Structural Timber Poles
Timber poles are utilised in structural construction to provide support for gravity loads and resistance against lateral forces. Not only serving a structural function, timber poles provide many aesthetic benefits, with their use in construction often complementing architectural designs aimed at harmonisation with the natural environment.
Timber pole construction is typically utilised to provide support for gravity loads and resistance against lateral forces. The natural appeal of timber ensures that its role is not purely structural however, with timber poles complimenting architectural designs aimed at harmonisation with the natural environment. The small number of footings required in pole frame construction also ensures minimal disturbances to the site.
With a double bearer system, poles can be spaced further apart than is usual, creating a more spacious building interior, that allows greater interior design flexibility. While poles are usually placed in a grid like system this is not compulsory and the flexibility of the application means the system can cope with a wide variety of designs, enabling designers to take full advantage of beautiful outlooks.
This article provides a comprehensive overview of the process involved in specifying, designing and constructing a solid timber pole construction.
Metal capping should be provided to any pole ends exposed to weather. This will prevent moisture entry and drying out of the ends.
Once the pole is on site, to avoid surface checks and a path to the inner untreated timber, it is imperative that it is not allowed to dry out too rapidly. This is best achieved by standing the poles as quickly as possible; the effects of sunlight exposure are less extreme when the pole is upright. A coat of a water repellent preservative or the application of a paint system can also be useful protection.
Cutting of timber in the embedded zone is not recommended as untreated timber may be exposed, hastening decay. All fixings in weather exposed areas should be 300mm above ground and cuts and drillings should have a preservative such as copper naphthenate emulsion applied before assembling the joint.
All fastenings into CCA treated timber should be hot dipped galvanised. Ensure elements such as bolts are well greased prior to installation. In any exposed situations, the bolt shank should be provided with a plastic sleeve or painted with a tar epoxy.
Pole structural design considerations are usually dependent on the geo mechanics of the site.
On steep sites, the integrity of the soil mass is maintained by the relatively few points in which the pole framing meets the earth. In addition, footings penetrate the often unstable top soil, providing a good foundation in the more stable underlying layers. A geotechnical engineer should be engaged prior to finalisation of design to report on foundation and stability of the slope. Depending on the steepness of the terrain, design alterations may be required to ensure the structure copes with increased wind forces. Refer to AS 1170, Part 2a, Loading Code - Wind Forces.
A reactive foundation is essentially a clay that significantly changes its volume in response to variations in surrounding moisture levels. Any structure built on such foundations will also experience this movement. Pole frame construction is particularly suitable for reactive foundations as the flexibility of the timber material and design methods can mitigate the effects of a reactive site. By increasing the foundation pressure through the use of small pole frame footings, swelling pressures can be resisted and foundations can be placed at a depth where the seasonal moisture variations are minimal.
Sites that have low bearing capacity are those with very soft clays and filled sites where the strength of the soil is low or the compaction has been so minimal that settlement will continue to occur for some time. For soft clay foundations, the poles must be founded in firmer soil, located further down or in the support generated by the development of skin friction. For foundations subject to ongoing settlement, support can only be generated by end bearing.
Pole frame construction is extremely adaptable to architectural expression. The ability to vary the placement of poles and the interchangeable relationship of poles and walls provide designers with a wide range of building forms to satisfy a variety of design requirements.
The following figures illustrate some of the possibilities.
Structures with limited room for bracing elements, such as those with high portions of glass, are most efficient with a roof less than 15°. There is a substantial increase in racking forces for roof pitches over 25°.
High roof areas will exhibit a significant temperature range. Various roof vent and turbines, throttled by dampers, may be installed in the roof to allow a degree of environmental control. The roof overhang should be given special consideration as it can protect the pole connections, cladding and windows, reducing maintenance.
To enhance the architectural effect internally, pole frames may be placed external to the walls. Where this occurs extra attention must be given to durability and flashing at bearers and floor joists. In the internal system, poles of lower durability and seasoned joists and bearers may be utilised, all but eliminating shrinkage problems and enhancing bearer pole connection. Poles can be set within the walls, provided shrinkage detailing ensure the walls remain weather proof.
