How does ripping lumber, cutting a board along its length, affect its grade or strength values? Structural lumber is graded or inspected which provides designers, through the National Design Specification® (NDS®) for Wood Construction, the strength/design values for the lumber members. This process is accomplished by visually inspecting the entire board or passing the board through a testing machine.
The lumber industry is overseen by the American Lumber Standard Committee (ALSC). This group is made up of lumber producers, distributors, and users. They review and approve the grading rules and design values that are submitted to them by the grading agencies and then published in NDS.
Each grade of visual graded lumber has specific requirements for grain direction and slope, lumber defect size(s) and distribution. See Figure 1 for examples of lumber defects from the Metal Plate Connected Wood Truss Handbook. [For all images, See PDF or View in Full Issue.] The same issues exist for Machine Graded Lumber. As reflected by the visual grading rules, there is a known relationship between lumber defect size(s) and distribution and strength values.
An example of these grading requirements from the Southern Pine Inspection Bureau (SPIB) is shown in Figure 2. Note the chart that gives the knot size limits, as an example, per width of board for this one grade of lumber.
When you rip a member and change the member width, the defect size(s) and distribution requirements change. For example, from the knot chart above, a 2-1/4” diameter knot is permitted along the center line of the wide face in a 2x6 while a 1” diameter knot is allowed in a 2x4 along the edge of the wide face. If you rip 2” from a 2x6 that leaves a 1-7/8” knot along the edge of the wide face, see Figure 3. What is the new grade of this member? A 2x4 No. 2 SP permits a 1-1/4” knot along the wide face so in this example this ripped board does not meet a No. 2 grade.
So caution should be used when ripping lumber used as structural members. No matter the amount being ripped the grade of the material likely will drop. Options are to send the truss design to your MiTek engineer for a repair or have a qualified lumber grader come out and inspect the new members. Note the grader should be able to inspect Machine Evaluated Lumber and provide a new visual grade to the material.
For additional information, or if you have questions, please contact the MiTek Engineering department.