Purchasing the correct grade of lumber to match your design drawing requirements is only one step in selecting the right lumber. Previously, we’ve discussed lumber quality in “Who is Responsible for Lumber Quality in Your Trusses?,” but let’s expand that discussion with this important concept of qualifying lumber. [For all images, See PDF or View in Full Issue.]
A particularly key step that’s often misunderstood is called qualifying the lumber. This step should happen early in the lumber picking process but, because of distinct reasons, it might be skipped. When that happens, unacceptable lumber can arrive at the saw. Then, at the saw, lumber qualification should take place, but unfortunately sometimes unacceptable lumber makes it through this step and reaches the table. Now at the table, decisions need to be made that will impact the quality of the component, and they will make the difference between ANSI/TPI 1 compliance and liability.
The first photo is an installation that everyone should agree is not acceptable. The heavily waned web has zero tooth capacity to transfer load from member to member. The reason this is not acceptable is because the plate must be embedded within specific tolerances to be effective. See Table 3.7-1, ANSI/TPI 1.
Let’s examine some key details that can help your facility understand the process and respond appropriately and practically. In the olden days, we just made sure we had lumber that was mostly defect free. Today, that might not be practical, especially where we have cost constraints.
The Key Points
- Just because lumber is correctly graded, that does not mean it is suitable for use in a component.
- Lumber grading is a function of capacity, so it’s not necessarily about practical use in trusses. A defect can pass a grading rule but still be unsuitable for component use (see photo).
- Selection of appropriate lumber must have some connection to the function of the member, but often in today’s operations it does not.
- Some linear saw operations make culling based on need impractical, so it’s often ignored.
- During automation, picking, loading, and transfer to the table, lumber is not evaluated and inappropriate pieces often end up at the table, resulting in lumber needing to be recut because the defects are not suitable or migratable.
- Component needs can vary depending on use, e.g., floors require different characteristics compared to roof trusses.
- A wood member not suitable for floors might be perfectly okay for roofs and vice versa.
- Down-grading the lumber due to negative characteristics reduces the value of the lumber once installed in the product.
The simple sketch of a floor truss edge shows how the surface plates are embedded. This is a limited narrow area of real estate. This narrow 1-1/2” wide surface needs to be fully utilized to transfer load. Higher plate tolerance helps make some defects under the plated area tolerable, but often plate placement and other factors will use up any gains from over-plating. This is why floor truss plates should be installed as close to the top of the top chord and as close to the very bottom of the bottom chord as possible to resolve the design forces.
In the final photo, excessive wane prevents any reasonable tooth connection. Please note that the face of the web exceeds half of the face of the member in the plated area. Even with a 20% fabrication tolerance, the tooth count will not meet the design requirements.
The Bottom Line
Meeting lumber grading rules does not automatically make lumber suitable for your application. Wane is a critical factor in that determination and must be considered by everyone involved in the process. It is always important that you qualify your lumber for the job you expect it to perform – your quality, safety, and liability depend on it.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with over four decades of experience in the structural building components industry. Glenn serves as a trainer-evaluator-auditor covering sales, design, PM, QA, customer service, and production elements of the truss industry. He also provides project management specifically pertaining to structural building components, including on-site inspections, expert witness and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis