Stress Graded Timber Short Course

This short course will provide you with the basic knowledge to specify graded timber for different applications. Although the focus is on structural lumber, appearance grade lumber is also covered. The course is aimed at architects, engineers, other built environment professionals, as well as DIY enthusiasts in South Africa who want to explore the use of wood in and around buildings.

short course

How to complete this course

The course is open access and the content is available to anyone with internet access. This course should be completed in the sequence of the numbered sections. We make use of four components in our course (a) sections with text and illustrations, (b) video clips to explain some of the concepts, (c) a short online quiz after each section to test your acquired knowledge and (d) a test, available to learners who wish to obtain a certificate of completion. Please contact us at info@thewoodapp.com with the short course name to complete this test.
To make the course accessible to most people, we’ve tried to limit the time needed to complete it (an estimated 2.5 hours). This course will give you an overview of selected building materials impact and the environment, and design strategies to incorporate timber in a sustainable way. For in-depth knowledge on some topics you might require input from other sources such as national standards, material suppliers, selected textbooks, articles, etc.
If you have any questions or suggestions, please contact us at

info@thewoodapp.com.

2. Stress grading of lumber

  • The different strength and stiffness properties
  • Stress grading basics
  • Which basic wood properties influence the strength and stiffness of lumber?
  • Lumber stress grading methods: Machine, visual grading and proof grading
  • The SANS grades used in South Africa
  • Long term strength and stiffness behaviour of graded lumber
  • Resawing, finger jointing, and chemical treatment
  • Using imported or ungraded lumber for structures

3. Appearance grading

  • SANS appearance grades
  • American Hardwood grading (NHLA) rules

4. Other Resources

Questions to be answered in this course

Typical questions we want to answer in this course are the following:

  • Why and how is timber graded?
  • What wood properties can I expect from a specific stress grade?
  • How is the quality of timber controlled and how can you be sure that the properties of stress graded timber will be consistent?
  • Can timber without a recognized stress grade be used in a structure?
  • Will the strength properties of timber stay consistent over time?
  • What is the difference between stress grades from different countries (i.e. SANS / EN / ANZ)?
  • What does appearance grade timber entail?

Introduction

Lumber is generally graded into either stress (structural) grades or appearance grades. As the names imply, stress graded lumber is intended for structural use while appearance lumber is used in applications where aesthetics is of importance. In reality though, the situation is more complex with structural components often having aesthetic functions while appearance lumber in most cases also have structural functions. For instance, lumber used in industrial and packaging applications are usually graded on appearance characteristics although their main function might be structural. This may sound irrational but in fact many visual properties (such as knots) also have a profound effect on strength characteristics. In fact, lumber is often stress graded using visual grading systems.

2. Stress grading of timber

Stress grading of timber refers to the grouping of timber according to its strength and stiffness properties.

The different strength and stiffness properties

When talking about “strength” of timber we actually refer to a number of related strength properties. There are seven mechanical properties typically used in designing a wooden structure – see Figure 1 below. The bending strength is often considered the most influential strength property, and the names of stress grades are therefore often related to the bending strength value of a grade. For instance, in South Africa the grade S5 refers to the allowable bending stress value of 5 MPa of that grade. Bending strength (fb) is sometimes also referred to as the modulus of rupture or MOR. The other properties that are considered important include the tensile strength parallel to grain (ft) and the stiffness or modulus of elasticity (E or MOE). The remaining properties are considered less important although they might be critical in specific applications.

Fortunately, strength and stiffness properties are usually related to each other. In other words, if one property such as stiffness is high for a specific grade, the other properties will most probably also be relatively high.

