Wooden Poles 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.

4. Specifications

4.1 SANS specifications

4.2 Sale of poles

4.3 Marking of poles

4.4 Quiz 4

5. Applications

5.1 Roof structures

5.2 Pole supported buildings

5.3 Road bridges

5.4 Other applications

5.5 Connectors and hardware

5.6 Quiz 5

Disclaimer
In the compilation of this module, free use was made of published information such as text, figures, drawings, tables, graphs, etc. As the use of such material is subject to copyright considerations, and the suitability and relevance of this content is in the process of being assessed, the content of this module is only reserved for personal use and the purpose intended. To adhere to copyright regulations, any publication of the module or parts thereof considered, is subject to obtaining the necessary copyright agreement from the publishers by the author. Photographs taken and figures, drawings, tables and graphs generated by the author, are subject to copyright as well.

Whilst all and the utmost care has been taken to ensure the accuracy of the information contained in this module, no warranty can be given regarding the use, suitability, validity, accuracy, completeness or reliability of the information, including any opinion or advice.

Wooden Poles Short Course Video 1

1. Overview

1.1 Use and advantages of wooden poles

Wooden poles have been in use since ancient times as it provides structural members with minimal processing required. Basic pole members are subjected to biological degradation, i.e. fungal decay or insect attack (or both), and therefore preservative treatment of such poles is required. The standard in South Africa is that all softwood (e.g. Pine) and hardwood (Eucalyptus a.k.a gum) poles used in loadbearing structures shall be preservative treated according to SANS 10005, The preservative treatment of timber and SANS 1288, Preservative treated timber. There are a few occasional exceptions where untreated poles (mainly poplar) are used to replicate historical farmsteads. For this document, it is accepted that all wooden poles and posts shall be treated against biological attack, unless stated otherwise.

Wooden poles have a large variety of uses, from 10-32 mm top diameter laths used in thatched roofs, 32-50 mm top diameter droppers used in fences, and poles with 50 mm top diameter upwards and with lengths of up to 18 m used for transmission lines and heavy construction. The demand for telephones poles has declined due to growth of cellular phone networks, however, the transmission pole market is still fairly strong in Southern Africa as the continent develops.

Sheds, shelters, stores, and many other structures can be built quickly, easily and cheaply with wooden poles. Poles from the stronger pines and eucalyptus species are more suitable for this purpose.
Pine and gum poles are also commonly used as vine trellis material and fencing. For laths and droppers only gum species are suitable. Complete fences can be made from wood by using laths, split poles, and slabs used to fill in the space between the horizontal pole railings. Attractive rustic effects may be obtained in this manner.

Wooden poles remain very popular due to various reasons:

1. Naturally renewable and available on a sustainable basis;
2. It is a carbon-neutral product;
3. Good insulation, resilience to wind and high shock impact;
4. Relative long life span (preservative treated);
5. Strongest timber evenly distributed at the periphery increases the effective moment of inertia;
6. Cheaper than steel or concrete poles;
7. It has a salvageable value after its useful life;
8. Easy to attach fittings or modify in-service;
9. Lasts better at the coast;
10. Requires less maintenance than steel poles;
11. Pole structures are easy build on steep sites as footings penetrate unstable soils and anchor underlying layers;
12. Footings are stable on reactive soils (clay) or low bearing pressure soils (wetlands and sand dunes).

The most important disadvantages are:

1. The use of preservative chemicals is required to improve the durability;
2. The appearance might sometimes be variable;
3. Variation in strength;
4. In the case of heavier large construction, wooden poles can usually not compete with the stronger materials such as reinforced concrete.

To be acceptable as a pole, it must

1. Be reasonably strong;
2. Have little taper;
3. Be reasonably straight;
4. Absorb the preservative chemicals readily;
5. Not develop seasoning defects such as shakes and splits easily;
6. Not have excessive spiral grain;
7. Not have excessive knots.

1.2 Resource of Poles

Poles can be manufactured from softwood (conifers) or hardwood (broadleaved) species. In South Africa, the softwood poles are all produced from certain allowable pine species and hardwoods from Eucalyptus (gum) species. It is obtained from full tree lengths or from plantation thinning and tree top sections. These thinnings and treetops provide an additional income for the forestry sector, and the availability of poles up to 3 m long, will always be easily accessible. Thinnings contribute further to the intermediate income of the plantation forestry industry.

Of the pine species grown in South Africa, Pinus canariensis and Pinus pinaster provided the best quality pole material, but it is not being planted any more. The availability of Pinus radiata, the next best pine species for pole production, is limited and also under pressure due to pest and disease threats. The other plantation grown pine species are deemed to be too weak for structural building or transmission poles and is excluded by SANS structural specifications. Gum pole material is far more freely available than pine pole in South Africa. Of the gum species grown in South Africa, Eucalyptus grandis and E. grandis hybrids are the most common species used for pole production.

Of the heavier transmission poles, the gum species such as Eucalyptus paniculata and E. cloeziana would be far more suitable due to their higher strength and natural durability but are no longer grown in sufficient volumes. As a result of reasons given above, Eucalyptus grandis and E. grandis hybrids have become the most used pole material in South Africa.
Pole lengths are sold from 1.5 m to 18 m and diameters can generally range from 50 mm to 200 mm.

1.3 Different types of poles

When pine poles are only debarked, it will be left with irregular diameter bulges. Pine poles and posts are often rounded or peeled to provide a smooth pole with no knots or other uneven protrusions on the surface. A peeled pine pole still has the taper of the tree and can also have sweep. This is also known as a peeler pole or tapered poles. A rounded or cylindrical pole is like a peeled pole; however, it has a straight and cylindrical form with a constant diameter. Rounded poles are smoother than peeled poles but are weaker and therefore not regarded as suitable for load bearing/structural applications.
The treatable sapwood zone/volume of gum poles in comparison to pine poles is much less and therefore gum poles are only debarked and branches trimmed flush with the bole, and not peeled or rounded.
Pole peelers rotate the pole against a cutter head. The rotating mechanism is angled to push the log length through the peeling machine. Logs are only held in position by smooth and spike rollers in the vicinity of the cutter head. This is dangerous as the ends travel through a wide arch when peeling a pole with sweep.

