Wood Strength: What Makes Wood So Strong?

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    It’s important to consider wood strength when choosing a type for your woodworking project. There are different strength properties to consider, too — like compression strength, hardness, and stiffness — each of which will play a role in determining whether a specific type is right for your coffee table, door, chair, or other projects.

    The success of your project starts with the strength of wood, and whether it’s enough to withstand the forces you’ll be putting on it for years to come. Here’s why it matters.

    Why Wood Strength Matters

    Do you want your piece to last for generations, or should you expect to replace it in a few months? That’s how much difference the type of wood can make. Consider a cheap piece of furniture you can purchase at a big box store compared to an antique desk that’s been in your family since your great-grandmother. Those are likely made with two different types of wood, with different mechanical properties.

    Longevity is all about taking bending strength, shear strength, and other properties into consideration. If you want your furniture to last, don’t choose a wood based merely on color, appearance, or current popularity. Choose based on strength.

    unsmoothed wood shelf holding decorations in a house

    Hardwood vs. Softwood

    Tree species are divided into two basic types: hardwood and softwood. Those are tricky terms, though, because hardwood isn’t necessarily denser. Those categories aren’t just about hardness, either: They’re actually mostly about reproduction, though there are other properties they have in common.

    Hardwood

    A hardwood tree is one that produces seeds with a covering. Examples include oak, ash, and walnut. This makes balsa a hardwood, despite being extremely lightweight and one of the least dense woods available. Hardwoods are often darker in color, and they  tend to be deciduous trees with broad leaves.

    Softwood

    A softwood tree is one that drops unprotected seeds that spread in the wind. They’re typically evergreen trees like pine, Douglas fir, and cedar.

    Both types have their place in building:

    • Softwoods are easier to grow and easier for a woodworker to manipulate.
    • Hardwoods are more durable.
    • If you’re looking for a piece that stands the test of time, you’ll probably want to use a wood species like oak, or walnut. They’re more expensive and more challenging to work with, but they produce beautiful, long-lasting pieces of furniture.
    • If you’re looking for a less expensive item, don’t…

    Keep in mind you will see differences in strength properties even among closely related species. Yellow pine is stronger and harder than white pine, for example.

    Black Walnut White Epoxy Coffee table in living room

    Factors That Impact the Strength of Wood

    There are different types of strength and specific ways to measure them, which gives us several factors that influence how strong the wood is overall — and whether it’s appropriate for your next project.

    1) Grain Direction: Working Parallel or Perpendicular to the Grain

    Wood is best able to resist forces that run parallel to the grain, while they’re more susceptible to stress that comes perpendicular to grain. It’s thus important to consider the grain direction when putting furniture pieces together to ensure the finished product is best equipped to resist stress. Uneven grain and knots are indications of a weaker wood.

    2) Specific Gravity

    Specific gravity is the measure of density, a property value found by dividing the weight of the wood by the weight of the same volume of water. A higher ratio indicates a high density and typically a stronger wood. Moisture content impacts this measurement, though, so it has to be considered along with density values. Typically, density is measured with a moisture content of 12 percent or less. Keep in mind that wood should be air- or kiln-dried to 12 percent before building with it. In many cases, a moisture content of 8 percent to 10 percent is preferable.

    3) Compression Strength

    This is how much stress a type of wood can bear under compression parallel to the grain before it reaches compression failure, or the collapse of the wood fibers under the force. This is measured in pounds per square inch (psi). A balsam poplar has a compressive strength of 4,020 psi, for example, while yellow birch sits at 8,170 psi. This is an important consideration for table and chair legs, which endure high levels of compression on an ongoing basis.

    4) Bending Strength

    Also measured in psi, this is the load a wood can bear under compression perpendicular to the grain. The maximum fiber stress for poplar is 300 psi, while the yellow birch can withstand up to 970 psi. Bending strength is also referred to as the “modulus of rupture” or “flexural strength.”

    5) Stiffness

    Also called the “modulus of elasticity,” this indicates how much the wood will give or sag under the weight of a load applied perpendicular to the grain and is measured in millions of pounds per square inch (Mpsi). Hickory offers a high modulus of elasticity at 2.16 Mpsi while the North American softwood white cedar is quite low at 0.8 Mpsi. This is important when building a shelf, for example, especially if you intend to place a heavy load on it.

    6) Hardness

    The industry standard for measuring the hardness of a species of wood is known as the Janka test. They take a ½” steel ball, and press it into a piece of wood until it is halfway embedded. And they measure the force needed to get it there. 

    This test is one of the best measures of the ability of a wood species to withstand denting and wear. 

    If you’re not sure whether the type of tree you’d like is right for your project, contact a woodworking professional for help. They can tell you whether the wood is up to the challenge of the stress you’ll be placing on the finished product.

    Get Woodworking Advice From a Professional

    Woodworking is both art and science, and it’s not just about aesthetics. You have to consider load and stress when choosing a type of wood to ensure it has the required hardness, shear strength, specific gravity, and more to hold up to both the intent of your project and wear and tear over time. Contact Makers Woodshop today to speak with an expert about strength and other properties of wood.

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