Figure 1. The Strength and Stiffness Properties Used for Designing Structures with Wood

Stress grading basics

Variability
Wood is a natural material with enormous variation in strength properties. To understand stress grading of lumber, it is essential to understand this variation and also how grading systems deal with variation. In Figure 2, the bending strength distribution of more than 2 000 pieces of SA Pine lumber that were tested at Stellenbosch University over the past decade can be seen. These pieces came from different regions, sawmills, and Pinus species. The weakest pieces had bending strength (MOR) values of less than 5 MPa, whereas the strongest pieces had values higher than 100 MPa. It is clear that it will not be efficient to use all this lumber as a single grade. An efficient stress-grading system will separate lumber into grades with similar strength values.

Characteristic strength values of a grade
An important question is what strength value engineers can use to design structures when wood has such high variation in strength? The value used by engineers is referred to as the “characteristic strength” of a grade. The characteristic strength of a specific grade is that strength value where 5% of all pieces tested will break. That means 95% of all pieces will be stronger than the characteristic strength. This characteristic strength value is also called the 5th percentile strength value. In Figure 3, the strength distribution of 100 pieces of an imaginary grade of lumber is plotted. For this grade, the 5th percentile value or characteristic value will be the value of the 5th weakest piece of lumber which will be 10 MPa. Take note that 5% of all strength values in a grade will be lower than the characteristic value. Does this mean that 5% of all structures are likely to fail? Fortunately not! As most structures form complex load-sharing systems and as points of weakness in structural members seldom coincide with points of maximum stress, the 5th percentile has been found adequate to ensure a universally accepted degree of safety. Additionally, there are safety factors included in the structural design process.

Allowable or working strength values of a grade
Take note that another strength value that is sometimes used to describe a stress grade, is that of the “allowable strength” or the “working strength” of a grade. This value is simply the characteristic strength value divided by a safety factor. For instance, the S5 grade of SA Pine has a characteristic bending strength value of 11.5 MPa. The allowable bending strength of this grade is the characteristic value divided by a safety factor of 2.22 which results in an allowable bending strength of 5.2 MPa. (This grade was called S5 because its allowable strength was close to 5 MPa).

Figure 4 shows how the SA Pine lumber from Figure 2 could potentially be separated into the three SANS grades used in South Africa. You will notice that there is still quite large variation of strength values even in the individual stress grades. There is usually a quite significant overlap in strength values between grades. For instance, quite a large percentage of the higher strength S5 lumber might be stronger than the weaker lumber of the S7 grade. The more efficient a grading system is, the better it will separate strength grades with less overlap between grades.

Figure 2. Bending Strength Variation of Ungraded SA Pine from Different Regions and Sawmills in South Africa

Figure 3. The strength distribution of 100 pieces of a single grade of lumber. The characteristic value for this grade will be the strength value of the 5th weakest piece (5th percentile). This is the value used by engineers to design structures with a specific lumber grade.

Figure 4. The black, red, and green curves represent the way a grading system can potentially separate the SA Pine timber from Figure 2 into three SANS structural grades, viz. S5 (black curve), S7 (red curve), and S10 (green curve). The dashed lines indicate the 5th percentile MOR value, or characteristic strength, of each grade.

Characteristic strength is a statistical value!
Timber users sometimes want to test a single board to find out whether it complies with the strength requirements of a specific grade. This is not possible since the way strength is described for a grade depends on the testing of a large number of boards and a statistical distribution’s results. Remember, 5% of the strength values of a grade will usually be lower than the characteristic strength. If you test a single board and it is lower than a grade’s characteristic value, one cannot state that it does not comply with the strength requirements of that grade.

Quiz

 

1. Which timber strength property is often considered the most influential or important property?
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2. A new stress grade is developed and the characteristic bending strength of the new grade must be determined. Forty (40) pieces of a timber from this stress grade is tested in bending in order to determine characteristic bending strength – the value used by engineers to design structures. The results are shown below ordered from minimum to maximum. What should the characteristic bending strength value for this grade be? [Note: in reality most standards require much larger sample sizes than 40 pieces]
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Which basic wood properties influence the strength and stiffness of lumber?