Figure 1: Pole pealer

Figure 2: Cutting head of pole pealer

Figure 3: Pole rounding machine

1.4 Agricultural, building and fencing poles and guardrail posts

Poles produced under SANS 457-2 (softwood) or SANS 457-3 (hardwood) are classified as “agricultural, building and fencing poles, or guardrail posts” and will have the same minimum characteristic strength value. In the case of pine poles all agricultural, building and fencing poles as well as guardrail posts are graded in the same way and are all marked as SANS 457-2.

In the case of hardwood poles, structural and agricultural poles can only be produced from Eucalyptus (gum) species and have different permissible defects than fencing poles and guardrail posts, that can be made of any hardwood, provided it complies (almost extensively gum poles). Structural and agricultural gum poles are therefore marked with an additional S, fencing poles with an F, and guardrail posts with an additional P to the 457-3 mark.

1.5 Utility poles and cross-arms

Utility poles are seen as the stronger and larger telephone and transmission poles and cross-arms, and are produced under SANS 753 (pine) and SANS 754 (Eucalyptus), and only the species specified in the standards may be used.

1.6 Split poles

Split poles are poles that are split through the centre (pith) and then treated. It is common in vine trellising and as fencing slats; however, it is not suitable in ground contact and is produced under SANS 1288.

1.7 Square posts or plinths

Square posts or plinths have been squared by cutting four perpendicular sides. The circumference of the tree, or wane, is often left so that a maximum dimension can be obtained from the tree. The term “plinth” is used for shorter members.  In the case of pine there is normally sufficient sapwood left to achieve the preservative treatment requirement for ground, contact (H4) applications. In the case of gum, square posts will at best be suitable for above ground applications since almost all of the treatable sapwood will have been removed, thereby restricting compliance to at most the H2 treatment to refusal allowed. Square posts and plinths are produced under SANS 1288.

1.8 Quiz 1

 2. Wood Properties and Characteristics

2.1 Hardwood and softwood

Softwoods belongs to the Gymnospermae class that includes plants whose seeds are naked and not enclosed in an ovule (like a pinecone). The wood is in general, as the name implies, physically soft. It is also referred to as conifers. As poles, it is necessary to know that the timber from softwood is, with some exceptions, generally lower in density, weaker in strength, more flexible, and less durable, but easier to pressure treat compared to hardwoods. Examples of softwood are pine, spruce (impermeable), and cypress.

Hardwoods belongs to the class Angiospermae and reproduce with flowers in which the mature seed is surrounded by the ovule, like an apple. The term hardwood can be a misnomer as there are several Angiosperms that are very soft, for example balsa, willow, and poplar. Hardwoods are usually darker in colour due to extractives in the central core (heartwood) and is, with some exceptions, higher in density, stronger in all strength modes, stiffer, more durable, but more difficult to pressure treat compared to softwoods. Hardwood poles in SA almost always consist of gum (Eucalyptus species).

2.2 Sapwood and heartwood

On the inside of the cambium, newly formed wood cells add to the sapwood through which the water and the nutrients flow up the tree from the roots to the treetop. The sapwood is also a place to store starch and sugars that can be used later to sustain the tree through periods of slow growth or dormancy.  Usually, as each new growth ring of sapwood is formed, an inner ring of older sapwood is itself “retired” and in transition to become heartwood. The cells die and get filled with extractives. The sapwood thus forms a sheath of relative constant thickness, like a pipe around the heartwood. However, due to the wider annual rings of the juvenile core, the thickness of the sapwood is a little more towards the top of the tree. The proportion of sapwood to heartwood varies according to species, age of the tree, rate of growth and the environment. Softwoods usually have more sapwood compared to hardwoods.

Figure 5: The distribution of sapwood and heartwood in different boards cut from a log (Jozca and Middleton, 1994)

The sapwood is physiologically active and responsible for sap transportation through the tree stem. Normally the sapwood of trees is not naturally durable because it contains organic foodstuffs (like sugars and starches) and is very porous. This allows the ingress of insects and fungi. Fortunately, preservatives can penetrate sapwood easily due to its porosity.

To protect the inner physiologically inactive heartwood, the tree resorts to three protective mechanisms to protect the “dead” heartwood in its centre: 

1. Firstly, it mechanically closes the interconnecting bordered openings in the cell walls by aspiration of the tracheid’s pits that are commonly found in conifers
2. It further deposits fungicidal tannins, phenols and other extractives like gum and resin in the disused cells to ward off insects and fungi, more so in broad-leaved (hardwood) species than in conifers and also more so in some species (Tamboti) than in others.
3. It also extrudes bladder-like tyloses “blow ups” into the vessels of broad-leaved timbers to prevent pathogen ingress.

These three mechanisms make the heartwood more naturally durable, with exceptions and in varying degrees, depending on species and age. This also makes the heartwood less water-conducing and therefore unfortunately also virtually impermeable or impregnable with wood preservative chemicals.

These deposits also increase the gross density, but it must be stressed that this does not increase the strength except in the compression mode. Sapwood thus has a higher moisture content, but dries easier and can also easily be impregnated with wood preservative chemicals due to its high water-conducing properties.

2.3 Grain spirality

The direction of the cells deposited in the tree, is referred to as the grain angle, in relation to the length-axis of the tree. This grain appears as spiral patterns along the tree length and is referred to as spirality or spiral grain. Spirality is not always confined to the juvenile core (see 2 d) but can occur throughout the stem. Depending on the severity or scale, wood with spiral grain has a lower strength and reduced stability that leads to warp. The bending strength of a beam is reduced by about 4% if the inclination is 1:25, and by approximately 45% if the inclination is 1:5.  Stiffness is also reduced, but to a lesser extent.

