Wood Strength and Structures


Figure 14.1. Subject a post to enough stress and the long fibres will crumple and bend over as illustrated in figure 14.2. The failure in the photographed example came from the pressure applied to the end of a steam-bent oak strip restrained by the end stops of a bending strap.

This is an excerpt from “Cut & Dried” by Richard Jones. 

A tree’s form, looked at as if it was an engineered structure, consists of a column and cantilevered beams. The trunk is a column and the branches represent beams. The wood forming the structure of a tree, from root tips to the leaves, must simultaneously withstand vertical and horizontal loads, as well as twisting, flexing and shearing stresses. The trunk, for example, is anchored to the ground by the roots and carries a vertical load but it also experiences twisting and bending as the crown moves in the wind; the trunk itself is therefore a column cantilevered from the ground. The branches, cantilevered from the trunk and similarly cantilevered from each other, have to carry the weight of the leaves, withstand gravity and cope with wind-induced side-to-side movement and other forms of bending and flexing. The strength and mechanical characteristics of wood developed as a means to support the living tree, not as a material for the use of mankind; it is fortuitous for man that trees evolved into such useful material that we have been able to take advantage of for millennia.

To cope with the mechanical stresses life throws at them, trees have developed wood as their supporting structure. Wood is anisotropic because it has inconsistent properties in different directions. Essentially this means the inconsistencies exist longitudinally, radially and transversely within a branch or log. Secondly, wood is heterogeneous, meaning its composition varies. It’s easy to see these anisotropic and heterogeneous characteristics of wood because of the obvious irregularity visible in the grain structure. Steel and plastic, on the other hand, are homogenous and isotropic materials, being uniform in composition with equal properties in all directions. Some engineers of my acquaintance take issue with this and insist sheet steel, for example, has a “grain.” This suggests it is perhaps measurably stronger in one direction than another, and bends more easily one way than another. I cannot verify this claim independently but I am inclined to believe the word of those who work the material on a regular basis.

Frequently-tested Strength Properties
The mechanical properties of wood indicate its strength properties. Testing of these properties takes place in both hardwoods and softwoods. The terminology used to describe the mechanical properties of wood identifies the applied stress, e.g., compression, tension, torsion (twisting) and shear and, secondly, the orientation of the stress to the wood’s grain direction. Briefly, the descriptions are as follows:


Figure 14.46. Architectural post-and-beam construction. Wooden post-and-beam construction supports a deck above boathouses. The deck is a dining area for the attached restaurant.

Compression Parallel to the Grain – This stress shortens the fibres lengthways, e.g., the weight of a building compresses the stilts (posts) of certain types of beach houses or boathouses, or the legs of chairs. A slim post can support a lot of downward pressure (weight) if there is a method found to prevent the post from either buckling or falling over, e.g., pedestal chairs with a supporting bracket-like foot.


Figure 14.2.

Compression Perpendicular to the Grain – 
An example of this is a loaded shelf that sags unless the shelf has additional structural support. A shelf under compression (loaded on the top face) leads to tension (stretching of the grain) on the bottom face. Some woods are more elastic than others. Ash is comparatively elastic and will recover better than pine. The elasticity of ash is one reason for its suitability for hammer and axe shafts, and baseball bats.


Figure 14.3. A wedge, such as an axe used to split firewood, illustrates tension force applied perpendicular to the grain.

Tension Perpendicular to the Grain – 
Cleaving or riving illustrates this form of stress (see figure 14.3), and is used to advantage in traditional country wood crafts, for example, those associated with coppicing such as fence and hurdle making and basket weaving. Riving or cleaving oak radially to create workable pieces of timber has a long tradition. Driving nails into wood applies the same stress, and some wood species require pre-boring prior to nailing to prevent splitting, whereas others generally don’t require pre-boring. Those species benefitting from pre-boring tend to be even grained but hard to very hard such as maple, or have distinctly soft spring growth and hard summer growth, such as some softwoods including Douglas fir, and ring-porous hardwoods such as ash and oak.

Meghan Bates

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