USES; 1.6 GRADE STAMP IDENTIFICATION
2.0 AUSTRALIA: 2.1 DETAILS OF MECHANICAL STRENGTH VALUES; 2.2 SPECIES USED; 2.3 DETAILS OF DIMENSIONS; 2.4 GRADE STAMP IDENTIFICATION; 2.5 GRADE COMPARISON
3.0 NEW ZEALAND: 3.1 DETAILS OF MECHANICAL STRENGTH VALUES; 3.2 SPECIES USED; 3.3 DETAILS OF DIMENSIONS; 3.4 GRADE STAMP IDENTIFICATION; 3.5 GRADE COMPARISON

Issue 3

1.0 EUROPE: 1.1 DETAILS OF MECHANICAL STRENGTH VALUES; 1.2 SPECIES USED; 1.3 DETAILS OF DIMENSIONS; 1.4 GRADE STAMP IDENTIFICATION; 1.5 GRADE COMPARISON
2.0 ISO: 2.1 DETAILS OF MECHANICAL STRENGTH VALUES; 2.2 SPECIES USED; 2.3 DETAILS OF DIMENSIONS; 2.4 GRADE STAMP IDENTIFICATION; 2.5 GRADE COMPARISON
3.0 BRITAIN: 3.1 DETAILS OF MECHANICAL STRENGTH VALUES; 3.2 SPECIES USED; 3.3 DETAILS OF DIMENSIONS; 3.4 GRADE STAMP IDENTIFICATION; 3.5 GRADE COMPARISON

Issue 4

1.0 SOUTHERN UNITED STATES: 1.1 DETAILS OF MECHANICAL STRENGTH VALUES; 1.2 SPECIES USED; 1.3 DETAILS OF DIMENSIONS; 1.4 GRADE STAMP IDENTIFICATION; 1.5 GRADE COMPARISON
2.0 CANADA: 2.1 DETAILS OF MECHANICAL STRENGTH VALUES; 2.2 SPECIES USED; 2.3 DETAILS OF DIMENSIONS; 2.4 GRADE STAMP IDENTIFICATION; 2.5 GRADE COMPARISON
3.0 OVERALL SUMMARY
4.0 REFERENCES

ISSUE 1

1.0 INTRODUCTION

In an effort to compare South African lumber prices internationally by grade, a simple method to compare grades was developed. This methodology forms the basis of this publication, wherein grades are compared on a strength basis by taking into account all four of the most important strength values in structural timber (modulus of elasticity (E), bending stress (Fb), tension parallel to the grain (Ft) and compression parallel to the grain (Fc)).

In this publication each country will be dealt with separately and structured in the following way:

The strength values of each structural grade will be shown.
Where possible the grading stamp given to each grade will be shown and explained.
Finally, the comparison between the foreign grades and the South African grades will be shown using the simple comparison method that was developed.

1.1 HOW THIS PUBLICATION SHOULD BE USED AND INTERPRETED

In considering the exportation of timber, it must be noted that this publication will only assist you in indicating the standard of timber on a strength basis. For example, this publication shows the South African Structural Grades like S5 being comparable with other international grades like the Australian Stress Grade F5, but this does not mean the specifications are identical, they are only comparable on an overall strength basis.

Note: In many instances, even if structural lumber has passed the required strength tests, other factors like the appearance play a major role in the successful sale of the timber and the prolonged satisfaction of the customers.

Thus, this publication should be used to give a comparable idea of the international grades with the South African grades to initiate the process of export. It will also make it easier to determine the competitive price of certain South African grades in each country by determining their local price per comparable grade.

1.2 THE RECOMMENDED PROCEDURE TO FOLLOW WHEN GOING INTO THE EXPORT MARKET

The following steps are recommended in preparation for entering export markets:

Locate the market.
Compare the grades in the desired export country with the South African grades using this publication. This will give a good understanding of the export grade.
Compare the price paid for the equivalent export grade with the South African grade. This will help determine a competitive price.
Obtain the required specifications of the export grade from the customer (dimension, appearance and tolerances).
Produce a sample batch and send it to the relevant export customer.
If the batch is found to be acceptable, continue with the export initiative.