Figure 7: Severe spiral grain in a pole

2.4 Juvenile wood and mature wood

The juvenile core in the centre region around the pith is sometimes slightly lighter in colour than the mature wood surrounding it. It displays a change in its properties proportionally away from the pith, but the zone ends gradually, after 7 to 12-year rings from the pith, irrespective of height in the tree. One such difference is the geometry and angles of the cells. The juvenile wood forms nearly a cylinder of wood around the pith of the tree. The top of a tree is exclusively juvenile wood as part of the sapwood (refer to Figure 8 below).

Figure 8: The position of the juvenile core, sap, and heartwood (Jozsa and Middleton, 1994)

The juvenile wood has a lower density due to a larger proportion of earlywood that has thin cell walls and large cell openings. Also, in the juvenile core, the grain angle is larger due to spirality.  Further, the angle of the microfibrils in the S2 cell wall, again in relation to the length-axis of the cell, is larger in the juvenile core. Microfibrils, with a diameter of ±25 nm, are deposited parallel in layers in the cell wall, of which the S2 layer is by far the thickest layer. Lastly, the juvenile wood contains plenty of small knots that are present even after artificial or natural pruning inside the juvenile core

Figure 9: The dual effect of a large S2 microfibril angle and spiral grain in juvenile wood (Jozsa and Middleton, 1994)

This lower density, more grain deviation due to spirality, larger S2 microfibril angle and presence of knots make the juvenile wood weaker in strength. The spirality and larger microfibril angle will cause the juvenile wood to shrink much more in length. 

The magnitude of the above-mentioned changes in juvenile wood properties can be summarised in the figures below.

Figure 10: Influences of juvenile core on timber properties (Haygreen & Bowyer, 1996)

The prevalence and scale of juvenile wood is actively reduced by genetic manipulation in nurseries and silvicultural practices in plantation forestry.

2.5 Reaction wood

Reaction wood is the term applied to woody tissues produced in certain parts of leaning tree stems and at the upper and lower sides of branches. Reaction wood is a factor that influences the shock resistance, strength, and stability of poles.

Compression wood is the reaction wood of soft woods. It develops typically on the underside (compression side) of leaning or malformed stems, branches and beneath branch insertion. This will cause an eccentric pith, situated to the side of the oval shaped poles. Eccentric growth rings containing an abnormally high proportion of late wood are indicative of its presence. It is also characterised by a more gradual transition between early wood and late wood than in normal wood.

Figure 11: Eccentric pith and growth rings of compression wood in pine

Some pines, like the locally grown Pinus taeda, is well known for the occurrence of abnormal compression wood, where reaction wood is formed in straight upright trees. Although different circumstances, the effect is the same as for normal compression wood. Abnormal compression wood forms complete or broken dark concentric circles around the entire cross section of the tree stem (Figure 12). The central core and often the outer ring underneath the cambium are not affected.

Figure 12: Increased degree of abnormal compression wood

Tension wood is the reaction wood of hardwoods. Tension wood is usually formed on the upper or tension side of leaning stems and branches, sometimes with little evidence of eccentric growth and in rare cases, it can also appear on the underside. The cross sections of leaning tree stems or branches are quite often eccentric with the radius of maximum on the upper side of the lean. Tension wood will have the same results on stability and strength as compression wood.

The S2 layer in reaction wood has a microfibril angle, which approach 45° to the cell axis (4-5 times bigger than normal wood). This angle is much larger than is found in the S2 layers of normal wood. As a result of this longitudinal shrinkage may be up to ten times greater in reaction wood, while transverse shrinkage is down to 75 to 50% of normal.

The combined effect of juvenile wood and reaction wood can be seen in the diagrams below.

Figure 13: A summary of combined juvenile wood and reaction wood

Due to a large S2 angle, when reaction wood breaks, it is an abrupt breakage, like a carrot or brittle materials. Normal wood will break with characteristic splintering.

Figure 14: Lack of splints with compression wood failure

2.6 Knots

Randomly distributed knots in a pole do not have the same localised weakening effect as in lumber, since the distribution of knots are symmetrical around the pith and the relative proportion of the pole consisting of knots. Knots may, however, be distributed as a knot whorl, typically in pine, where the knots are surrounding the pith at the same vertical height. This has a large effect on the strength.

Figure 15: Branch whorls (left, Mercado Florestal) result in knot whorls in a pole

Tight (sound, live, fixed) knots are created when a living branch is overgrown by the main stem of the tree. Because the living cambium sheaths the tree, such knots have growth rings in common with the main stem, meaning that tight knots will not fall out as the wood dries. Tight knots still reduce the strength of the wood of which they are a part. 

Loose knots (dead knots) are created when a dead branch is overgrown by the main stem of the tree. In this case the branch is simply surrounded by new growth and does not become an integral part of the main stem. Upon drying, such knots can fall out of the wood of which they are a part and spoil the appearance. Again, strength reductions related to slope of grain can be significant. Fungal infestation is common at dead knots.

 The picture below shows a branch that has been overgrown by the main stem of the tree. For a time, the branch was living while it was being overgrown. Note the continuous growth rings in this region; this part of the branch creates an intergrown or tight knot. At some point, the branch died. It did not fall from the tree, however, and continued to be overgrown or encased by the main stem. This part of the branch is not an integral part of the surrounding and will fall out if the piece were cut thinner.

Figure 16: An overgrown knot, indicating live and dead knots (Snyder, 2008)

In the case of poles, the knots will reduce the strength in the following ways:

• cells are deposited at angles as it is trying to grow around the branch. This is called grain distortion or grain deviation and causes a weak spot.
• It is common in some conifer species to have branch whorls, where many branches exit the stem on tree height. This will mean a lot of localised disruptions at the knots, emphasised by the size and number of branches (see Figure 15).
• A volume of reaction wood, more or less the same size of the knot, forms below the branch.
• More distorted grain is evident where the branch enters the stem at a small angle.
• Larger branches will mean larger knots, more grain distortion and results in weaker timber. There is also grain distortion where a branch has been pruned (knot occlusion).
• Resin accumulates at knots together with bark inclusion as timber grow over the branch or knot. The resin has no influence on the strength, but makes it messy and difficult to paint over, spoiling the appearance.
• The localised end grain dries out easily and, being weak, results in splits on the face of a pole.
• Surface roughness is a problem for certain applications.
Trees tend to reduce its support to branches as it gets older. The branches will die and are detached from the trees by means of self-pruning. Usually, the tree length can be divided as a rule-of-thumb in thirds. The bottom third will have plenty of clear wood, the mid third will have loose dead knots and the top third will have smaller live tight knots. Radially, the small live knots are more common in the central knotty core.