There is only one way in which we can be totally sure how strong a piece of lumber is and that is to test it until it fails – which will make the piece useless for further use. In lumber stress grading we therefore depend on measuring other related properties non-destructively to predict the strength. There are many different properties that can influence the strength and stiffness of lumber, but three basic wood properties overshadow the rest, viz.: (a) wood density; (b) knot / grain deviation properties; and (c) microfibril angle. Wood density and knot properties can be measured in different ways but microfibril angle is not easily measurable. Take note that the importance of each of the three properties differs between species and the specific strength property we refer to. For instance, knot properties are usually very influential in Pinus and other softwood species in terms of lumber strength. Contrastingly, in hardwoods such as Eucalyptus, knot properties are usually less influential.

Wood density
At a microscopic level, wood can be compared to a pipe system which transports water (sap) from the roots of a tree to the leaves (see Figure 5). Each “pipe” is a string of interconnected cells, each with a wall and a cavity through which sap flows. The wood density is effectively the relative volume of cell wall material to cell cavity volume. Wood with large cell cavities and thin cell walls have low density, and wood with small cell cavities and thick cell walls have a high density. Wood density can vary from as low as 100 kg/m3 (balsa wood) to 1 200 kg/m3 (kingwood) and sometimes varies tremendously within a tree. High density (or heavy) wood is usually stiff and strong while low density (or light) wood is usually flexible and weak.

Figure 5. Wood density depends on the relative ratio of wood cell wall volume to cell cavity volume. High density (or heavy) wood is usually stiff and strong while low density (or light) wood is usually flexible and weak.

Knots and grain deviation due to knots
Knots are considered as one of the main strength reducing characteristics in lumber. Interestingly, the knot material is not responsible for the strength reduction, but rather the grain deviation around the knot. A tree is a magnificent structure and the way the wood grain from the stem goes around and interlocks with the grain of a branch ensures that branches in a living tree do not affect the strength of the stem that much. However, when we cut or saw through this complex grain structure, the branch (or knot) becomes a weak point. The tensile strength parallel to the grain of clear SA Pine is usually above 100 MPa whereas the tensile strength perpendicular to the grain is about 4 MPa – a massive 25 times difference in strength! Straightness of grain, therefore, has a profound effect on strength properties. In Figure 6 and Figure 7 the effect of knots on grain deviation is clearly visible, even on wood where no knots can be seen.

Figure 6. Grain deviation caused by knots have a large effect on the strength properties of wood. Parallel to grain tensile strength is roughly 25 times higher than perpendicular to grain tensile strength.

Figure 7. Even though there are no knots visible on this wood, the grain deviation due to an adjacent knot will cause a reduction in strength.

Microfibril angle
Softwoods such as SA Pine mainly consists of cells called “tracheids”. Each tracheid is made up of a cell wall containing various layers made up of microfibrils. The microfibrils of the main layer are arranged in a spiral around the cell cavity or lumen (see Figure 8). The angle at which these microfibrils are arranged is called, unsurprisingly, the microfibril angle. Wood with a small microfibril angle is usually stronger and stiffer than wood with a large microfibril angle. Studies on South African grown Pinus patula have shown the relative influence of density and microfibril angle on stiffness of timber to be similar.

Figure 8. Microfibrils are arranged at an angle (θ) to the vertical axis of a softwood tracheid cell. Microfibril angle has a large influence on the strength and stiffness of wood.

Measuring the microfibril angle of wood is complex and cannot be done fast enough to be useful when grading lumber. But other properties of wood such as the acoustic properties, correlate well with microfibril angle and can be used for grading.

Variation of density, microfibril angle and knots in a pine tree stem
In Figure 9, the usual variation of density, microfibril angle, and knot properties in a pruned SA Pine tree stem can be seen. A high wood density, low microfibril angle, and small knots will result in the strongest and stiffest lumber. Therefore, lumber close to the bark from the pruned section of the stem will usually be the strongest while lumber close to the pith of the tree will be the weakest.