Typically it is the size of the room that will dictate pole spacing and economics, and usability and aesthetics tend towards as few poles as possible. A secondary but equally important consideration is the size of both the bearers and the joists. Increased sizes allow accommodation of increased spans and together with the strength limitation of the bearer-pole connection, this can dictate the upper bound to the construction.
Embedded pole sizes are generally based on their deflection in response to lateral load to ensure they supply a portion of the racking resistance. Note that connections that are cut into the pole can adversely affect pole strength. In instances where poles are fixed to base plates, the pole size is usually more than adequate for the design loads.
The following outlines the elements requiring consideration during the design process.
When the pole is surrounded by an unyielding backfill, the structure may be designed with base fixity and a consequent reduction in bracing elements. The embedded depth required for development of significant moment would range from 1.5m to 2.5m. The backfill must be easily compactable down the side of the pole in a space nominally 100mm wide and up to 2.4m deep. Suitable materials for backfilling would include gap-graded fine crushed rock or river gravel with 10mm maximum aggregate size. For hardwood poles, a no-fines (10mm maximum aggregate) concrete should provide structural support while allowing drainage of moisture away from the pole.
For the three systems illustrated in the figure below, an impervious plug of clay or pre-mixed asphalt should be placed at the top to prevent surface water infiltration. Gravel is used to remove moisture from around the pole and dissipate it into the soil. Gravel and no-fine concrete are not useful backfills in soft clays as the soil particles may squeeze through the voids allowing base movements.
Ordinary concrete is an alternative backfill but is suitable only for use with treated pine poles. If the pole shrinks away from the concrete, pouring dry sand down the crevice may restore fix and stability. The crack can then be sealed using a bead of silicon based sealant. In the gravel backfilled poles, the effective width of the backfill pushing into the soil is usually about 80% of the hole diameter, while the concrete based backfills are fully effective in distributing the load over the full diameter. That is, the effective width of the laterally loaded pile, as far as the geo mechanics design only is concerned, is 80% and 100% of the hole diameter respectively.
The vertical load on pole foundations is concentrated due to the large grid and the often multi-storey nature of the building. For this reason, it is imperative that a Geotechnical Engineer is engaged to investigate the proposed foundation soil. Details of concrete footings and timber soil plates are given in the AS 1684 Timber Framing Code (Table 2, EF 14) for single floor loading. In foundations of stiff clay or better, a no-fines concrete plug 200mm deep should suffice to spread the load to limit settlement and, at the same time, allow dissipation of any moisture from the pole. It is important that all disturbed material is removed from the hole.
Regular inspection for signs of distress should be conducted and provision should be made to pour creosote or other preservatives down any shrinkage crevices as they develop. These gaps should then be sealed with a gap filling caulking material.
Securely fixing a base plate and a pole is difficult due to the irregular shape of poles, shrinkage and fastener skip. As such, poles to be seated on base plates will be designed from the outset as columns with pinned bases. During erection some level of fixing is essential as it is necessary for the poles to stand vertically without aid until the bracing elements are added. The following figure demonstrates some typical base connections.
There are several pole design options that are largely dependent on the type of base fixing required. For embedded poles, sufficient fix and stability can usually be achieved to allow a design of inter-connected cantilevers. The varying degrees of indeterminacy will be dependant on the floor arrangement and interconnection of poles.
The strength group of the specified pole will dictate the stress grade of the poles in accordance with AS 1720.1. (Refer table 6.1). By reading from tables 2.3 and 2.4 of the same standard, other working stresses and properties can be determined. Section 6 of AS 1720.1 covers the design of round timber and provides additional stress modification factors.
For domestic load conditions, sawn structural framing should be in accordance with AS 1684 Timber Framing Code.
The use of grid allows accurate setting out, especially on difficult terrain and more effectively locates the framing members to resist lateral sway. The use of continuous member, wherever possible, produces a stiffer structure and the fewer splices makes construction much simpler. If the grid chosen is non uniform and the floor framing is unseasoned then the one depth of member (to suit the largest span/spacing) should be used to the whole structure to simplify construction and to reduce differential timber shrinkage.
