Understanding Wood – Hoadley
Chapter 4 - Strength of Wood
Chapter 4 of Understanding Wood explains the properties of wood relating to strength and how to orient the boards to make best use of them in furniture and other types of construction.
There is only one topic for this lesson.
How to Orient Wood for Strength
It takes enormous amounts of compression along the grain to damage wood. This is why pieces of wood cut with the grain running length-wise make for good table legs.
It is important that legs are thick enough to avoid flexing under a load.
Softer woods are especially susceptible to this type of damage. If dent resistance is important then hard, dense woods should be used.
Ash can be damaged if compressed across the tangential face (tangent to the growth rings). This is also true of any other ring porous wood. Ring porous refers to species of woods that have the largest pores concentrated in the earlywood.
Wood is so susceptible to damage when this type of force is applied that furniture-makers should take great care to avoid subjecting their furniture to it. On the positive side, this quality of wood also permits rapid material removal under the right circumstances.
It is highly unlikely anyone will ever see tension induced failure along the length of the grain in their furniture.
Pound for pound, wood is more resilient against bending than many other structural materials. It is therefore very useful for making beams and joists.
Wood does break when bent too far. This is especially the case when the grain doesn’t run all the way along the length of the board.
Wood can resist quite a bit of shearing force parallel to the grain before failure occurs. It will fail at some point though. It is however, much stronger in this regard than it is in tension across the grain. This is important to remember when doing any kind of wedged joinery.
Another example of shearing force being applied parallel to the grain.
Perpendicular shear can only by removing thin slices of wood from the end of a board. Attempts to shear the middle of a board will just result in compressive force type damage occurring instead.
Reducing span by 20% approximately doubles stiffness. Increasing span by 25% doubles deflection. Increasing depth increases stiffness.