Figure 9. The strength and stiffness of wood is usually a function of density, microfibril angle, and knots. The typical variation of density, microfibril angle, and knots in South African grown Pinus patula are illustrated above. Generally, density increases from pith to bark while the microfibril angle will decrease from pith to bark. Knot properties vary as a function of pruning and stem height.

Timber stress grading methods: Machine, visual grading and proof grading

Commercial stress grading systems measure one or more properties of wood that is related to strength in order to separate timber into grades. As discussed before, influential properties that are related to strength include the knot characteristics, density, and microfibril angle. One important property that is often used in design, namely the stiffness or modulus of elasticity (MOE), can be measured directly without destroying the timber.

Timber grading systems in sawmills are often categorised as either machine grading or visual grading systems. As the name implies “machine grading” includes grading where mechanical or electronic equipment is used. Proof grading is considered a special type of machine grading. Visual grading is where a human operator does the grading by visually evaluating the timber. To estimate the mass of a piece of timber, i.e. to evaluate the wood density, (visual) grading operators usually lift up the board.

Most sawmills in South Africa do visual grading, and this is probably true for most countries in the world. Visual grading is usually based on the knot properties and the density of timber. The specific rules for visually grading South African softwood timber are explained in the section below.

Machine grading of lumber can use a number of different methods to measure properties of interest. Modern machine grading systems can use many different types of sensors and electronic systems. The oldest method which is still often used is measuring the stiffness or MOE of lumber mechanically. This is done by applying a load to a piece of lumber, measuring the deflection of the lumber, and calculating the MOE.

Multi-sensor grading machines where a number of properties are measured at the same time are becoming more popular. These machines typically measure acoustic or wave properties of wood, measure knot properties with X-ray scanners and optical cameras, as well as grain angle using laser scattering. Mathematical algorithms can then be applied to predict the strength of each piece of lumber.

A specific type of machine grading that is sometimes applied, is called “proof grading”. In this case, a bending or tensile testing machine applies a load that is equal to or higher than the characteristic value of a grade. Any piece of lumber that is weaker than the grade characteristic value will fail and be discarded. In a sense this is a very safe or conservative type of grading since other types of grading allow 5% of pieces to be weaker than the characteristic value. Proof grading, on the other hand, will result in no weaker pieces in the specific strength mode that was tested.

Lumber grading technology and the advanced technology used in these systems are evolving fast. Regardless of which technology is used to grade lumber, the most important aspect is that the graded lumber has strength properties similar to, or better, than the characteristic values for that grade.

Quiz

 

1. Which of the following timber properties does NOT influence their strength and/or stiffness properties?
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2. Which wood will generally be the strongest in bending in a pruned SA Pine tree?
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3. In which type of grading system the weakest pieces, having a strength below the characteristic value of the grade, is broken?
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The SANS grades used in South Africa

NB: The SANS grades are summarised in this section. The relevant SANS standard should be consulted for detailed requirements of each grade.

In South Africa, we mostly use grades defined in a South African National Standard (SANS standards). These standards refer, in most cases, to grades for Pinus (pines) or softwoods grown in South Africa. More recently, Eucalyptus grades were also developed specifically for green or wet graded lumber. Most of the standards refer to structural grades or grades used for structures, houses, and roof construction with only one standard focused on appearance and industrial grades (SANS 1783-3). The different grades and standards are described in Table 1 below.

Table 1. The Different Types of Lumber Grades Described in SANS Standards

It is important to notice that specific grades are linked to specific dimensions. Stress graded structural lumber must be at least 38 mm or thicker and 50 mm or wider. Lumber of 25 mm or 16 mm thickness is therefore not allowed in a structural grade.

Strength and stiffness properties of SANS structural grades
The strength and stiffness properties of the structural grades described in the previous section, can be seen in Table 2. Different grading methods can be used to obtain these structural grades but regardless of the grading method, each grade need to conform to the mechanical properties listed in this table. Take note that the EH grade refers to young, green Eucalyptus lumber. These strength values are valid for the lumber in both the green and dry condition.

