Wood

Wood is a fascinating material in terms of both its structure and mechanical performance. It is an ecologically friendly construction material since it is a renewable resource and, indeed, the energy required to produce unit weight of timber is lower than for any other building material. Wood is also cheap – it costs sixty times less than steel per tonne. Wood is a low-density, cellular polymeric composite and consequently does not fall into any one category of material.

Internal Structure:









Fig. 1 Diagrammatic illustration, showing the principal structural features, of a wedgeshaped
segment cut from a mature hardwood tree [after J.M. Dinwoodie, J.Microscopy,
104(1), 1975].

Wood, predominantly, has a structure comprising parallel, columnar cells (see figure 2). The elongated cells can be considered as fibres, embedded in a matrix of the polymer lignin. The cell walls contain helical windings of cellulose microfibrils.

















Fig. 2 Simplified structure of the cell wall showing orientation of the microfibrils in each of
the major wall layers [after J.M. Dinwoodie, J.Microscopy, 104(1), 1975].

The exact microstructure of wood depends on whether the material is derived from coniferous trees (softwood) or broad-leaved trees (hardwood). In both types of wood, 90-95% of the cells are aligned along the vertical axis, while the remainder are in the radial directions. There are no cells in a tangential (or hoop) orientation. The distribution of cells is different in all three principal sections (in cross section, tangential section and radial section as illustrated in Figure 1) and wood is therefore very anisotropic. There are four different types of cells in hardwood.

• Parenchyma cells (200 - 300 μm in diameter these cells are responsible forcarbohydrate storage and can be aligned horizontally or vertically);
• Tracheids (which perform the function of both storage and support);
• Vessels (responsible for conduction, these are cells whose end walls have beendissolved away and they are short - 0.2-1.2 mm, and wide - approx. 0.5 mm);
• Fibres (these provide the principal source of support and they are long (1 -2 mm)with an aspect ratio of 100:1).

However, when observing the microstructure of a hardwood sample in practice it is normally only easy to see the vessels, fibres and parenchyma cells, since the tracheids are very few in number.

(source: University of Cambridge, Department of Materials Science, Microstructures and Mechanical properties of Wood, December 2008)














Wood-Cedar-Eastern-Juniperus Virginiana-cross-section














Wood-Cedar-Eastern-Juniperus Virginiana-longtitude-section














Wood-Cedar-Eastern-Juniperus Virginiana-radical-section

Image courtesy of MicrolabNW Photomicrograph Gallery


Bonding Force











The bounding force inside a wooden volume base on the cellulose cells' wall.

















Sectional view of a soft wood plank (Conifers which may have scale-like (cedar wood) or needle-like leaves (pines)). Image courtesy of Paulo Monteiro, University of California Berkeley, Introduction to Wood lecture.


















Sectional view of a hard wood plank (Broad leaf trees (oak)). Image courtesy of Paulo Monteiro, University of California Berkeley, Introduction to Wood lecture.

As of the two above image, we can see wood is best for bearing pressure along its fiber direction because the pressure is distribute equally along its cells' wall. Tension can be distribute pretty good along the cell's axis too, but a significant length before the brace's end must be consider to avoid splitting. Build up from straws packed fiber cells, wood provide a certain level of bending but not so much because the fiber cell would shrink. For longitudinal, tangential and radial directions, wood's ability to stand cutting force is worst.

Shrinkage:













Image courtesy of Paulo Monteiro, University of California Berkeley, Introduction to Wood lecture.

Sawing Lumber:

















Image courtesy of Paulo Monteiro, University of California Berkeley, Introduction to Wood lecture.


Non-Conventional Usage



Alvar Aalto's Paimio Chair.

By sticking many layer of plywood under a constraint bending force, Aalto had successfully force wood to bend into curvature but also keep its elastic and aesthetic.



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