2.7 Growth stresses

Distortion and splitting in log ends and sawn boards due to the release of growth stresses are common in many gum species and some other hardwoods. It can be devastating in terms of its effect on end splitting of poles and distortion in split poles.

The tissue in the growing tree stem is subjected to stresses that occur from a slight shrinkage in length of the individual cells in the final phase of maturation, located underneath the cambium layer underneath the bark. Each cell layer is deposited in a state of tension, with the cumulative effect that will pose an increased compression force towards the centre core of the tree. The growth stress in vertical standing trees and equivalent felled trees differ only slightly, in contrast to wind stresses.

Figure 18: Ends splitting due to growth stresses

A log, processed into a split pole after being cut, will immediately shorten along the grain at the bark side, whereas boards cut from the centre pith of the log expand in length after being cut.

As a result, common problems to poles are: 

• Heart cracks and heart shakes/splits that are longitudinal separations of the wood, shown in Figure 18.
• Brittle heart/core is compression breaks and forms in the central core of the stem. It is the combination of high compressive forces, low wood density and knottiness. Brittle heart has a carroty surface in a cross-sectional breakage and large numbers of broken cells in softer wood.

Anti-split nail plates are hammered onto the ends to counteract end splits that usually form after cross-cutting (growth stress related end splits) and as a result of rapid moisture loss from the cross-cut ends (drying stress related end splits). Growth stresses in poles (and in sawn logs) are generally much more difficult to control than drying stresses.

The use of anti-split nail plates is compulsory when treating hardwood poles (SANS 457-3) with waterborne preservatives (CCA) or transmission and telephone poles (SANS 754) with either CCA or creosote and must cover a minimum area as specified in the relevant standard.

Figure 19: Extreme growth stresses where an anti-splitting plate could not prevent splitting

2.8 Cell collapse

Cell collapse in the central core around the pith, is common when cell walls plasticise under an elevated temperature during the initial stages of kiln drying of poles. It is magnified by large capillary forces experienced in impermeable hardwoods.
The strength of poles will be less affected, though the preservatives could bleed from the timber after preservative pressure treatment.

Figure 20: Cross cut of a pole showing severe cell collapse

2.9 Quiz 2

3. Preservative Treatment of Poles

3.1 Durability and treatability

Some timber species are naturally more durable than others.  The natural durability of timbers cannot, however, be compared without considering the environment in and the conditions under which they are generally used.

As a rule, sapwood is regarded as non-durable (class 5) whereas the heartwood can be rated as slightly more durable to highly durable. Many insects (such as Lyctus and Hylotrupes) bore and breed in sapwood only, and heartwood is usually also less accessible to fungi.  Moreover, the heartwood of some species contains toxic extractives and volatile oils and is less permeable to water due to resin, tyloses and aspirated pits in the heartwood. The heartwood of hardwoods is regarded as more durable because of the extractives and a higher density, compared to softwoods. In general, the softwood (pine) and hardwood (eucalyptus) species grown in South African plantations and used for poles are regarded as non-durable and must be preservative treated to increase its durability and expected service life.

An international natural durability classification system is in use, and it is based on the probable service life expectancy of impermeable and unpenetrated heartwood.

Poles when used in a structure, shall be treated against biological attacks. Poles are usually treated to for a specific end application where it will be exposed to a specific hazard class. A summary of the treatment hazard classes can be seen below. For more detailed descriptions see SANS 10005: 

H2 – Low hazard applications: Internal structural applications (i.e., roof trusses)

H3 – Moderate hazard applications: Exterior above ground applications (i.e., exposed decks)

H4 – High hazard applications: In – ground contact applications (i.e., planted poles)

H5 – Very high hazard applications: Fresh water and heavy wet soil applications (i.e., wetland pilings)

H6 – Extremely high hazard applications: Marine applications (i.e., sea pilings) 

Additional information can be obtained from the SA Wood Preservers Association (SAWPA)’s website at https://sawpa.co.za .

3.2 Treatment methods

The specification where the treatment requirements are specified is SANS 1288, Preservative treated timber. The requirements of main importance for each hazard class are the depth of penetration (in mm) and preservative retention (in kg preservatives per m³ timber). Factors that influence these requirements are the level or risk of exposure, biological agents it is protected against, preservative type, specific use of poles and whether it has sufficient sapwood to comply to the penetration and retention requirements.

Only pressure impregnation processes can be used to treat poles for hazard classes H2 – H6 when creosote or water-based copper containing preservatives such as CCA is used. The required penetration and retention of preservatives are easy to control and assess with this treatment method. In the case of water-based borates   it is possible for poles used in H2 hazards to be treated by means of a simple dip-diffusion process, provided that the poles are still in a green state. Water-based borates can also be applied to timber by means of a pressure impregnation process (seasoned) or pressure diffusion process (semi-seasoned). Creosote hot-cold open tank treatment, although still acceptable, is rarely performed.

3.3 Wood Preservatives

The preservatives used for treatment of poles are mostly a coal-tar based creosote and water based mixtures of copper-chromium-arsenic compounds (CCA). This document will only discuss these preservatives. Environmentally friendly CCA alternatives, preventing the use of heavy metals and arsenic, are available, though very limited in use. Borates, mentioned above, are also rarely used for pole treatment. Safety information regarding the use of preservative treated wood can be obtained from https://sawpa.co.za/safety-information/.

