Since the beginning of time, wood or lumber has been used in construction. It’s a widely available, renewable, organic, composite resource for carpenters. In a primitive sense, wood is valuable because it’s easier to manipulate and process compared to other construction materials, like metals.

Strength by the Axis

Lumber is distinguished by the grain size and direction of the fibers, along with the associated orthotropic properties. Figure 1 shows three unique axes which offer different mechanical properties. The longitudinal axis provides the greatest tensile strength as it runs parallel to the tree stem, but greater compressive strength is found in the tangential direction perpendicular to the grain.

Figure 1. Fibers found within a section of wood

Tree Types & Traits

Coniferous trees yield softwood. Hardwood is from deciduous trees. Hardwood traditionally has a higher density, which directly correlates to greater strength, durability and a longer life span. Typically, species for construction are oak, mahogany, maple, cherry and walnut. Hardwoods are best for flooring and furniture making. Softwoods lack the same densities and strength but offer faster growth rates and material flexibility. Cost-effective softwood planks account for roughly 80% of all timber usage in construction. Most often you’ll see spruce, pine and fir applications in interior walls, roofing, framing and structural support.

The Cut Matters

The way lumber is cut defines its best use. One of the most common is the rip cut, which saws parallel to the grain. A rip saw chips splinters of wood, much like a chisel, rather than using shear force to cut. The sharp and serrated blades create a flat finished surface instead of following the grain pattern. Experienced mill workers use a visual grading system to examine the characteristics that affect the mechanical properties of cuts. The key considerations are knots or holes, slope of grain and warping.

Knots break up the continuity of the grain during sawing, which considerably impacts the strength in tension, and to a lesser extent, compression. The type of knot in a section of wood doesn’t matter since all knots have equivalent strength effects. Knots near the edge of a member are more restrictive when you’re managing bending stresses. While knots are easily visible, the slope of the grain may be tougher to analyze to the untrained eye. Since the fibers are not parallel to the edges, the slope of grain devalues the mechanical properties and encourages warping with moisture changes.

Quality Rankings

Softwood lumber is graded No.1 through 5, ranging from construction grade to economy grade. For soft lumber, it’s a spectrum of knot quantity and size as well as paint-ability. At the higher end, there’s often a large amount of waste and other defects which deter from structural usage. Lumber is also graded on appearance, a scale of A to D select for furniture making. A specimens have no blemishes. D have few but minor, noticeable blemishes. Beyond this scale, you’ll see 1 and 2 common, for boards with tight knots.

Hardwood lumber gradings also differ between firsts and seconds (FAS) and selects for different furniture and fixture usage. FAS requires an 83.3% or greater total of clear wood, to meet the minimum dimension requirements. FAS One Face follows the same classification, but only one side needs to be FAS while the other can be common grade. The selected grade is the same as FAS One Face but with different minimum dimensions. Finally, the No. 1 common grade requires a 66.6% clear face, while the No. 2 common requires 50% based on the poorest face.

Grain Patterns

Plain-sawn logs are cut into horizontal pieces from the log parallel to the pith. Plain sawing provides a unique appearance due to the natural grain distribution of the tree. It is also the most cost-effective method as it utilizes as much of the log as possible. With the plain cut, shear stress is distributed parabolically across the grain.

Plywood & Veneer

Plywood is an engineered wood. Flat sheets are created by gluing together thin layers of wood, alternating the direction of the longitudinal grains. The manufacturing process begins by harvesting the raw lumber. Limbs are cleared off, the tree is felled, then transported, debarked and cut into manageable sections.

For veneer, logs are heated and soaked before thin sheets are peeled by a lathe. The peeled sheets are cut into appropriate sizes and visually graded. Higher graded veneers become strong exterior faces. The middle layers add thickness, separate the outer layers and help resist bending. An odd number of layers provides a balanced, mostly symmetrical design as the orientation of grain angles differ.

A drying method reduces the moisture content of the veneers to maintain consistent dimensions over time. Once dry, the grains are arranged at right angles, to increase durability. The face veneer is placed on top of the glued interior sheets which are then heated and placed under pressure, while the adhesive cures. Once dry, the plywood sheets are trimmed to their proper sizes.

Figure 2. Plywood Veneer Orientation and Quantity

More Engineered Varieties

Another type of engineered wood is chipboard, made from compressed wood chippings and waste byproducts of other manufactured timber products. The waste elements are combined and bound by resin, adhesives and other additives. Typically, larger sized chips are used in the center surrounded by smaller ones. The wood is then subjected to high temperature and pressure to bond all pieces together. Dimensional cuts, post treatments or veneer facings are added for specific usages.

Many other types of engineered woods exist for specific needs. An oriented strand board follows a similar process as chipboard but is made of cross-oriented sheets of rectangular flakes. The external layers point toward the strength axis. It’s widely used as a construction material, now becoming more common than plywood. Cross-laminated timber is like plywood, with the key difference being sawn lumber rather than peeled veneer. The boards are oriented perpendicular to the adjacent boards, usually with an odd number of layers.

Load Bearing Joist Materials

Another strong structural composite lumber is glulam, a stress-rated engineered wood. With properties comparable to steel, it is more than ordinary dimensional lumber. While it is similar to cross-laminated timber, it’s load bearing, with layers of parallel grains, giving more strength in one axis.

Figure 3. Glulam and CLT Comparison

Heat and pressure treated high quality laminations of wood are glued together, hence the name “glulam.” Composite I-joists or I-beams are versatile, and common structural materials. The flanges are made of laminated veneer lumber or solid-sawn lumber, while the web is made of plywood or oriented strand board. The combination results in an economical, lightweight, and supportive material.

Today’s lumber is more than what it may appear to the uninitiated. While it’s an old, widely used natural resource seen everywhere, advancements in technology and new manufacturing practices to keep this vital resource in construction. With boosted mechanical properties, it remains competitive with man-made materials and hard metals.

To tie this all back to our work in the land development world here at Howell Engineering, while we may not propose lumber in our details and specifications for stormwater management systems, it is important to understand a widely used material when discussing projects with architects and contractors.