1.3 GRADES AND GRADING SYSTEMS THAT WILL BE COMPARED

This publication will cover the grades and grading systems used in most of the more important softwood timber producing countries in the world. This includes: Australia, New Zealand, Southern United States, Canada, Europe and ISO standards.

1.4 DETAILS OF THE STRENGTH COMPARISON METHOD USED

The details of the method used to compare the strength values of international grades with South African structural grades is simple in conceptand enables the comparison of more than one strength value at a time.

The strength values used for each grade include: Bending Stress (Fb), Tension Parallel to the Grain (Ft), Compression Parallel to the Grain (Fc) and Modulus of Elasticity (MOE) (E).

These values were chosen due to the fact that they are considered the most important areas of strength required in a piece of timber for use in construction.

The method of comparison will now be shown using an example where the South African Structural Grade S5 will be compared with the Australian Stress Grade F5.

a) Step 1: The South African S5 mechanical properties are determined:

Table 1

Table 1

* // = parallel

b) Step 2: The Australian F5 mechanical properties are determined:

Table 2

Table 2

c) Step 3: The S5 grade is compared by each mechanical property, proportion wise to the mechanical properties of the F5 grade. This is done by dividing F5(Fb) by S5(Fb), F5(Ft) by S5(Ft), F5(Fc) by S5(Fc) and F5(E) by S5(E). These proportions are displayed in tabular format as follows:

Table 3

Table 3

To help explain how to interpret the above proportions, the MOE strength proportion is taken as an example. Table 3 shows that the MOE strength value of the F5 Grade is 0.885 the proportion of the S5 Grade which means the F5 Gradeís MOE strength is only 88.5% that of the S5 Grade.

d) Step 4: The penultimate step produces the single number required to compare the two grades. The four proportions calculated in Step 3 are averaged out in Step 4 to give an overall proportion which is used to do the comparison (see table 4).

Table 4

Table 4

The two grades can now be compared on the average proportion of the four mechanical strengths. The best way to interpret this is to remember the closer the proportion gets to 1 the closer the two grades compare. In this case the match is very good, the F5 grade only being 1.042 times stronger than the S5 grade.

Note: When this method is applied to countries with more grades than South Africa, some of the other grades will fall in-between the South African Grades. To accommodate this, if the overall average proportion (represented by a blue curve in Figure 1) falls within 0.15 of 1 (0.85) it is defined as being within an Absolute Limit (the Absolute Limit is represented by a thin red line in Figure 1). If the overall proportion falls within 0.1 of 1 (0.9) it is defined as being within the Acceptable Limit (The Acceptable Limit is represented by a thin blue line in Figure 1).

There is no need for an upper limit since there is no technical reason why a grade should not be used below its structural capabilities.

For example:If the above example included an extra Australian grade and the Overall Average proportion for the comparison with the South African S5 grade was calculated to be 0.85 it would still be considered a match within an absolute limit. If it was calculated to be 0.9 it would be an acceptable match.

d) Step 5: In this publication not only will the results be displayed in the tabular form shown above but it will also be shown in a graphical form.

The graph shown below (Figure 1) shows the comparison between the South African S5 grade to the Australian F-Grades. The thick blue curve represents the overall average proportion calculated in Step 4. The thin red line represents an absolute limit and the thin blue line represents an acceptable limit as shown in Step 4. The horizontal black line represents a perfect match. As seen in Figure 1 the blue curve passes over the black horizontal line (equivalent to a perfect match ñ a proportion of 1) at the F5 sector, again showing that the S5 and F5 grades are a close match.

Figure 1

2.0 MECHANICAL PROPERTIES OF WOOD

The strength and resistance to deformation of a material are referred to as its mechanical properties.Strength is the ability of a material to carry applied loads or forces, e.g. the ability of a roof truss to hold up a roof. Resistance to deformation is determined by the amount a material can be compressed, distorted or bent under an applied load e.g. the truss in a roof has to be able to resist deformation in all the mentioned areas so that the roof does not lose its shape.