Table 2. The strength and stiffness properties related to SANS structural grades (values obtained from draft versions of SANS 10163-1 and SANS 1707-1).

Marking of SANS graded lumber
If a piece of lumber has been graded according to a SANS grade, it must be marked according to very specific requirements. For structural grades the following must appear on the mark: (a) the manufacturers name or trade mark, (b) the stress grade and method, (c) in the case of mechanically stress graded lumber, the date or batch number of the stress-grading, (d) in the case of fingerjointed lumber the letters FJ in a colour that represent a specific exposure class of the adhesive used, as well as the date and batch number of fingerjointing – see SANS 10096 for more information, (e) in the case of lumber that was pressure treated for improved durability, the H class will appear – see SANS 1288 for more information. Table 3 shows the mark on a piece of graded structural lumber. The ● mark after the S and before the 5 indicate that the lumber was visually graded.

Table 3. Lumber graded according to the SANS standards must be marked. This visually stress graded lumber mark (left) contains the manufacturer name (Merensky L), the stress grade (S5), and the certification mark (SATAS). The position of the ● on the mark indicates the method of grading – see table on the right.

Visual stress grading rules (SANS 1783-2)

Visual stress grading of pine lumber using SANS 1783-2 is by far the most used grading method in South Africa. The two main properties that get assessed to assign a grade to a piece of lumber are wood density and the knot characteristics (see Figure 10 and Figure 11). Take note that the minimum density allowed for a stress grade is 360 kg/m3 (for S5) and the maximum circumferential distance of knot-cover, at the worst section of wood, is the sum of one face width and 1.5 edge width of a board.

Figure 10. The wood density and knot requirements in SANS 1783-2 grades. These are the two principal grading criteria in visual stress grading of pine. The f refers to the face width of a board and e to the edge width of a board (SANS 1783-2, 2012).

Figure 11. Different Knot Types. A Knot Whorl is Found on Pith Boards.

Take note that there are many other defects that also get assessed, such as machine damage, splits, resin infiltration, excessive grain angle, wane, checks, warp (bow, twist, spring, cup) and decay. These properties are often linked to the influence they have on the manufacturing of products from lumber and not necessarily the strength or stiffness. For instance, if a piece of lumber has severe deformation such as twist, it will be very difficult to manufacture a roof truss from it. Apart from knots, warp is the characteristic that is arguably responsible for the most downgrade of lumber (see Figure 12).

Figure 12. Maximum Permissible Warp Values for Structural Lumber According to SANS 1783-2 (2012)

Green graded Eucalyptus structural lumber

A unique structural product that has been developed in South Africa, is green graded Eucalyptus lumber. This product is normally manufactured from timber, harvested from short rotation Eucalyptus trees. It has been finger-jointed and proof-graded while unseasoned or green (with a moisture content above 30%). Its intended end-use is nail-plated roof trusses that are manufactured while the lumber is still green, and drying of the lumber occurs in the roof space after construction of the roof. Users of this lumber must be aware that the lumber will undergo some changes like shrinkage and checking while drying in the roof space. SANS standards (SANS 1707-1 and 2) have been developed for the product and is currently in the draft stage.

Figure 13. Green Graded Eucalyptus Structural Lumber in Roof Trusses

Battens and brandering grades

Battens and brandering are products used to be fixed against beams and joists in roofs for the attachment of ceilings, and on roof trusses for the support of tiles, slates, and thatch. These products also perform structural functions, but are described in a separate standard (SANS 1783 Part 4). The dimensions in which battens and brandering are manufactured include 38×38 mm, 38×50 mm and 50×50 mm.

Long term strength and stiffness behaviour of graded lumber

An important question is whether graded lumber will keep its strength and stiffness properties over the lifetime of a structure. In other words, if a wood structure can support a specific load just after construction, will it still be able to support the same load 50 years later?

