This past week we looked at a fence system, built around a stone pathway and its separation from the public space at a bike trail and the adjoining private property, at the roadway. Today we will continue looking at these details and how the wood grain and the related position of a fastener significantly changes the strength of the connection. As well, the strength of the connection is impacted by the distance from the fastening point in the edge of the individual wood member. Wood has a much higher tendency to split where fasteners are driven into or close to the end-grain. Last week we talked about the improved connection and mounting at a cross green direction through a board. End-grain lacks the same strength resistance for both pull-out and shear strength.
Pullout strength refers to the ability of the fastener to resist being pulled out of the wood due to tensile forces acting along the length of the fastener. This strength resists the connection from separating under load. Shear strength relates to the fastener’s resistance to shearing or lateral forces that try to cause one piece of wood to slide past the other. The screws or bolts generally have sufficient shear strength to prevent this type of movement and maintain the integrity of the joint, but not all wood and wood grain direction and position have comparable resistance strength. These brackets help distribute the forces against a larger area of the boards and change the grain direction in relation to the fastener.
The reason for this difference in strength lies in the cellular structure and fiber orientation of wood itself, the direction of the grain in relation to the shaft of the fastener. Wood is composed of long, tubular cells called “tracheids” or “fibers” that run parallel to the trunk or stem of the tree. These fibers are primarily responsible for the strength and rigidity of the wood, acting like tiny straws bound together. If you’ve ever seen a fake karate master brake boards too easily, it’s because boards also break easily in line with the grain. It’s the same principle at work.
When a force is applied parallel to the grain (longitudinally), the fibers work together to resist the load, distributing the stress along their length. However, when a force is applied perpendicular to the grain (across the end grain), the fibers offer far less resistance, as the load is concentrated on their ends, which are relatively weak and prone to splitting.
By using structural steel corner brackets, the orientation of the fasteners can be optimized to take advantage of the wood’s inherent strength in the cross-grain direction. Instead of driving fasteners through the end grain of one board, the brackets allow the fasteners to be positioned away from the edge, where the grain pattern is stronger and more continuous.
With the 90° angle of the bracket, the fasteners can now engage the cross-grain of both boards, effectively creating a cross-grain-to-cross-grain connection. This configuration capitalizes on the wood’s strength in the cross-grain direction, where the fibers are continuous and can better resist the shear and tensile forces imposed by the fasteners.
The cross-grain strength of wood is generally higher than its end grain strength because the fibers are continuous and can distribute the load more effectively. When a fastener is driven into the cross-grain, it encounters the side surfaces of the fibers, which offer greater resistance and holding power compared to the ends of the fibers encountered in end grain connections.
By using these structural steel corner brackets, the fasteners are strategically positioned to take advantage of this cross-grain strength in both boards, resulting in a significantly stronger and more durable connection. This approach not only enhances the overall structural integrity of the wood framing but also reduces the risk of splitting, cracking, or premature failure at these critical joints.
In essence, the particular supplementary use of steel corner brackets, in combination with an understanding of wood grain orientation and fastener placement, allows for the creation of robust and resilient connections that maximize the inherent strengths of wood while mitigating its weaknesses. This synergistic configuration allows for better spacing, better grain connection, and a more stable and long-lasting wood framing system. In result, it is capable of withstanding the demands of various loads and environmental conditions.
Unlike the typical structural fasteners installed at the decks at the back of houses, structural brackets of this specific type are actually not required by the building code, for use in fences. Particular type of installation goes above and beyond the typical off the shelf, big-box store type products and materials, and has an additional cost but it’s easy to argue that that cost is recouped over time because of the increased longevity of the fence due to the stronger structural connections.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