Mechanical properties are usually the most important characteristics of wood products whenused for structural buildings. Structural application may be defined as any use where mechanical properties are the primary criteria for selection of the material, e.g. in roof trusses where timber is chosen for its strength and not for its appearance.

The term strength is often used in a general sense to refer to all mechanical properties. This can lead to confusion, since there are many different types of strengths. It is for this reason that an attempt has been made to compare certain grades on more than one strength value.

Note: The term stress is used to describe the force one member imposes upon another.

As mentioned before, four mechanical strengths were looked at and these were chosen as they are considered to be the most important in structural uses.

2.1 BENDING STRENGTH (Fb)

This value determines the load a beam will carry before it breaks. The value given to a grade regarding this strength will be a minimum value, which must be exceeded by the timber to be classified in that particular grade. For example, in order for a piece of timber to qualify for an S5 grade it must have a minimum bending strength (Fb) of 11.5 megapascals.

2.2 TENSION PARALLEL TO THE GRAIN (Ft)

This is a strength value given to a piece of timber that shows how much force can be applied before it breaks when being pulled along its length.

This is a very important strength criterion for the bottom member (chord) in a roof truss and in the design of connections between structural members.

For a piece of timber to qualify for an S5 grade it must have a minimum tension parallel to the grain (Ft) strength of 6.7 megapascals.

2.3 COMPRESSION PARALLEL TO THE GRAIN (Fc)

Compression strength relates to the ability of a section to support a given load acting across that section. In other words it is the strength value given to a piece of timber such as a wall-plate supporting the roof trusses.

For a piece of timber to qualify for an S5 grade it must have a minimum compression parallel to the grain (Fc) strength of 18 megapascals.

2.4 MODULUS OF ELASTICITY (E)

The Modulus of Elasticity of a section of timber is a measure of its resistance to a change of shape or size under the action of various forces on the section. In other words it is a measure of the stiffness of a beam.

This is regarded as one of the more important strength values looked at in structural timber apart from bending strength.

For a piece of timber to qualify for an S5 grade it must have a minimum modulus of elasticity (E) of 7800 megapascals.

3.0 FACTORS AFFECTING THE STRENGTH OF TIMBER

Due to the varying nature of wood and the major differences between wood species there are many factors that will affect the strength of a section of timber. For the purpose of this publication only the most important factors will be mentioned.

3.1 DENSITY

The strength of clear (defect free) wood is closely related to its density, which in turn, is closely related to the thickness of the cell walls: the thicker the cell walls the stronger the wood. However, the effect of density is almost completely overridden by the effects of other defects such as knots.

Wood density is affected by the following:

The environment: density reduces with a decrease in mean annual temperature.
Species: each different wood species will have a different density.
Position of wood in the tree: density increases radially from the pith to the outer wood and decreases with increasing tree height.
The age of a tree: density increases with tree age.

3.2 KNOTS

Knots are the most common defect that reduces the strength of timber. Knots in sawn timber are simply cross-sections through branches. Where successive growth layers of a tree have had to pass around a branch there are zones of sharply sloping grain so that the primary forces in a board are not parallel to the grain. Knots are therefore points of weakness. Thus the effect of a knot in many cases can be considered equivalent to that of a hole. In other cases the knot may have a greater effect than a drilled hole because of the before mentioned distortion of the grain that accompanies it.

The method of assessing strength loss is based on the relationship between the knot size and grain distortion. The larger the knot, the greater the distortion, the greater the loss of strength.

The position of the knot also has a large effect on strength. A knot on the top or bottom edge of a board is more severe than the same knot found in the centre of a board.

3.3 SLOPING GRAIN

Sloping grain is a defect which occurs when the grain in the board is not parallel to the edges of the board. The slope of grain is expressed as the length in centimetres through which there is a one centimetre deviation in the grain.

Sloping grain in a board can be caused by:

The way the lumber is cut from the log.
If a crooked tree is sawn.
If a tree with spiral grain is sawn.