Research has shown that lumber which is under a constant load, loses strength and stiffness over time. Fortunately, for graded structural lumber the percentage strength loss is fairly low. For clear grade, defect-free wood (not often used in structures), the loss in strength is more profound and can be as high as 40% of the initial bending strength. This strength and stiffness loss occurs during the first 10 years of load after which it remains more or less constant. In the timber engineering field this strength loss is referred to as the “load duration effect”. To allow for this expected strength loss of timber, timber design codes include a load duration factor. This is a well-known phenomenon and timber structural engineers allow for it as a standard practice in their analyses. Take note that this strength loss occurs only for a limited time, and older timber structures will not keep losing strength over time. This strength loss is linked to cyclical changes in moisture content of wood which typically occur during different seasons or in application fields where the humidity of the environment often changes.

Permanent deflection, creep, or sagging of lumber is closely related to the “load duration effect”. It is also related to cyclical moisture content changes in wood. Where structures have been designed according to a timber design code such as SANS 10163, and the load duration and deflection limits have been considered, excessive creep should not occur in the long term.

Figure 14. Creep or sagging of lumber, and even failure, can occur over time due to too high stresses on a member combined with cyclical changes in moisture content of wood. Where the structure was designed according to a design code such as SANS 10163-1, this should not occur as this load duration / creep phenomenon is considered in the design analysis. (Picture: https://floridafoundationauthority.com)

The moisture content of lumber is another factor which can have an effect on the strength and stiffness properties of wood. Wet or green timber, where the moisture content is above 20%, is weaker in certain strength modes than dry timber. Compression strength particularly is badly affected by high moisture content.  In timber engineering, this weakening of lumber due to high moisture content is called the “moisture content effect”. This is also a well-known phenomenon and timber structural engineers allow for it as a standard practice in their analyses. In the SANS 10163-1 design code a reduction in compression strength parallel to grain of 33% is allowed when the moisture content of the member is expected to go above 20%.

 

Figure 15. High moisture content reduces the compression strength of wood. This “moisture content effect” is included in structural design codes such as SANS 10163-1.

Quiz

1. Which of the following dimension timber pieces cannot be graded into a SANS structural timber grade? Dimension given as thickness X width X length.
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2. Which of the following is NOT required on the compulsory mark that must be applied on SANS stress graded timber?
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3. What is the minimum density allowed for SANS stress graded timber in South Africa?
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Resawing, finger-jointing, and chemical treatment

Resawing – When a piece of graded lumber is re-sawn or split in two (edgewise or in thickness direction), the resultant pieces must be graded again and the original grade does not apply anymore. This is mainly because the size/defect ratio has been altered. Take note that this does not apply for cutting a longer piece of lumber shorter – then the original grade still applies to both pieces.

Finger-jointing – Finger-joints are usually stronger than the weakest sections in a piece of lumber (i.e. sections where there are knots). It is, therefore, acceptable for structurally graded lumber to be finger-jointed. However, the adhesive used should comply with the end-use conditions of the lumber. If lumber is to be used externally, adhesives must be suitable for external conditions. Also, for SANS graded lumber, finger-jointed lumber must comply with SANS 10096 and must also be marked according to the specifications in that standard (see Figure 16).

Chemical treatment – Graded lumber used in structures in some areas of South Africa must, according to law, be chemically treated to improve its durability. The areas where this must happen is specified in SANS 10005 and is mainly the coastal regions of South Africa. The marking of such timber is specified in SANS 1288 (see Figure 16). The level of treatment is according to hazard or H-classes and is as follows:

H0: interior lumber (only insecticide, no fungicide)

H2: interior lumber, above ground

H3: exterior lumber, above ground

H4: lumber in ground contact

H5: lumber in fresh water

H6: lumber in sea water

Chemical pressure treatment of structural lumber with a water-based preservative does affect the strength properties of the lumber somewhat. This effect is taken into consideration during the structural design process according to the design code SANS 10163.

