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May 2024 #16298 Page #35
How Do You Determine Fabrication Tolerances During Design?
By Glenn Traylor
ow does a component manufacturer determine what fabrication tolerances should be used during truss design? To answer that question, we first need to clarify what is meant by “fabrication tolerance.” Per ANSI/TPI 1–2014, the guidance is
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3.3.2 Fabrication Tolerance. All Truss joints shall be designed using a fabrication tolerance specified by the Truss Manufacturer. The fabrication tolerance correlates to the quality control factor, Cq, as defined in Section 6.4.10. Utilizing ANSI/TPI 1 to design trusses means that designers have available to them a calculation when they specify connector plates that is provided by their plate manufacturer. There are other software or manual methods that can be used to design trusses that are not provided by the plate supplier, but those situations are exceedingly rare. The fabrication tolerance calculation is built into the software, so it will automatically adjust the required plate size up to allow for flexibility in plate placement and defects in the plated area. The reason for this tool is that assembling a truss requires some latitude and room for adjustment depending on conditions and materials. Importantly, the following items should be considered: 1. Variations in lumber quality. 2. Impact of wane. 3. Variations in lumber dimensions. 4. Equipment capabilities. 5. Skills and development of truss builders. 6. End user expectations and job-specific requirements. 7. Component manufacturer reputation.
Variations in Lumber Quality When evaluating lumber quality, lumber grading rules considerations should not be confused with fabrication tolerance. Grades are selected and calculated using design values. Member size and plate selection are determined using published values. While each grade of lumber has its own manufacturing tolerances, they can vary significantly between lumber manufacturers but should be controlled by grading rules and regulations. For more on design values, see the April 2023 article by David Conner, VP of Timber Products, Truss Design Value Comparison Tables. Continued next page
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