Properties of creosote

• These preservatives are relatively resistant to water leaching
• Creosote is therefore uniquely suitable for industrial end applications used under exterior conditions.
• Freshly treated poles tend to bleed during the first few months after treatment, especially when exposed to hot weather or strong sunlight. This is more common with the full cell pressure treatment method and when drying defects such as cell collapse and honey combing formed during seasoning.
• Creosote is oily and dark in colour and has a characteristic strong odour.
• Creosoted poles can stain plaster and other absorbent materials with which it is in contact.
• Creosote poles does not burn down in grass veld fires, although it is highly flammable directly after treatment,
• Freshly creosoted pole can normally not be painted. A bituminous-based aluminium paint or epoxy-based paint can be used on the timber after it has been exposed for a few months, but other paints can be applied only after some years of exposure.
• No change in the dimensions or in the shape of timber results from creosote treatment.

The CCA is applied as a water-based solution. The chrome serves as a fixing agent to allow the wood cellulose to bind chemically to the arsenic (as insecticide) and copper (as fungicide), becoming insoluble and leach resistant.

• After having been re-dried, timber treated with these preservatives is odourless and clean and is suitable for interior and exterior use.
• CCA treated timber is light green and after drying it can be painted and glued.
• These preservatives are non-flammable.
• The preservatives may cause corrosion of certain metals, but treated timber, after a fixation period of 7 days, is generally not corrosive.
• Wetting during and subsequent re-drying after treatment with these preservatives can cause grain-raising and the treated timber lacks dimensional stability. The ends of structural hardwood poles need gang nails to prevent splitting.
• The strength properties of freshly treated CCA poles are reduced because of the timber having been wetted during treatment. However, the normal working stresses assigned to a specific grade are not affected. Wet poles used in a load-bearing application, can lead to excessive deflection because of creep. If the timber is re-dried after preservation treatment, there will be no effect on the strength and stiffness of the poles.
• After re-drying, CCA treated poles are more susceptible to damage by veld fires than creosote poles because of afterglow.
• When timber that has been treated with these preservatives is machined, slight blunting of tools could be experienced.
• CCA treated timber should be stacked for at least one week to allow for sufficient fixation time for the CCA chemicals to chemically bond to the timber.
• Never burn CCA treated timber. The arsenic is chemically fixed to the wood. By burning the treated wood, the wood degrades in the burning process and the ash that remains behind contains the arsenic, previously bonded to the wood.

3.4 Preservation challenges of various products

Poles cannot be treated with preservatives while it still has bark on it.  Not only does the bark have to be removed, but it is important that the cambium layer below the bark should also be removed.  Preservatives do not penetrate timber that still has the cambium intact.  Therefore, it is essential that pole material is debarked as soon as possible after felling otherwise the cambium layer starts to dry out on the tree and becomes difficult to remove.

Debarking poles can be done manually or mechanically.  This depends upon the number, sizes, [lengths and thicknesses] and type of wood being prepared for poles.

Timber treaters should take care not to remove too much sapwood when rounding/peeling logs, as is evidenced in Figure 21 below. If too much sapwood is removed, there is not enough permeable sapwood timber left to obtain the required preservative retention and the penetration depth is also limited.

Furthermore, the eccentric pith will mean that one side of the pole will have more juvenile wood, which is characterised by length shrinkage and swelling. This will cause sweep in the pole. Lastly, a large amount of the strongest timber, underneath the bark is removed and the pole will be weaker, and the asymmetric removal of strong and stiff fibres will cause irregular deflection under compression stresses, and add the sweep mentioned above, which may lead to buckling of poles under load.

Figure 21: Rounded pole with sapwood removed and asymmetric distribution of juvenile wood

It is for this reason of insufficient sapwood, that square posts and plinths are rarely treated for H4 applications. There is not enough and thick enough sapwood to comply with the required retention and penetration requirements. Further, the process of peeling or rounding is not performed on hardwood poles since the sapwood in hardwoods has a limited thickness.

3.5 Best practices

When a pole is cross-cut, the central impermeable and unpenetrated heartwood of the pole will be exposed to wood destroying insect and fungal attack. To mitigate this, order the pole lengths as such that no crosscutting is done after treatment. If a pole, deemed to be planted in the ground, must be cut, then it is best to cut the thin end. The fully treated big end shall be planted in the ground and it will offer more resistance against biological attacks and offer strength at the critical ground level. The top end can also be cut at an angle to allow rainwater to run off. Lastly, if it is unavoidable to cut/groove/drill surfaces after treatment and it exposes unpenetrated surfaces, it can be dipped into, or painted with creosote or a suitable remedial and supplemental brush on wood preservative to provide some sort of prevention of biological attacks. If creosote is used for remedial purposes, the creosote must be heated, to reduce the viscosity and thus improve the penetration of creosote.

Figure 22: When treated poles are cross-cut, rot can set in the exposed impermeable unpenetrated heartwood of the pole core

Similarly, it is preferred that all poles are machined, drilled or shaped before the pole is treated. This is especially important in slab-gaining of poles.

3.6 Quiz 3

4. Specifications

4.1 SANS specifications

The following SANS specifications apply for the use of wooden poles in South Africa:

Building and fencing poles:

SANS457-2, Wooden poles, droppers, guardrail posts and spacer blocks – Part 2: Softwood species, specifies the material and physical requirements (only following SA Pine species P. radiata, P. pinaster and P. canariensis)

SANS457-3, Wooden poles, droppers, guardrail posts – Part 3: Hardwood species, specifies the material and physical requirements (only suitable Eucalyptus species may be used for agricultural and structural poles)

Utility poles:

SANS 753:  Pine poles, cross-arms and spacers for power distribution, telephone systems and street lighting (only SA Pine P. radiata, P. pinaster and P. canariensis).

SANS 754:  Eucalyptus poles, cross-arms and spacers for power distribution and communications systems

Preservation:

SANS 1288, Preservative-treated timber, specifies the minimum preservative treatment requirements.

SANS 10005, The preservative treatment of timber specifies the approved preservatives and treatment processes to be used, defines the hazard classes, and specifies handling and safe use as well as specific areas where treated timber must be used in South Africa.