Bearers can be used in pairs to minimise connection loads and the resulting smaller sections makes them easier to lift. It is common to use 45 or 50mm wide double bearers, but these should be restrained by solid blocking between each pair at the third points and some care may need to be taken in exposed locations to prevent cupping.
As per the following figure, large cantilevers may be supported by knee braces.
In order to provide support to flooring and to maintain a rigid structure, joists should be placed both sides of the pole. This provides a load path that enables forces in the floor diaphragm to pass via these joists into bearers and then into the poles, while at the same time accommodating shrinkage. The flooring should not be fixed directly to the pole.
The use of roundwood as building elements, other than as columns, poses some complications, largely due to difficulties in marrying one round shape to another. The consistent fitting of the joint at fabrication is usually not practical given the difficulty of the site and the weight of these components.
The following table and figure shows that the load carrying capacity of a double bearer pole connection is significantly influenced by the bearer species.
Double Seat Capacity
Floor Area Supported m2
S3 or SD6
It is to be noted that when seasoned timber is exposed to the weather or high moisture conditions it should be regarded as unseasoned for determining both its size and connection.
The cutting of the pole for these seats reduces the bending strength of the pole at this point by both a reduction in permissible bending stress and section modules. The strength capacity at the notch is less than 30% of the intact pole for bending in a direction perpendicular to the bearers. Also, where the seat is exposed in timbers below Durability Class 1, the durability of the joint will suffer. In these situations, bolted connections without cut-in bearing seats are recommended. Cutting the pole should be kept to a minimum but where cuts are necessary, the area should be flood coated with a preservative such as copper naphthenate.
Pole framing construction is a form of post and beam construction where wall framing is subject to lateral loading only. This loading may be significant in high wind areas. Refer to AS 1684 Timber framing code for wall framing timber sizes. Some typical jointing situations are illustrated below, for situations where the poles and walls are in-line. Where the wall framing forms a part of structural bracing system, adequate tie down must be provided between the bracing wall and the floor, roof or pole.
The load may be carried from bearer to pole through a variety of bolted joints. Refer to the following table for design loads of bolted joints.
From the tables, it can be seen that bearer-pole connections relying on bolts alone are very limited, with the best achieving support of less than 10m2 of floor area.
Bull-dog connectors may be used to increase the capacity of softwood joints but are difficult to press into hardwood. Split-sharing connectors and shear plates may also be used to increase the joint strengths and the grid size but are difficult to install in round timber.
The steelwork should be hot dip galvanised and exposed connections should also be tar epoxy coated. Both the corbel and the steel seat connections are limited by compression perpendicular to the grain in the bearers.
Horizontal movement is caused by a variety of external factors making bracing essential to avoid resultant structural damage.
The structure can be designed to resist wind with a limiting displacement of height from ground/300. Then following construction, if ongoing vibrations are present, external bracing can be retrofitted.
The stiffness of a variety of bracing elements are defined in manufacturers published test data. In addition to requiring sufficient resistance to displacement, bracing systems must have a natural frequency as far removed as possible from that causing the exciting, otherwise the structure resonates.
The natural frequency of cantilevered poles forming a Pole Platform is about 2Hz (2 cycles/s) which is similar to that generated by walking. The cross-bracing maintained in a tight condition will bring a structure frequency to near 6 Hz. This means that annoying motions are more economically overcome by using bracing than by providing more or larger poles.
Common bracing systems used in pole construction are illustrated in the following figure.
Embedded poles deflect in proportion to the height to the third power, meaning short poles are most effective in combating these racking loads. The following table gives bracing strengths of embedded poles.
The following table and illustration demonstrate typical design loads and details for cross-braces.
Bracing walls are normal stud walls with structural sheeting of diagonal timber or steel members concealed under the lining. They should be located as close to the poles as practically possible and these elements should be distributed uniformly throughout the building so that horizontal diaphragms of the floor and ceilings are effective.
