STEP lecture A4 Objectives

P. To provide the concept of wood as a cellular, anisotropic material. To present the Technical University basic definitions of moisture content and density. To introduce shrinkage of and their implications in structural engineering. To present the

background for the understanding of modification factor, Summary

The concept of wood as a composite material is presented. The microstruc- ture of the wood cell wall is discussed with a view to explaining the anisotropic nature of and mechanical properties of wood. Important features of macrostructure included; keywords are growth rings, juvenile and reaction

wood, grain deviation,

Density is the single most important physical characteristic of wood. The variability is discussed the concept of characteristic density presented.

Water is always present in wood. The amount of water has a profound influence on all wood properties. Moisture content and fibre saturation point are defined and the sorption isotherm presented. Anisotropic shrinkage and swelling are introduced and their engineering implications discussed. Different types of caused by drying presented. An overview of strength relationships and the influence of moisture on the failure mechanisms of wood

wood based is discussed.

Wood and wood based experience a significant loss of strength over a period of time. For permanent loads, wood wood based materials are assigned strength values that are 60% or less of their term strength. This duration of load effect is discussed and moisture dependency is described.

Introduction

Wood is a natural, organic cellular solid. is a composite made out of a chemical complex of cellulose, lignin and extractives. Wood is highly anisotropic due to the elongated shapes of wood cells and oriented structure of the cell In addition, anisotropy results differentiation of cell sizes throughout a growth season and in part a preferred direction of certain cell types ray cells).

The structure of cell walls, the aggregation of cells to form clear wood and the of structural timber represent three structural levels which all have a profound influence on the properties of wood as an engineering material. For instance, of cell wall provides the explanation of why

and swelling of wood is normally to 20 times larger in the direction than in the longitudinal direction. The microstructure of clear wood the key to understanding why wood is 20 to 40 rimes stiffer in longitudinal direction the transverse direction. The macrostructure of knots, fibre angle etc. provides the explanation of why tensile strength along the grain drop from more than for clear wood to less than 10 for timber of low quality.

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Wood is obtained from two broad categories of plants known as hardwoods deciduous trees) and softwoods (gymnosperms, conifers) (Figure

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right:

The observation of wood without optical aids shows not only differences between softwoods and hardwoods differences between species, but also differences within one specimen, for sapwood and heartwood, and Iatewood, the arrangement of pores and the appearance reaction wood. All these phenomena are the result of the development and growth of wood Softwoods and hardwoods differ in cell type (Figure 2).

shows a relatively simple structure as it consists of 90 to 95%

which are long (2 to 5 and slender to 50 cells with flattened or tapered, closed ends. tracheids are arranged in radial files, and their longitudi- nal extension is oriented in the direction of the stem axis. In evolving from earlywood to the cell walls become thicker, while the

become smaller. At the end of the growth period tracheids with small cell and radial diameters are developed, whilst at the beginning of the subsequent growth period tracheids with large cell and diameters are developed by tree (Figure This difference in growth result in a ratio between density and earlywood density as high as 3: 1.

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Resin

Figure 2 Models of a a block, for

storage and the transport of take place within parenchyma cells which in are predominantly arranged in radially running rays (Figure 2).

Resin canals are longitudinal radical cavities within the tissue of most softwoods.

The tree sap stream from one to another is by small openings or recesses in fibre wall known as pits. A predominant type in softwoods is the bordered pit. These not let water move freely but they also act as valves to prevent the spread of air into sap cells, in which case the water columns, extending the roots to the crown, would and the would

die. pits function during drying of timber.

Capillary forces are developed upon retreat from the cell lumens the pits, and the pit effectively to seal off the pit openings. This not only the drying of wood; it also may impede greatly susceptibility to later treatment. Such pit aspiration is the reason why spruce, for example, is very difficult to impregnate.

anatomy is varied and than that of softwood, but most concepts are analogous. Hardwoods have a basic tissue for strength containing fibres and fibre Within this strengthening tissue, conducting vessels are distributed, large These vessels are long pipes ranging a few up to metres in length and consisting of single with open or perforated ends. Diffuse-porous and ring-porous hardwoods can be by the arrangement of the diameter of the vessels (Figure 3). Hardwood fibres have thicker cell and smaller those of softwood twcheids. The differences in thickness and lumen diameters between earlywood and are not as as in softwoods.

The number of parenchyma cells in hardwoods is higher than in softwood.

Hardwoods often have very large rays and particularly in tropical hardwoods are percentages of longitudinal parenchyma.

Some basic features of the wood cell are found to be among many different wood species. The basic substance of wood cell wall is cellulose which is aggregated into larger units of structure called fibrils.

These, in turn, are to entities known as microfibrils. The of cellulose chains contained in each has been to be

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in the range of 100 Lo 2000. The cellulose in a is embedded i n a

of and by

of of pore

cross section. A, (red B,

layered structure of the fibre cell wall is illustrated in Figure 4. Between the individual cells there is a layer, the (ML), which glues the cells together to the tissue. The middle is rich in lignin and pectic substances virtually free of cellulose. In the ( P ) the cellulose microfibrils are arranged in a irregular network. In normal wood tissue, the

consists of layers and The outennost

layer, S,, is very thin to 0,2 and exhibits an average angle (for the layer as a whole) of about 70". The of the secondary wall is up of the layer, which is typically several thick. The are usually oriented to fibre at a relatively angle (5 to Within the the are arranged with a gentle slope but not in a strict order.

4 Key: cell

S,, of

From viewpoint, the is an ingenious construction.

The dominant layer axially oriented bundles of rnicrofibrils very an initiative under

effectively takes up tension forces. In compression the bundles of are turned into slender columns which are then prevented from buckling by the inner and outer reinforcing layers of S, and microfibrils having a more gentle slope.

Growth rings

For softwoods and ring-porous hardwoods there is a relationship between the width of growth rings and density, Softwoods tend to produce high density Iatewood bands of a relatively constant thickness. Most of variation in growth ring width is caused by a variation in the thickness of the low density early wood bands. For most softwoods, therefore, density decreases increasing ring width. This explains why ring width is included as a grading parameter in many visual grading rules currently used in Europe. However, caution be exercised when using such The density level for a given ring width is dependent on soil type, conditions, practice etc. Therefore, for softwood of origin, ring width does not predict density any real accuracy (Figure 5).

0 I 2 3 4 5 6 7 8

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Ring-porous hardwoods such as oak and ash are characterized by a high concentra- tion of open vessels produced during spring. The width of these rings is relatively and the variation in growth ring width is caused by a variation in the thickness of the high density bands of fibre This is why density increases with ring width for most ring-porous hardwoods. is no such relationship for diffuse-porous hardwoods such as poplar beech.