Building codes:

SANS 10082: Timber frame buildings

SANS 10163-1: The structural use of timber Part 1: Limit-states design

SANS 10163-2: The structural use of timber Part 2: Allowable stress design

Table 2: Pole strength in bending and stiffness

Defects listed in specifications:

Sapwood width, decay, ring shake, face checks, end checks, cross fracture, mechanical damage, crook, sweep, knots, surface spiral grain, slab-gaining, finish of top ends of poles, nail plating.

4.2 Sale of poles

Poles should preferably be treated in the shape and size it is intended to be used, with a complete envelope of sapwood, but if squared posts are used, a large amount of sapwood shall be present. Complete sapwood penetration is required for cylindrical poles equal to the minimum sapwood as required for poles. Also refer to SANS 10005, Section 12.3.

Poles and droppers are always treated and therefore also specified in a South African National Standard (SANS). A round dropper is like a pole, though it is a short length that is intended for use as a support in fences and has a top diameter of between 25 mm and 50 mm. Poles are classified according to their top end diameter and is often colour coded. Various products are specified in the SANS standards. Table 3 is adapted from SANS 457-2, with additional information for SANS 753.

Table 4 is adapted from SANS 457-3, with additional information for SANS 754

Droppers are sold in the following lengths: 0.9 m, 1.1 m, 1.2 m, 1.35 m, 1.4 m, 1.5 m, 1.8 m, 2.1 m and 2.4 m

Poles are sold in the following lengths: 1.2 m, 1.5 m, 1.8 m, 2.0 m, 2.1 m, 2.4 m, 2.7 m, 3.0 m, 3.6 m, 4.2 m, 4.8 m, 5.4 m, 6 m, 6.6 m, 7.2 m, 7.8 m, 8.4 m, 9 m, 9.6 m, 10.2 m, 10.8 m, 11.4 m, and 12 m.

SANS 753/4 includes lengths of 14 m, 15 m, 16 m and 18 m.

Selling of non-structural poles

SANS 457 or SANS 1288 can be used to manufacture treated wooden poles (excluding utility poles). The distinction between SANS 457 and SANS 1288 is that SANS 1288 only covers preservative treatment criteria when SANS 457’s physical strength or visual requirements cannot or will not be met. As a result, SANS 1288 poles are considered non-structural and should not be used as structural poles. Building poles, agricultural poles, fencing poles, and guard rail posts are among the structural pole types and end uses listed in SANS 457-2 and SANS 457-3. 

SANS 1288 allows for the treatment of poles since 2000. This was to encourage the use of treated poles that were not in line with SANS 457 in applications such as ranch style outdoor furniture or other applications where the purchaser requested a crooked/curved natural looking pole. Proper preservative treatment and protection against biological agents such as decay fungi and wood destroying insects would still be ensured in accordance with SANS 1288.

 As a result, the SANS 1288 pole treatment allowance is only for special requests/orders and needs, not for general retail purposes where an end-user/consumer could buy a pole and then use it in an application where, due to non-compliance with the regulations, it may fail and cause harm or damage due to non-compliance with the physical strength requirements specified in SANS 457.

4.3 Marking of poles

South African Standards specify marking requirements for any preservative treated product that claims to comply with the respective Standards (SANS 1288, SANS 457-2/3 and SANS 753/4). The following information must be contained in the marking:

• A unique plant identifier or trademark. This is usually a name or acronym that identifies the treatment plant where the timber was treated. A register of treatment plant trademarks is maintained by the third-party product certification body with whom the treatment plant is certified, i.e., the SABS or SATAS, which will also be indicated.
• SANS standard designation. This number is usually found below the quality mark and indicates the SANS standard the treated timber complies with, e.g., SANS 1288, SANS 753, SANS 754 or SANS 457. In the case of SANS 457-2 (hardwood poles) the additional letter refers to the class or grading, i.e., S – Structural /agricultural, F – Fencing and P – Guardrail post

A hazard class which identifies the level of treatment applied (for example H4).

Date or year of manufacturing of poles, the last two digits (for example 13 for 2013).

For poles and round wood, the information is mostly applied on 25 mm metal tags (all pine poles and Creosote treated hardwood poles) or anti-split end plates (CCA treated hardwood poles). The information may be presented in several layouts, but most are done as set out in the following diagrams:

South African Standards specify marking requirements for any preservative treated product that claims to comply with the respective Standards (SANS 1288, SANS 457-2/3 and SANS 753/4). The following information must be contained in the marking:

• A unique plant identifier or trademark. This is usually a name or acronym that identifies the treatment plant where the timber was treated. A register of treatment plant trademarks is maintained by the third-party product certification body with whom the treatment plant is certified, i.e. the SABS or SATAS, which will also be indicated.
• SANS standard designation. This number is usually found below the quality mark and indicates the SANS standard the treated timber complies with, e.g. SANS 1288, SANS 753, SANS 754 or SANS 457. In the case of SANS 457-2 (hardwood poles) the additional letter refers to the class or grading, i.e. S – Structural /agricultural, F – Fencing and P – Guardrail post
• A hazard class which identifies the level of treatment applied (for example H4).
• Date or year of manufacturing of poles, the last two digits (for example 13 for 2013).

For poles and round wood, the information is mostly applied on 25 mm metal tags (all pine poles and Creosote treated hardwood poles) or anti-split end plates (CCA treated hardwood poles). The information may be presented in several layouts, but most are done as set out in the following diagrams:

Figure 23: 25 mm metal pole markers and anti-split end plates (50 mm >)

The number of poles marked (at the time of leaving the treatment plant) is for:

• Poles and half rounds: each pole shall be marked, preferably on the top end.
• Laths and droppers: One in a bundle, but in the case of loose laths and droppers at least 10%.

5. Applications

Pole structures were traditionally used to build low-cost houses, sheds, barns, and industrial buildings. It has become more popular in recent times to be used as roof structures with thatch cover in eco-style resorts. The poles are versatile for modification though the connection points can be problematic. This section will discuss various applications of poles in structures. Most of the information was gained from the SALMA Wood Manual Pamphlet 2.7 “Poles and Pole Structures”.