Figure 16. If treated, timber should be marked with the relevant hazard class (H2 above left). If finger-jointed, it should contain the letters FJ (above left) and the date of manufacture (above right).

Quality assurance and auditing of SANS stress graded lumber

 

How can the designer and end-user of a building be sure that stress graded lumber will conform to the published grade properties?

According to the national building regulations of South Africa any building and any structural element or component must be designed to “provide strength, stability, serviceability and durability”. The regulations governing the whole building process from the design to the finished structure is set up to ensure that buildings conform to this requirement.

Stress grader accreditation

To stress grade and mark lumber according to SANS standards, the stress grader needs to be certified by an accredited certification and accreditation auditor. The auditor will ensure that the stress grader has the necessary equipment, skills, and facilities to stress grade lumber according to a specific SANS grade standard. Once this has been established the grader will receive approval to use his own unique mark which can be applied to graded lumber (see Figure 16).

Certification and accreditation of auditors

Stress grading certification and accreditation auditors also need to be accredited by the South African National Accreditation System (SANAS). It is the only national body responsible for carrying out accreditations with respect to conformity assessment, as mandated through the Accreditation for Conformity Assessment, Calibration and Good Laboratory Practice Act. Examples of two organisations that have been accredited to perform lumber stress grading certification and accreditation in South Africa include SATAS and the SABS (see www.satas.co.za and www.sabs.co.za).

Regular auditing of a stress grader

The stress grading certification and accreditation auditor must ensure that all their clients that have been accredited, comply with the relevant SANS standards. To this end they perform regular audits on the stress graders to ensure that all the rules in the standards are adhered to. The mark on stress graded lumber contains both the name of the stress grader as well as the name of the certification and accreditation auditor (see Table 3). Where non-compliance is noticed on stress graded lumber, end users can contact the stress grader and / or the relevant certification auditor and lodge a complaint.

 

Continuous testing of stress graded lumber

Since 2021, all stress graders using SANS standards will be required to perform continuous strength and stiffness testing on a sub-sample of their graded lumber according to the SANS 1783-5-2 standard. To the best of our knowledge, South Africa is one of only two countries (New Zealand being the other) where destructive testing and a statistical process control method is applied to ensure the quality of ALL graded lumber.

The current set of regulations governing the quality of South African stress graded lumber is arguably of the most stringent in the world. This should provide peace of mind for users of lumber graded according to the various SANS stress grading standards.

 

Using imported or ungraded lumber for structures

In some instances, architects, home owners, or builders may want to use ungraded lumber, imported wood species, or lumber from species that does not appear in any South African stress grading standards in a structure. The question is then which strength and stiffness values can be used for design analyses? Firstly, since these lumber resources do not appear in any local standards, the use of it in a structure needs to be approved by an accredited professional structural engineer.

If imported lumber has been stress graded according to an overseas standard, the engineer simply needs to obtain the grade characteristic values for the specific grade being used. Obtaining the relevant international grading standard will also be useful as it will specify other valuable information that might be required such as acceptable warp, defects, and moisture levels.

Some lumber species may not have a stress grading standard (i.e. indigenous species such as yellow wood, tropical species, or most hardwoods). In this case, engineers might have different approaches. They can use grading rules for a similar species, get the lumber visually graded by an experienced grader, and use the characteristic values for the similar species. There are risks with this approach, especially if no mechanical testing data is available for the species used. Testing a subsample of the lumber at a laboratory can minimise such risk. A safer approach will be to get lumber proof tested. In this case, each piece of lumber is tested in a bending or tensile test machine to the characteristic stress value that will be used by the engineer.

3. Appearance Grading

Unlike structural grading, is appearance grading usually not a legal requirement for any product class and hence the grading process is less regulated. Clients might have very specific requirements and sawmill processors will adapt their grades to satisfy the largest proportion of their clients. Producers of appearance grade lumber are therefore less likely to make use of SANS grades and certification bodies for this product class. In this section the SANS appearance / industrial grades will be described as well as a short section on hardwood grading according to the National Hardwood Lumber Association (NHLA) in USA.