3.4 MOISTURE CONTENT

As wood dries below the fibre saturation point, most strength and elastic properties increase. It might be expected that this would occur since, as water is removed from the cell wall, the long-chain cellulose molecules move closer together and become more tightly bonded.

3.5 DECAY

Decay is generally prohibited in grades of lumber used for structural purposes because it is impossible in many cases to try and estimate the extent to which the strength has been decreased and how much further the weakening effect will increase with time.

4.0 METHODS OF STRESS GRADING FOR STRUCTURALTIMBER

Due to the many factors reducing the strength of timber the strength can vary considerably from board to board. To take advantage of the stronger boards and eliminate the weaker ones, structural timber must be graded. The process of grading for strength is called ìstress gradingî and involves some form of non-destructive test to predict the strength of each board.

There are three main methods of stress-grading:

Visual stress grading.
Mechanical stress grading.
Proof grading.

4.1 VISUAL STRESS GRADING

Visual stress grading relies on the grader to asses the effect of defects on the strength and stiffness of the timber. Research has quantified the strength-reducing effects of defects such as knots and slope of grain. Similarly the relationship between the density and the strength of the timber has been established and the information has been used to write visual grading rules. The grading rules, therefore, lay down limits to the maximum size of the knots and general slope of grain and the minimum density permitted in a given grade.

4.2 MACHINE STRESS GRADING

Machine stress grading relies on the relationship between the stiffness and the strength of the lumber. Generally the higher the stiffness the higher the strength, but this relationship differs between different species.

All the machines used in machine stress grading work on the same principle that flexes the board without breaking it. Some machines will apply a known constant load to the timber and measure the deflection while other machines deflect the timber a known amount and measure the load required to meet this deflection.

The latest machines use computer systems to calculate the stiffness and indicate the grade by automatically spraying the lumber with coloured dyes, which are coded, to the various grades.

The advantages of mechanically stress grading are:

It is objective. Visual grading is unavoidably subjective.
It produces a more uniform product.
It largely eliminates the regional and silvicultural differences in strength characteristics of structural lumber that occur in visual grading.

4.3 PROOF GRADING

Proof grading (testing) determines if all pieces of a pre-graded consignment of timber meet the assigned minimum stress levels. Pre-grading may be done by machine or visually. A proof-grading machine applies a pre-determined load to a piece of lumber. If the piece does not break, it is regarded as being up to grade.

In other words proof grading is really just a grade testing method to determine whether the pre-grading was done correctly and to prove whether a certain defect still falls within the required strength category of the grade.

The next issue (Issue 2) will start by going into detail on the South African structural grades. This issue will also start the comparison process by beginning with South Africa versus Australia and New Zealand.

ISSUE 2

1.0 SOUTH AFRICA

1.1 STRUCTURAL GRADES

Structural timber in South Africa is used in building construction and where strength, stiffness and straightness are the primary requirements.

Most grading for structural timber in South Africa is done according to the visual stress grading method. The grading of timber using this method cannot be considered an exact science because it is based on visual inspection of each piece and thus relies on the judgement of the grader. The South African visual stress grading rules provide for a 5% variation between grades. In this system defects capable of physical measurement are judged solely on the basis of their dimension.

Grade descriptions based upon the poorest pieces allowed in each grade are desirable in the interest of keeping the grading rules simple. As a result the average quality in any grade is usually far better than the minimum described.

The grading system employed in South Africa is applied so that the most obvious defect is looked at during inspection. Each defect has a written specification, which can be found in the SABS Sawn Softwood Specification.

The defects that are covered include:

Warp (spring, bow, cup and twist)
Cross fracture
Insect damage
Knots (through face, edge, whorl, cluster, combination and superficial splay)
Wane
Density
Sloping grain
Decay
Surface checks
End splits

The South African structural grades have gone through quite an evolution. This publication is not a history lesson though. Today's structural grades consist of three primary stress grades, which have been graded by one of the methods previously mentioned. The three grades include:

Stress Grade 5 (S5)
Stress Grade 7 (S7)
Stress Grade 10 (S10)

1.2 SPECIES USED

The most common pine species being sawn in South Africa can be seen in Table 1. Even though there are different pine species in use, they still fall under the same structural strength specifications. Table 1 also shows the average strength value and density for the species found in South Africa. All values are in Mpa.

