Roundwood refers to naturally round wood such as posts and poles which are often used in structural applications. Roundwood is relatively inexpensive because it requires very little processing.
High strength for weight
The density and strength at the outside of a log is higher than at the centre. In the production of sawn timber, much of this strong outer wood is removed and wasted.
In roundwood this is retained, at the outside of the structural member where it is most needed.
In addition, knots in logs are naturally reinforced by nodal swellings, and this is retained in unshaven roundwood.
For these reasons the design code NZS 3603 gives higher characteristic stresses for roundwood than for sawn timber.
High strength for cost
Because there is less processing, less waste and higher strength material, roundwood offers more load carrying capacity for the same cost than sawn timber, and much more than glued or laminated products.
These differences give roundwood advantages in utility structures where price is relatively important compared to appearance, and in those structures that require strength, rather than stiffness.
Poles are available in a wide range of sizes. The diameter is usually called up as “Small End Diameter” (SED) with common sizes being from 125 to 350mm, in 25mm increments.
Lengths are usually in 0.6m increments up to 6.0m long, then in 1.0m increments up to 12.0m or more. Sizes up to 18m in length and 600mm in diameter are available as special orders.
Radiata pine is very suitable for treating as it has a large layer of sapwood.
The heartwood at the centre of most logs is difficult to treat and the penetration of the chemicals is often low (10 – 12 mm).
In general the core of a pile or pole does not become well treated. An alternative to radiata pine was Corsican pine, but it has not been planted for some years due to its slow maturing time, and susceptibility to dothistroma needle blight.
The strength of a radiata pine log is partly related to the rate of growth (the slower the growth the stronger the log).
Forests with closely spaced trees tend to have slower growth rates, with smaller knot sizes and hence yield higher strength poles than more intensively managed forests.
Excellent poles were obtained from early pine forests planted in a close spacing regime, and not pruned nor thinned. In the past, such forests (after growing some 30 to 40 years) produced thin, tall, straight trees (20 m to 30 m) ideal for the pole and post industries, but these forests are now rare.
Faster growing managed forests produce pole material for larger sizes above 200mm SED, with some of the best poles coming from slower growth trees planted in sand dunes.
Unless the designer is confident of the supply of high strength poles or the construction process is in a very controlled situation, it is wise to design for poles of the lower strength group.
Non availability of high strength poles during construction can delay the project and involve substantial redesign and disruption.