Unseasoned hardwood is often used in Australia due to its high joint carrying capacities. In conventionally framed multi-storey structures, the shrinkage in the bearers, joists and wall plates cause the whole building to move downwards as the timbers seasons over the first few years of life. In pole frame construction, each level is virtually fixed and shrinkage in the framing will result in movement in the top plates and floor or roof above.
Joists should be constructed to accommodate this movement and in addition cornices should be shadow fixed to conceal the movement internally.
To accommodate the shrinkage all bolts passing through the poles should have sufficient thread to permit later tightening. 40mm is usually sufficient. Structural square washers should be used at all bolt head to timber interfaces as per the following table.
By driving the small end of the pole down, the stiffness of the system is significantly enhanced. Reasonable alignment is achieved if the foundation is fine grained and homogeneous.
Pre-boring to 90% of the small end diameter will improve the accuracy of the driving process. Note however, that irrespective of the detail in this process, driven pole alignment is unsuitable for pole frame construction and only practically suited to pole platform construction, where misalignment of poles can be compensated for with the use of overhung cantilever and where the location of the bearer on top of the pole can be varied.
For potted poles, embedment of between 1.5m and 2.4m are common. Note that the bored excavation needs to be sufficient in diameter to facilitate positioning and plumbing of the pole, with additional room to allow compaction of the backfill.
Pole framed construction is common in exposed sites and thus the likelihood of water penetration is heightened. Extra attention to flashing is necessary around openings and corners and to penetrations through the cladding for joists and bearers.
Ease of intended maintenance practices should be a key factor considered during design planning.
Timber is an excellent material choice for structural applications. Being a natural product it compensates for any defects in its form (caused by injuries and knots) by shaping its fibres around these defects to ensure they are of little structural consequence.
Note that when the timber is sawn the inbuilt growing stresses and internal defects become more significant and the allowable stresses in the sawn timber are reduced to about 50% of that log. As a consequence, one of the best ways to maximise the efficiency of timber is through timber poles.
Poles for structural use can be prepared in a variety of ways and AS 2209 Timber poles for overhead lines describes acceptable limits for roundness and straightness, knot size, permissible checks and end splits.
Timber pole and beam durability is defined as follows:
• For in ground contact poles: Timber durability class 1 only. Alternatively, poles containing sapwood that can be effectively pressure preservative treated to a H5 level as per AS 1604 and AS 2209.
• For roof poles, mounted on stirrups or stools clear of ground contact under exposed conditions: Durability class 2 or a H3 level preservative treatment is acceptable.
• For above ground contact in protected environments: Any durability class of timber is acceptable, provided any lyctid susceptible sapwood is treated to H1 level in accordance with AS 1604.
The most common preservative treatment is CCA (Copper Chrome Arsenate). To avoid difficult and costly pole replacement as the result of decay, appropriate quality control is essential to ensure treatment is performing to an adequate level. Independent checks can be arranged through the preservative treatment suppliers or the State Forest Services.
Softwood Poles (Pine): Pine softwood poles accept treatment well, are lighter and have a reliable structural service history. During processing the logs are debarked and often shaved to achieve a more uniform shape. Note that shaving will effect the strength of the timber as smaller diameter poles (less than 225mm diameter) are composed of less dense wood. Allowance should be made for this by reducing permissible stresses and stiffness as per AS 1720.
Hardwood poles: Double treated poles will extend the life of in ground, embedded hardwood poles. Alternatively additional ground maintenance can be applied by flooding the area with creosote and diffusion chemicals such as Koppers NC Oil Emulsion, facilitated by a polythene tube embedded in the backfill. End splitting is best controlled by the use of end sealers combined with nail plates or metal collars.
Pole shape: The bow in a pole is called the ‘sweep' and is defined in AS 2209. For a select grade pole, an acceptable single sweep grade pole 10m long is 70mm. When you combine this with some non-circularity and taper, compounded by placement tolerances, it is unlikely that the pole is in an accurate position at each level. The following figure demonstrates some typical methods for overcoming this issue.
Sawn Framing: All sawn framing structural timber should comply with the relevant Australian Standard, stress grades and quality assurance programs.