Table1:

Most common pine species used by the South African structural timber industry

Table 2.1

* Source: SABS Board Grading Course 41.06

1.3 DETAILS OF DIMENSIONS

By definition, structural timber in South Africa has a nominal width of at least 50mm and a nominal thickness of at least 38mm. The exceptions, in terms of the minimum width, are battens and brandering, which can be as narrow as 38mm nominal width.

In terms of structural timber, the most common dimensions are termed the 'Bread and Butter' sizes, due to their popularity.

The most common South African structural lumber dimensions (including battens and brandering) can be seen in Table 2.

Table 2:

Most Common Sizes of South African Structural Pine Available

Table 2.2

* Source: SALMA Timber Manual

1.4 DETAILS OF MECHANICAL STRENGTH VALUES

The strength values of the structural grades are at the heart of this publication. The values that will be given here are the ones used to compare the South African structural grades with other international grades. These values are a standard for each grade and represent the minimum strength values required for a given structural board to qualify for a certain grade. The strength values are the same for each board irrespective of dimension.

The strength values given here apply for all methods of grading i.e. the values are the same for visual, mechanical and proof graded timber. The minimum strength requirements for the different mechanical properties are given in Table 3. The Black Cross (XXX) reject grade is also included.

Note:

The colour coding used for each of the South African Stress Grades will be used throughout this publication.

Table 3:

Minimum Strength Requirements (Mpa) of South African Stress Grades

Table 2.3

* Source: SABS 563 & SABS 1245

1.5 GRADES SUMMARY AND END USES

The South African timber industry also makes use of various other grades of timber for a variety of differing uses. This short section expands on the differing grades for the reader's interest. Table 4 focuses on the three broad grade categories (Structural, Appearance and Utility), the specific grades found within those categories and examples of the typical uses for those grades.

Note:

The focus of this publication is on the Structural Category of timber within each country.

Table 4:

Typical Timber Grades Found in South Africa with Examples of End Uses

Table 2.4

1.6 GRADE STAMP IDENTIFICATION

The grade stamp used to identify South African certified (certified by the South African Bureau of Standards - SABS) structural timber is very simple. It includes the type of grading system used (Visual, Mechanical or Proof), the stress grade it falls under (S5, S7 or S10), the SABS standard mark (SABS 563) and the mark of the sawmill that produced the timber.

The stamps shown in the following figures show everything except the producers mark.

The first stamp shows that it is visually graded (two dashes either side the SABS mark) and it is an S5 Stress Grade.
The second stamp shows that it is mechanically graded (one dash to the right of the SABS mark) and it is also a S5 Stress Grade.
The third stamp shows that it is proof graded (one dash to the left of the SABS mark) and it too is a S5 Stress Grade.

2.0 AUSTRALIA

2.1 DETAILS OF MECHANICAL STRENGTH VALUES

The Australian Stress Grades are based on strength groups and provide for the basic working stress for design purposes. The Australian Stress Grades are graded visually or mechanically and both methods can be used in proof grading.

The visual grading is conducted using similar visual strength indicators as the South Africans, thus the Australians have various grade descriptions that limit all common defects or imperfections, which affect strength and describe timber at the lower limit for each grade.

Mechanical or machine stress grading relies on the relationship between the stiffness and strength of the timber. Generally the higher the stiffness, the higher the strength, but this relationship differs between species.

The Australian grading process has three steps:

The timber species being graded is classified into a species strength group (for the purpose of this comparison, strength group SD6 for dry radiata pine will be used).
The timber is then sorted into five structural grades (Structural Grade Numbers 1 through to 5).
These Structural Grades are based on strength groups and provide for basic working stress in bending for design purpose. These stress grades are termed "F" grades. The "F" grades are aligned to the five structural grades on the basis of in-grade testing results.

