Reviewing: Strength of Wood

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

compressing a board along the grain

Wood is highly resistant to compression along the grain.

 

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.

 

 

 

 

 

thin board flexing as it is compressed along the grain

A board may resist compression along the grain but it will flex if too thin.

 

It is important that legs are thick enough to avoid flexing under a load.

 

 

 

 

 

 

compresssing the faces of a board

Wood tends to dent easily when compressed across the faces.

 

Softer woods are especially susceptible to this type of damage.  If dent resistance is important then hard, dense woods should be used.

 

 

 

 

 

 

compressing ring porous woods

Compressing ring porous woods can cause de-lamination.

 

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.

 

 

 

 

 

tension being applied across the grain

Wood readily fails when under tension across the grain

 

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.

 

 

 

 

 

tension being applied along the length of the grain

Wood will almost never fail due to tension along the grain.

 

It is highly unlikely anyone will ever see tension induced failure along the length of the grain in their furniture.

 

 

 

 

 

 

applying force to bend a stick

Force being applied to bend a stick

 

Pound for pound, wood is more resilient against bending than many other structural materials.  It is therefore very useful for making beams and joists.

 

 

 

 

 

bending a stick to the point of breaking

This stick was bent too far and broke.

 

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.

 

 

 

 

 

shearing force being applied across the grain

Wood has poor resistance against shear across the grain.

 

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.

 

 

 

 

 

shearing force being applied along the grain

Wood is fairly resistant against shear applied with the grain.

 

Another example of shearing force being applied parallel to the grain.

 

 

 

 

 

 

applying shearing force across the end grain

Wood is very resistant to shear force applied across the end 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.

 

 

 

 

 

descriptive view of a wooden beam

The length and depth of a beam have large impact upon its stiffness.

 

Reducing span by 20% approximately doubles stiffness.  Increasing span by 25% doubles deflection.  Increasing depth increases stiffness.

 

 

 

 

 

 

 

 

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