BRE - British-grown Douglas fir: Growth rate and density relating to visual grading and strength class attribution

Organization: BRE
Publication Date: 1 April 2014
Page Count: 8
scope:

Introduction

Douglas fir (Pseudotsuga menziesii), like many timber species grown in the UK, is a native of the North American continent and originates in the area of California through the west coast mountain ranges up into Canada. In the US it is called 'Oregon pine' rather than 'Douglas fir'. The UK name comes from David Douglas, who first sent seed back to Britain in 1827, and the 'menziesii' part of the botanical name derives from Archibald Menzies, who sent botanical samples back to Kew whilst exploring the US with George Vancouver in 1793.

Douglas fir is a relatively minor timber species in the UK, like larch, with each species accounting for about 5% of the structural timber produced. In the US and Canada it was traditionally used for railroad construction and general construction work, due to the size of the trees and the durability of the timber in that environment. The natural durability of the material, as classified by BS EN 350-2:1994[1], varies based on origin, with the North American supply classified as moderately durable in resistance to fungi, whilst European-grown material is classed as moderately to slightly durable. The heartwood has a warm, slightly orange hue whilst the sapwood is pale, almost white. Figure 1 (righthand image) indicates the general appearance of the timber when planeD.

For its strength Douglas fir is stiff, with a high modulus of elasticity. This is an important factor for structural timber design as serviceability criteria, governed by stiffness, are generally the design-limiting factor, rather than strength. Douglas fir is finding increasing application on localised bespoke projects as structural framing (Figure 2). It is also available in large, relatively clear baulks suitable for timber engineering applications.

One of the current difficulties encountered when using this species is that the strength attributions for the visual grades in BS 4978:2007[2] are low: the general structural (GS) grade is C14 and the special structural (SS) grade is C18. Typical strength grades used for structural timber in construction are C16 or C24, so whilst we have to rely on visual grading, this has the risk of precluding Douglas fir from these valuable applications.

In BS 4978:2007, rate of growth (ie width of the growth rings) is used as a selector for density on the premise that a slow rate of growth equates with higher density, as density is related to the amount of wood substance present. This is a wellestablished relationship, but is often stated without a detailed understanding of how trees grow. Douglas fir grown in the UK tends to be fast-grown, which manifests as wide annual growth rings. Unfortunately this means that excellent-quality material with minimal strength-reducing characteristics is downgraded from the SS grade to the GS grade, simply due to rate of growth.

A careful look at how trees grow indicates that within the growth ring there are two components of growth: the earlywood and the latewood (Figure 3). The earlywood is produced in the spring when the tree starts to grow and is used to transport water up the tree. This material tends to be relatively low-density. Later in the growing period latewood is produced in preference to earlywood. The function of the latewood is structural support for the tree. This is where most of the wood mass of the growth ring is to be found, and in consequence this part is dense. Growth rings that comprise a good mix of earlywood and latewood have good density.

References

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