Due to the nature of wood, lumber characteristics can vary in every piece. Even “in grade” lumber can have drastic variations in performance. How can we deal with and adjust for natural defects and variability? We have discussed roof trusses in previous articles, including “Does Your Truss Plant Understand the Importance of Tooth Count?” and “Who is Responsible for Lumber Quality in Your Trusses?” Now, let’s take a closer look at floors. [For all photos, see PDF or View in Full Issue.]
Floor Trusses
Here’s an example. Traditional 4 x 2 floor trusses are somewhat unique in that they depend on narrow faces, normally 1 ½”, to transfer loads to chords then ultimately bearings. This can present a slightly different challenge compared to 2 x 4 roof trusses, where we have wider surfaces. The photos illustrate Plate Placement drawings for a typical floor truss. The circles numbered 1,2,3 represent the allowable defect each connection can have while still providing adequate tooth holding to complete the mission. The smaller the circle, the less the allowable defect. Circles 1 and 2 allow very little defect while 3 is a little more generous. Due to the narrow face, we cannot neglect identifying defects under the plated area given its impact on joint performance.
In the next photo of an end condition (Bearing), notice points 4 and 5. A very small amount of defect is allowed. More defect is allowed on the bottom chord. This sort of condition does not allow for much defect including wane situations.
Key Factors
So, the question is how do we deal with defects in the chords and, more importantly, the webs? Let’s point out some considerations and specifics.
- Every piece of lumber can vary even within the same tree.
- Lumber certified “in grade” still needs to be qualified for its particular use.
- Lumber quality/characteristics can create drastic variations in performance.
- Not every piece of scrap is suitable for webs.
- Better surfaces are required on floor webs than the lumber grade requirements may indicate.
- Each piece must be evaluated by the truss builder.
- Finding a quality source for your web material is key.
The photos show a lumber manufacturer collecting webs to be stacked on a pallet for sale to regional component manufacturers.
Take a very close look at the quality of these webs! Caution: Most lumber manufactures do not meet these high standards, but Wood Specialties located in Washington, GA has been raising the bar for 3 generations and 50 years!
The Bottom Line
You don’t have to have perfect materials, but high standards make good quality and reliable trusses a lot easier to manufacture.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost 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 and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.