 Pole planting techniques

There are a few correct pole planting techniques that will be discussed in this chapter. Incorrect planting technique can cause premature degrading caused by water traps decreasing the durability. Planting a pole must be done correctly as to stabilise the deck by sufficient anchoring. The SAWPA website also have useful information: https://sawpa.co.za/how-to-plant-a-pole/. In the former document, the importance is stressed not to create a “cup” in which a pole is set. It is either with a solid concrete footing block below the pole or a concrete collar around the pole at ground surface level.

Collar pole plant technique: This is a very common technique to fix a pole and add structural stability. After a 300 mm diameter x 600 mm deep hole is dug in the soil, the pole is placed in the centre of the hole and a low strength concrete mix (10 – 15 mPa) is poured around the pole. Make sure the pole is plumb and supported in the correct position with three stakes. Skew planted poles can be aesthetically unacceptable and can cause instability as well.  Choose two faces at right angles to each other that can be used as reference for plumbness in case of tapered poles. Stakes can be removed after a few days to work on the pole. Make sure there is no concrete under the pole when planting. This will set in a cup shape that will trap water and lead to pole degrading.

Figure 24: Collar type pole foundation diagram (make sure the bottom pole end is not covered by concrete)

Footing pole planting technique: Instead of using concrete, back filling material is compacted around the pole that stands on a precast concrete footing. Correct size footing will decrease pole pressure and sinking effect, increasing deck stability. Place a concrete footing in the base of a 600 mm deep hole. The width will depend on the footing size; the area of the concrete footing can be 300 x 300 mm to 600 x 600 mm, and concrete can be 50 mm thick. This footing must be completely set and dry before the pole is planted. Position the pole in a plumb upright position. Back fill with compactable material with low clay content – G7 materials works very well. Only fill a 150 mm layer after the previous layer was properly compacted. Fill and compact layers until level with the surface. Correct backfilling material and technique will increase deck stability significantly.

Figure 25: Diagram showing precast concrete footing post foundation type

5.1 Roof structures

Poles can be combined as a shell-type roof structure, typical for thatch cover, or as a truss arrangement for a regular roof.

The shell-type roof structures, with poles meeting at the apex, is used for smaller buildings. Some designs proved challenging, resulting in issues such as roof sagging. It is recommended that a reputable contractor or structural engineer is to be used. These shell-type structures are common as thatch cover. More information about thatch roofing can also be obtained from the Thatchers Association of South Africa, at www.sa-thatchers.co.za, and SANS 10407.

Roof trusses are mostly constructed of lumber but can also be constructed of poles. Figure 24 indicates a scissor truss with a span up to 6 m, and constructed from poles, diameters more than 100 mm, and with the use of bolts, thicker than 13 mm diameter, as fasteners. Sideways stability is weak, and trusses should be well braced. Longer spans requires a bottom chord with a central vertical member between the apex and the bottom chord.

Figure 26: Scissor truss with a span up to 6 m (SALMA Manual)

Half round poles can be used as trusses with spans less than 6 m, as indicated in Figure 27 below.

Figure 27: Truss with a span up to 6 m, using half round poles (SALMA Manual)

5.2 Pole-supported buildings

The correct procedure for planting a pole was specified at the beginning of this section. Other foundation types that keep the poles away from ground contact are described. These methods require very low structures of additional anchoring to adjacent walls. Examples of it is galvanised steel brackets which are often elevated above the ground and fixed to a concrete base. The pole will thus be away from ground contact. The brackets can have a square or rectangular clamp, or it can be a hidden plate. All of them use bolts to fix the pole to the support. Concrete footings and brick/concrete piers can also be used.

It is suggested that geotechnical engineers are consulted where heavy pole structures are planned on problematic sites such as reactive soils, soils with low bearing strength, and steep sites. Reactive soils are usually clay soils that change volume according to moisture content variation. Pole construction mitigates this to a degree. Poles can be planted deeper, where seasonal moisture variation is limited. The construction process also should not encourage moisture to travel deeper into the soil. In low-bearing pressure soils, like wetlands and marshes, the poles, also called piles, are commonly driven into the soil.

Pole-supported buildings consist of pole platform construction and pole frame construction. Pole platform construction consists of planted poles that extend to the bottom of the floor framing and assist only as foundations for platform type frame structures. This method of building is like the foundations of timber decks, which is covered in “Timber deck building“.

Pole frame construction

Pole frame construction involves poles to function as columns from the foundation to the roof (see Figure 28). Note that poles can be indoors (on the left) or outdoors (on the right) to enhance the architectural effect. The poles carry all vertical loads and internal walls do not carry any load and can be placed anywhere.

Figure 28: Pole frame construction (SALMA Manual)

The architectural design of type of construction can enhance the natural environment in which it is positioned. With the low environmental impact, it can be positioned anywhere, even across water, permitting the correct preservative treatment.

More care should be taken from driving rain as water penetration is easy at openings and corners. More attention must be taken at flashings and windows. This is especially true where the poles are located externally (Figure 28 on the right).

Poles are usually put on a grid, which will determine the positioning of internal walls. Note that designs can be adapted to accommodate more open plan designs.

Roof covers can be metal sheeting, tiles, or shingles. A large roof pitch will result in a bigger temperature variation, and warmer lofts are common with a high roof pitch. The racking forces, however, will be larger in this case.

Low-rise portal frames

Low-rise pole portal frames are simple structures, and they are possible to be constructed by the owner, and further allow for additions and modifications. This low-cost solution is popular for agricultural and industrial buildings, e. g. economic sense. Poles serve as stable framework for the walls and roof. Large open spaces are available and permit access to bulky equipment. Diagonal bracing needs are limited when poles are planted between 600 mm to 900 mm deep.

Pole rafters, purlins and rails for cladding are irregular in diameter can create problems to supply an even flat surface to carry roofing or cladding. Half-rounds or scarved poles (poles with one longitudinal flat surface) could be considered to solve this problem.

There are three different structural systems in portal frames

1. Braced portals

The bracing is an essential part of the structure of the system. Figures 29 and 30 indicate the use of eaves braces and rafter ties. Spans range between 6 m and 9 m, with bay widths from 3 m to 4.5 m. Eaves heights are between 3 m and 5 m.

