Wood Species and Properties
The selection of wood species for construction projects significantly affects the strength, durability, appearance, and cost of the finished structure. Softwood species from coniferous trees are the primary structural materials in North American construction. Douglas fir is one of the strongest softwoods available, with a modulus of elasticity of 1.9 million psi and maximum bending stress of 1,200 psi for Select Structural grade. The high strength-to-weight ratio of Douglas fir makes it ideal for long-span beams, roof trusses, and heavy timber construction. Southern yellow pine offers comparable strength properties and is widely available in the southeastern United States at lower cost than Douglas fir.
Spruce-pine-fir is the most commonly used species group for dimensional lumber in residential construction. The group includes several species that are combined under a single grade because their properties are similar. SPF lumber has good workability, holds nails well, and is available at moderate cost. However, its strength is lower than Douglas fir or Southern yellow pine, limiting its use in long-span applications. Hem-fir species group provides a balance of strength, workability, and appearance for both structural and finish applications.
Hardwood species from deciduous trees are used primarily for appearance-grade applications such as flooring, cabinetry, and millwork. Oak is the most popular hardwood for flooring due to its hardness, prominent grain pattern, and durability. The Janka hardness rating for red oak is 1,290 pounds, while white oak rates 1,360 pounds. Maple with a Janka rating of 1,450 pounds is harder and more resistant to wear but has a subtler grain pattern. Cherry with a Janka rating of 950 pounds is softer but prized for its rich color that deepens with age.
Grading and Quality Standards
The grading of lumber establishes quality standards that allow designers to specify the required strength and appearance for each application. The National Grading Rule divides dimension lumber into two categories: structural light framing and structural joists and planks. The grade of a piece of lumber is determined by the size, number, and location of knots, slope of grain, wane, and other natural characteristics. Select Structural grade has the fewest defects and the highest allowable stresses. Number 1 grade has slightly more defects but is still suitable for most structural applications. spread footing design for residential buildings. pile foundation capacity testing and verification. Douglas fir lumber strength properties. Number 2 grade allows more defects and is commonly used for general framing where maximum strength is not required.
Visual grading is performed by trained graders who inspect each piece of lumber and assign a grade based on established rules. Machine stress rating uses mechanical testing to measure the stiffness of each piece and assign a grade based on the measured modulus of elasticity. MSR lumber provides more reliable strength properties and allows designers to use higher design stresses than visually graded lumber of the same species. The MSR grade is stamped on each piece along with the measured E-value and assigned bending stress.
Moisture Content and Dimensional Stability
The moisture content of wood significantly affects its strength, stiffness, and dimensional stability. Green lumber freshly sawn has a moisture content of 50 to 100 percent or more. Kiln-dried lumber is dried in controlled conditions to a moisture content of 15 to 19 percent for framing lumber and 6 to 12 percent for finish lumber. Wood shrinks as it loses moisture below the fiber saturation point of approximately 30 percent moisture content. The shrinkage is greatest in the tangential direction, about half as much in the radial direction, and negligible in the longitudinal direction.
The dimensional changes from moisture content variations must be accommodated in building design to prevent problems. A 2×10 floor joist that shrinks 1/4 inch in depth across its width can cause drywall cracking at ceiling joints and uneven flooring. Green lumber used in wall framing can shrink enough to cause nail pops as the wall height decreases by up to 1/2 inch per story. Using kiln-dried lumber with moisture content below 19 percent minimizes these problems. The lumber must be protected from wetting during construction and allowed to acclimate to the building interior conditions before finish materials are applied.