SANS appearance grades

The SANS appearance grades are grouped together with packaging in a product class called “Industrial grades” and described in SANS 1783 Part 3. The following are the grades defined in this standard:

– Clear grade: an appearance most suitable for the manufacture of high-quality furniture and mouldings;

– Semi-clear grade: Intended for the manufacture of furniture, and for joinery purposes;

– Cutting grade: suitable for the recovery of clear grade material for remanufacturing purposes;

– Appearance grade: Intended for the manufacturing of products such as furniture, flooring, shelving, and for joinery purposes;

– Utility grade: Intended for the manufacturing of products where appearance is of no real importance;

– Packaging grade: Intended for the manufacturing of products such as pallets and crates.

The most common sizes for industrial grades are 25 mm and 38 mm thickness by widths of 76 / 114 / 152 / 228 mm in lengths of 0,9 m to 6,6 m.

American Hardwood grading (NHLA) rules

South Africa has a limited hardwood resource grown for saw logs and does not have any standard SANS grading rules for appearance grade hardwoods. A significant volume of high-value hardwood timber is, however, imported to South Africa annually, and it will be beneficial to have a good understanding of hardwood grading systems in use internationally. There is an effort by some organisations to standardise hardwood grading rules internationally. A brief overview of the hardwood grading rules as developed by the National Hardwood Lumber Association (NHLA) of the USA will be given here.

The major difference between the NHLA hardwood grading rules and South African softwood grading rules, is that hardwood is often produced in random widths and a board is graded (mainly) according to its potential to recover clear components from it. The main reason is that most knots in appearance grade hardwood products are unacceptable. If a sawmill removes the knots and cuts large boards into smaller clear dimension products, there will be thousands of different dimensions in stock at a sawmill. Every secondary manufacturer will require different dimensions from a sawmill for its products and it will be virtually impossible to satisfy the market demand. By keeping boards as wide as possible and only grading them into a few grades according to the potential of the board, every secondary manufacturer can process wide boards into the specific dimensions as required. In this way, the whole hardwood manufacturing process is both simplified and made more efficient as secondary manufacturers design their products and processes to recover the maximum volumes from the wide, random width boards they purchase. They also have a much better ability to cut wide boards into precise dimensions than what a sawmill has.

The NHLA hardwood grades used are the following:

Clear Face Cuttings

No.1 Common

No.2B Common

FAS

No.2A Common

No.3B Common

FAS One Face (F1F)

No.3A Common

Sound Wormy

Selects

Sound Cuttings

 

The detailed rules for each grade can be downloaded from the NHLA website at www.NHLA.com. Figure 17 shows some of the rules for four grades.

Figure 17. Hardwood grades according to the NHLA rules depend largely on the potential to cut out clear components from a large plank. Four grades are illustrated above with the potential clear cuttings that can be recovered from each. For FAS grade (top) at least 83.3% of the plank should be recoverable as clear pieces. (From: Walker, 1993)

1. Which hazard class timber can be used in ground contact?
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2. How can ungraded timber of a species for which no grading rules exist, be legally used in a structure in South Africa?
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3. The defining principle of hardwood appearance grades such as the NHLA grades, is:
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4. Other Resources

For additional information on lumber grading, you can refer to the following resources:

Australian stress grades: https://www.woodsolutions.com.au/articles/structural-grading

Canadian and USA softwood grades: https://cwc.ca/how-to-build-with-wood/wood-products/lumber/grades/

National Hardwood Lumber Association grading rules (USA):

https://www.nhla.com/nhla-services/hardwood-industry-rules/

European and British strength grading: https://blogs.napier.ac.uk/cwst/quick-summary-timber-strength-grading/ and https://ttf.co.uk/courses/timber-trade-topics/strength-grading/

Timber Engineering handbook: Thelandersson, S; Larsen, J.L. Timber Engineering. John Wiley and Sons Ltd. 2003.

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