The minimum strength requirements for the different mechanical properties are given in Table 5 for each Australian Stress Grade ("F" grade). Unlike the South Africans who have three Stress Grades, the Australians have six common Stress Grades.

Table 5:

Minimum Strength Requirements (Mpa) of Australian Stress Grades

Table 2.5

*Source: AS 2858-1986 standards

2.2 SPECIES USED

As mentioned in the previous section the relationship between stiffness and strength differs between species. For this reason the values used for the Australian comparison are based on Douglas fir and Radiata pine species, which are classified into the species strength group SD6.

Douglas fir is a North American grown species, which is imported into Australia, while Radiata pine is a species grown in Australia as well as imported from New Zealand.

2.3 DETAILS OF DIMENSIONS

The typical dimensions used by the Australian structural timber industry differ from those used in South Africa.

The most common Australian Structural Lumber dimensions (including battens and brandering) can be seen in Table 6.

Table 6:

Most Common Sizes of Australian Structural Softwood Available

Table 2.6

* Source: Ministry of Forestry

2.4 GRADE STAMP IDENTIFICATION

The Australian Standards denote the labelling of a grade by the use of a stamp, label or ink spray as branding. When branding visually graded structural timber this labelling is applied no less than 600mm in from the end of each piece of timber. The colouring is specific to each "F" grade and is as follows:

F4 = Red F5 = Black F7 = Blue F8 = Green F11 = Purple

The visually graded brand shows:

The "F" grade (in the required colour)
AS 2858 (denoting the Australian Grading Standard used)

The machine stress graded timber colour brand applied by the machine shows:

The mill identity
The manufacturer
"S" to show the piece is kiln-dried
piece dimensions (mm)
M.S.G. showing the piece is machine stress graded
AS 1748 (denoting the Australian Grading Standard used)

2.5 GRADE COMPARISON

The Australian Structural Stress Grades ("F" grades) are now compared with the South African Stress Grades ("S" grades) using the methodology explained in ISSUE 1 of this publication.

Note:

In the comparison tables, when the overall average proportion is shaded it means a match between the international grade and the South African grade (the shading will match the colours used in Table 3).

2.5.1 South African S5 Grade Vs the Australian "F" Grades

This subsection will compare the South African S5 Stress Grade with the Australian "F" Stress Grades (Figure 1 & Table 7).

Table 7:

Comparison between South African S5 Grade and Australian "F" Grades

Table 2.7

Figure 2.1

Figure 1

2.5.2 South African S7 Grade Vs the Australian "F" Grades

This subsection will compare the South African S7 Stress Grade with the Australian "F" Stress Grades (Figure 2 & Table 8).

Table 8:

Comparison between South African S7 Grade and Australian "F" Grades

Table 2.8

Figure 2.2

Figure 2

2.5.3 South African S10 Grade Vs The Australian "F" Grades

This subsection will compare the South African S10 stress grade with the Australian "F" stress grades (Figure 3 & Table 9)

Table 9:

Comparison between South African S10 Grade and Australian "F" Grades

Table 2.9

Figure 2.3

Figure 3

2.5.4 Overall Summary of South Africa Vs Australia

To conclude the comparison between Australia and South Africa Table 10 depicts a summary of the detailed comparison shown above. The Overall summary compares the grades from the Absolute limit.

Table 10:

Overall Summary of South Africa Vs Australia

Table 2.10

3.0 NEW ZEALAND

3.1 DETAILS OF MECHANICAL STRENGTH VALUES

The New Zealand structural grades are based on strength groups and provide for the basic working stress for design purposes. A piece of timber is deemed to fall into a particular stress grade on the basis of visual, machine or mechanical methods of assessment.

The visual grading is conducted using similar visual strength indicators as the South Africans, thus the New Zealanders have various grade descriptions that limit all common defects or imperfections, which affect strength and describe timber at the lower limit for each grade.

Unlike the Australian structural grading system the New Zealand system uses three different structural grades (No.2 Framing, No.1 Framing and Engineering). Each of the structural grades has basic working stresses, which are assigned to them.