Figure 29: Braced mono-pitched roof portal (SALMA Manual)

Figure 30: Braced pitched roof portal (SALMA Manual)

Figure 31: Bracing with lumber connected to the pole structure (SALMA Manual)

2. Unbraced portals

Unbraced portals are used in industrial and agricultural pole buildings. Figure 32 indicates the use of eaves braces and rafter ties. Spans are shorter than 6 m, with bay widths from 3.6 m to 4.5 m reckoned to be the most economical. Since there is no racking, the fixing of the pole is critical to ensure good racking. Pole columns are often replaced by a pole lintel to support the rafter and to create a larger space between columns.

Figure 32: Unbraced mono-pitched pole building (SALMA Manual)

3. Steel tied portals

Round steel bracing rods with screwed ends are successfully used to provide good racking strength. The screwed ends are good to facilitate connection and fine-tune the alignment of the portal members. The steel rods only provide tension, and the structure should be symmetrically braced since wind suction on sheathed roofs reverse stresses on the tie rods. A tied rafter pole building can be seen in Figure 33. This arrangement limits sagging of the rafters and allows either longer spans or the use of smaller diameter poles.

Figure 33: Mono-pitched steel-tied rafter building (SALMA Manual)

A pitched double tied rafter pole building is indicated in Figure 34. There is also a tie rod across the apex, which can be tensioned with a turnbuckle. This tie resists the horizontal widening of the eaves and reduces bending moments and deflection.

Figure 34: Pitched double steel-tied rafter building (SALMA Manual)

A double-span and tied rafter pole building is shown in Figure 35.

Figure 35: Pitched double-span steel-tied rafter pole building (SALMA Manual)

The fastening of the steel ties to the pole structures are indicated as circles in Figures 33-35. The detail of these connections is indicated in Figure 36. All bolts and nuts of the tie rods must be tightened, and portals must be realigned a year after the pole construction.

Figure 36: Detail of steel rods for tied portals (SALMA Manual)

Bracing is required to give racking strength between the portals, in the longitudinal direction of the building (perpendicular to the plane of the portal frames). The resistance will partly be provided by the embedded poles and further by knee braces, as in Figure 37 A, or full cross-bracing across one of the end bays, as in Figure 37 B. The purlins must resist wind loads and the racking strength must be improved by roof bracing at both ends as in Figure 37C.

Figure 37: Longitudinal bracing of pole buildings (SALMA Manual)

Industrial buildings can also be constructed using poles. An example of poles used for crane girder columns and roof columns are shown in Figure 38. Both these columns are tied together and buttressed with poles. The crane girder spans 20 m and the crane rails are 9.3 m off the floor.

Figure 38: Buttressed pole columns at Weza sawmill (SALMA Manual)

5.3 Road bridges

Poles are often used as road bridges and consist of two basic components: first the superstructure, consisting of the upper framework for the bridge span, deck and rails, and second, the substructure transmitting loads to the supporting rock and soil (see Figure 39).

Figure 39: Buttressed pole columns at Weza Sawmill (SALMA Manual)

The super structure is made by positioning poles alternately tip to but along the length of the bridge (along the traffic flow). It is then banded with steel cables. Under these poles are shorter poles that run across in a transverse direction (perpendicular to the traffic flow) to distribute loads. The deck is formed by spiking lumber or half-rounds across the longitudinal poles. These boards should preferably be creosote treated to ensure stability if this would be the top surface. Otherwise a waterproof membrane is positioned across the deck and covered with bitumen or other suitable material. The length of the bridge is limited by the availability of the length of poles.

5.4 Other applications

Other applications include jetties, walkways landscaping walls and retaining walls.

5.5 Connectors and hardware

The curve of the pole can be reduced by using notches as in Figure 40. Notches reduce the strength of a pole, and it can reduce the longevity of the pole if a thick portion of the treated sapwood is removed. Notches will also provide a footing to support cross members.

Figure 40: Notches in poles to support rafters or joists (SALMA Manual)

The most common fasteners to be used on pole structures are nuts and bolts. The complexity of the roundness of poles makes it very difficult to use nails. Nails are good to use as temporary fastening while preparing for drilling or use of additional hardware to use nuts and bolts. Lumber is effectively attached to pole structure by using bolts and nuts.

Figure 41: Bolted joints (SALMA Manual; Pole and Post Buildings)

The bearing capacity of bolts and nuts is improved by using standard washers, though the best practice is to use spike washers / tooth plates or split ring connectors. Split ring connectors can carry close to three times the normal load of bolts. Spike grid connectors operate on the same principle, though can transfer more loads. When a threaded rod or bolt is fastened, the spikes are squeezed into the timber. Note that various configurations are available with different curvatures to accommodate poles.

Figure 42: Tooth plates, split ring connectors and spike grid connectors to increase carry capacity of pole joints (TECO Products; Expamet; Milspec Anchors)

A pole that serves as a beam on top of a pole, typically as a ring beam, can be fastened with a metal strap to tie the beam down. The strap can be perforated to allow easy fastening with clout nails. Figure 43 indicates this system.

Figure 43: Steel straps used to fasten a horizontal beam on top of a pole (SALMA Manual; iStock)

Nailed flitch plates are used to arrange pole timbers into a structural frame. These plates are inserted into longitudinal cut grooves. The nails are driven through the plate of a recommended thickness of 1 mm (see Figure 44 (b)).

Figure 44: Flitch plate connections (The Handcrafted Life; SALMA Manual)

Various metal brackets are on the market, such as in Figure 45, which is in essence a curved truss hanger that accommodates poles.

Figure 45: Flitch plate connections (MiTek; Building Strap and Ties)

Metal brackets are common to join a few poles at nodes in a pole structure. This is typical at the ridge / apex of roof structures. Figure 46 indicates such a system, with a central joint in the bottom chord.

Figure 46: Steel supported ridge joint with central joint of the bottom chord (Journal of Asian Architecture and Building Engineering)

5.6 Quiz 5