Lumber is classified by species and grade. The grade of the lumber, which ranges from Stud Grade to Select Structural, is based upon multiple factors including the grain of the member; the number of knots, checks (cracks that cross the grain), shakes (cracks along the grain), splits (separation of wood grain); and density. The strength capacities for lumber vary depending upon the species and grade (Figures 1 and 2). Although one species of lumber may be better suited for supporting gravity loads, another may be more suitable for lateral loads (i.e. wind). The species and grade of lumber has a profound effect on the serviceability of the wood structures and, most importantly, structural integrity.
Building codes change regularly, and, over the years, the strength of some wood species, as listed in the building code, has decreased. For example, the bending strength of No. 2 Southern Pine was reduced between 20% to 26.67% from 2005 to 2015 (Figures 2 and 3). Using the material properties based upon information from previous codes could result in using inadequate materials, adversely affecting serviceability and structural integrity.
Load Path Defects
The concept of load path is an exercise of structural engineering in its simplest form. A force applied to any structure will get to the ground, eventually. The objective of the structural elements is to provide a safe path for the force to do just that, without damaging the structure and putting the occupants at risk. Although the load path is considered during design and construction operations, load path deficiencies are the result of using inadequate materials to support heavy loads (Figure 4 and 5). The result of load path deficiencies isn’t always a sudden failure of structural members, but rather damage to the aesthetic materials such as drywall or veneer. However, these telltale signs indicate a loss of serviceability and potential for life-safety hazard.
Modifications to structural members afford another opportunity for defects in wood construction. Modifications to structural members are done on a regular basis for a number of reasons. Whether it is to change the layout of the framing due to unforeseen circumstances, or to accommodate new mechanical/electrical equipment, modifications to existing wood structural members may be necessary. Modification defects occur when alterations are made to the existing structural elements with a disregard for structural integrity.
In scenarios in which sawn lumber must be modified, the fix is often easy and non-intrusive. Modifications to proprietary products (i.e. trusses, I-joists, etc.), on the other hand, are more invasive and, at times, more expensive. A truss, for example, is designed for a specific purpose. Modifying a truss by moving or eliminating a single component, as is often seen in attic spaces to accommodate mechanical/electrical equipment, results in a redistribution of loads (Figure 6). Without proper modification directives given by the design engineer, this redistribution of load results in the application of more load to elements and connections that are not suitable. These inadequate elements and connections will fail resulting in the eventual loss of serviceability or even failure of the structure (Figure 7).
The advantages of wood construction still offer economical and feasible options for residential and light commercial construction, just as it has for centuries. With proper design and construction practices, wood construction can serve as a long-term solution for our construction needs for centuries to come.