Tensile Strength of Wood

Tensile Strength Of 2×4

Let’s use the tensile strength as an example. Deflections of a beam under load depend on its cross-sectional area in the plane defined by the axis of the beam and the direction of the applied load.

The larger the cross-sectional area, the more the beam is able to resist the load.

For dimensional lumber the cross-sectional area of a 2×4 is its length, L, times its thickness, 1.5″.

When you stand the 2×4 up, it has a cross sectional area of L times its width. For dimensional lumber this is 3.5″.

The tensile strength of wood is commonly termed, measured and represented as a  strength property.

Tensile strength in building lumber is measured in two main ways. The tensile strength parallel to the grain and the tensile strength perpendicular to grain.

Additional measurements are often made to assess work-to-maximum-load in bending, impact-bending strength, and hardness. 

These properties are generally broadly grouped according to the categories of hardwood and softwood.

Tensile Strength Perpendicular To Grain

This refers to the resistance of wood to force acting across the grain that tends to split a member. 

Tensile Strength Parallel To Grain

That is the maximum tensile stress sustained Parallel to the grain. Across a variety of species of clear wood, relatively few data exist on tensile strength along the grain. The following table (sourced from Mechanical Properties of Wood: David W. Green, Jerrold E. Winandy, and David E. Kretschmann) provides the parallel-to-grain tensile strength of a few wood species tested at green. Note that decreased moisture content (12%) increases tensile strength from these values up by 32% in hardwoods and 12% in softwoods.

Modulus of rupture is often substituted for tensile strength of small, clear, straight-grained pieces of wood in the absence of enough tension test data. 

Modulus of rupture is usually used as an indicator of tensile strength for clear specimens.

Compression Failures

Products with visible compression failures have low strength characteristics, especially in tensile strength and shock resistance. Compared to matched clear wood, the tensile strength may be one-third lower in wood with compression failures. Even a small amount of weakening, visible only with a microscope, may result in brittle fractures and severe strength reductions. Because compression failures have low strength, many safety codes require certain structural members, such as ladder rails and scaffold planks, to be entirely devoid of these failures.

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