The minimum strength requirements for the different mechanical properties are given in Table 11 for each New Zealand structural grade.

Table 11:

Minimum Strength Requirements (Mpa) of New Zealand Structural Grades

Table 2.11

3.2 SPECIES USED

Sawn timber products in New Zealand are categorised by grading rules - New Zealand Standard NZS3631:1988; New Zealand Timber Grading Rules. The standard divides all species into three groups: native softwood, hardwoods (New Zealand grown exotic and imported) and exotic softwoods (New Zealand grown exotic and imported). For the purpose of this comparison the third group is used as it is the most common group used to produce structural timber in New Zealand using Radiata pine.

3.3 DETAILS OF DIMENSIONS

The typical dimensions used by the New Zealand structural timber industry differ from those used in South Africa, but are almost identical to those used in Australia.

The most common New Zealand Structural Lumber dimensions (including battens and brandering) can be seen in Table 12.

Table 12:

Most Common Sizes of New Zealand Structural Timber Available

Table 2.12

* Source: NZS 3601:1973

3.4 GRADE STAMP IDENTIFICATION

The New Zealand convention denotes the labelling of a grade as branding. Timber grade branding in New Zealand is not covered by a standard, but is a matter of convention or agreement between the supplier and the purchaser.

In general, little grade branding is done, except for paint on the end of a timber bundle. Conventions for colour coding visually graded timber are as follows:

No. 2 Framing = Yellow No. 1 Framing = Black Engineering = Silver

In terms of mechanically stress graded (MSG) timber the AS/NZS 1748:1997 - Product Requirements for Mechanically Stress-Graded Timber is used. In this case all MSG timber is marked at least once per piece with the following information:

Name or identification mark of the organization grading the timber
Stress grade
If seasoned, a mark to indicate this
Number of the standard

3.5 GRADE COMPARISON

The New Zealand Structural Grades are now compared with the South African Stress Grades ("S" grades) using the methodology explained in ISSUE 1 of the publication.

Note:

3.5.1 South African S5 Grade Vs New Zealand Structural Grades

This subsection will compare the South African S5 stress grade with all the New Zealand structural grades (Figure 4 & Table 13).

Table 13:

Comparison Between South African S5 Grade and New Zealand Structural Grades

Table 2.13

Figure 2.4

Figure 4

3.5.2 South African S7 Grade Vs New Zealand Structural Grades

This subsection will compare the South African S7 stress grade with all the New Zealand structural grades (Figure 5 & Table 14).

Table 14:

Comparison Between South African S7 Grade and New Zealand Structural Grades

Table 2.14

Figure 2.5

Figure 5

3.5.3 South African S10 Grade Vs New Zealand Structural Grades

This subsection will compare the South African S10 stress grade with all the New Zealand structural grades (Figure 6 & Table 15).

Table 15:

Comparison Between South African S10 Grade and New Zealand Structural Grades

Table 2.15

Figure 2.6

Figure 6

3.5.4 Overall Summary of South Africa Vs New Zealand

To conclude the comparison between New Zealand and South Africa Table 16 depicts a summary of the detailed comparison shown above. The Overall summary compares the grades from the Absolute limit.

Table 16:

Overall Summary of South Africa Vs New Zealand

Table 2.16

ISSUE 3

1.0 EUROPE

1.1 DETAILS OF MECHANICAL STRENGTH VALUES

The stresses assigned to grades of structural timber can have an important influence on their international competitiveness. The assigned stresses must represent their properties as accurately as possible, yet some form of grouping is necessary, simply to make their use in design easier.

The European Strength Classes are based on strength groups and provide for the basic working stress of timber for design purposes. These strength class systems are not specific as to whether they refer to visually or machine graded timber, but for the purpose of this document it is assumed to be for both. The minimum strength requirements of the Eurocode system can be seen in Table 1.

Table 1:

Minimum Strength Requirements (Mpa) of European (EN 338) Strength Classes

Table 3.01

*Source: Eurocode EN338