Basic Strength Properties of Wood

Structural designs that take advantage of wood’s natural strength properties are economical, attractive, and easier to design and detail.

Wood’s strength depends on the direction of the grain. The technical term used to describe this variance is “orthotropic,” which means having different strengths in three planes perpendicular to each other. Here’s an easy way to understand the orthotropic properties of wood: Visualize a tree trunk as structurally similar to a bundle of drinking straws. Push on the long ends of a bundle of straws, and the straws can take a good amount of pressure and still stay in shape. But fold the straws in half, and they deform easily.

Like a bundle of straws, wood grain is strong in the longitudinal direction. Think of columns of tree trunks holding up roof trusses as a classic example of this longitudinal strength. Wood is not as strong, however, when loaded perpendicular to the grain. Too much tension perpendicular to the grain pulls wood fibers apart and can lead to failure. For this reason, notching wood beams, the use of large diameter fasteners, or hanging loads all need the attention of a qualified design professional. Remember, it’s better to use multiple small fasteners rather than one large fastener because concentrated loads can over-stress wood’s capacity.

APA – The Engineered Wood Association provides detailed recommendations for drilling and notching engineered wood products.

Wood Connection Design Best Practices

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Proper connection details are critical to the structural performance and serviceability of timber-framed structures.

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Moisture and Environment

Wood is sensitive to moisture changes. It expands in moist or humid environments and shrinks in dry environments, so it’s important to design connections that allow for movement. Connections that aren’t detailed correctly for wood shrinkage can cause splitting. Detailing connections with slotted holes allow wood movement without causing stresses perpendicular to the grain. Wood shrinkage can also result in loose connections, while swelling can deform connection hardware. Well-designed wood connections take into account these natural tendencies.

Find illustrated guidance on detailing connections and avoiding common connection errors in APA’s Glulam Connection Details construction guide, available as a free download. Guidance for connecting side-by-side glulam beams is also available.

Exterior connections must drain moisture, since trapped moisture provides ideal conditions for fungal growth and decay. To ensure they last, exterior connections must be properly vented or drained. Avoid leaving wood in direct contact with concrete, masonry, or grout, which are porous and wick moisture. One solution is to leave a 1/2-inch air gap between wood and these materials.

Wood in wet or harsh environments, especially salty ocean air, or wood treated with preservatives or fire retardants requires corrosion-resistant connections.

The Nitty-Gritty on Nails

Nails come in a wide variety of types and have many different names, and to complicate matters, there is no standard for nail nomenclature. For example, different nails with the same pennyweight are not interchangeable; a 10d common nail has a diameter of 0.148 inch, a 10d box nail has a diameter of 0.128 inch, and a 10d sinker nail has a diameter of 0.120 inch. Avoid confusion and ensure correct nail size by specifying diameter and length, with an optional description of nail by pennyweight and type (e.g., 0.148” x 3” [10d common]).

Looking for more in-depth guidance? Find details on wood connection design best practices at www.apawood.org/connection-design-solutions.