How Do You Determine Fabrication Tolerances During Design?

How 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

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, C_q, 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.

Impact of Wane

Wane is the most common non-conformance situation according to a recent evaluation of a cross-section of truss manufacturers I conducted. Wane can vary from lumber manufacturer to lumber manufacturer. It’s impacted by how closely the lumber is graded as well as their sources for raw materials and availability due to regional limitations. Component manufactures can control wane by culling lumber, buying from reputable suppliers, or specifying grade controls such as “prime” or other grades that intentionally limit wane. Selecting “better” grades generally costs more, so that has to be considered when deciding on cost vs. benefit. For more on this consideration, see my article on Is Perfect Lumber Required to Make Great Trusses?

Variations in Lumber Dimensions

Lumber grading rules allow for variations in lumber dimensions. Generally speaking, these controls are respectful of the products’ use, but that is not always the situation. An example is resawn lumber applications. Sometimes these lumber variations exceed the capability of specific assembly equipment capabilities but are still “on grade.” Lumber dimension variations will not affect fabrication tolerance directly, but they will impact member to member gaps and embedment.

Equipment Capability

Not all equipment is created equally. It can vary with age, maintenance, or design. An example is cutting on linear saws vs. component saws. Linear saws clamp closer to the cutting blade creating more accurate cuts. Component saws by design need to hold lumber farther from the cut and this can be significant with scarf cuts and shallow angle cutting, allowing for variations due to twist, bow, and lumber. This is apparent in variations in butt cuts and scissor bottom chord variations. Sometimes we should adjust our designs to suit our equipment and capabilities. For an article discussing design considerations, see Are We Designing Our Components to Benefit Our Truss Builders and End Users?

Skills and Development of Truss Builders

New plants, poorly supervised plants, and plants that do not utilize an effective 3rd party review process will experience more difficulties related to the skill level of their truss builders. Likewise, even new employees at efficient plants will face a learning curve. Thus, less knowledgeable builders and poorly managed workers may need more latitude and higher fabrication tolerances.

End User Expectations and Job-Specific Requirements

Some end user customers and some municipalities require higher standards. Certain products may have significant variations in performance. An example would be town homes vs. custom-built homes vs. agricultural applications. A tile floor on a high-end home might have a significant flatness requirement. A stairwell advance to a second floor might have a line-of-sight performance requirement that limits variation in deflection. Even some church organizations have specific fabrication requirements which they will specify in their contract documents.

Component Manufacturer Reputation

Some truss fabricators have built their business by making quality a “marketing strategy.” Years ago, North Carolina-based Shoffner Industries experienced phenomenal growth by focusing on quality and using the best materials available. Business decisions encompass calculations beyond simple math, and the role that reputation plays in those decisions will vary between companies.

The Bottom Line

Fabrication tolerances are not intended to make allowances for undergrade materials or damaged connectors, but sometimes higher tolerance needs can help determine the actions to be taken when resolving issues. The preceding list considers many issues that impact the decision of what tolerance to use, but the process of determining the fabrication tolerance is not simple.

So, what is the best way to make this decision? The decision of what fabrication tolerance to use should be based on a review of in-house inspections and critical plate inspections dictated by ANSI/TPI 1. By reviewing this data, decisions can be made by adjusting tolerances based on compliance with these inspections. If most inspections rarely find issues, then the fabrication tolerance can be reduced. Doing this can result in reduced plate costs. If repeated inspection failures are found, then increase the tolerance slowly and witness the improvement.

In making a decision on fabrication tolerance during the design process, consider the following:

• Fabrication tolerance can vary from zero to the sky’s the limit.
• The higher the fabrication tolerance, the bigger the plate and the more you will spend on connectors.
• Using a higher fabrication tolerance may be a good insurance policy to reduce risk and improve fabrication non-conformance.
• Most CMs use two different fabrication tolerances, one for roof trusses and one for floors.
• Angulation tolerance should not be confused with fabrication tolerance. It is limited by +/-10° unless design requires a more restrictive angulation as specified by the truss designer or the design (see ANSI/TPI 1–2014 Section 3.7.3).
• Floor designs by nature are easier to control.
• The most common fabrication tolerance is remarkably high because, for the most part, the fabricator has not changed the default setting.
• A recommended tolerance goal is 5% for floors and 10% for roof trusses.
• Lasers can help CMs use lower fabrication tolerances.
• Older equipment might require higher fabrication tolerances.
• As training for truss builders and output improves, fabrication tolerance can be reduced with careful monitoring.
• When controls are lacking, or experience is limited, a higher fabrication tolerance such as 20% should be established until adjustments are made.
• Reliance on your 3rd party auditor and their regular physical presence to your facility goes a very long way in making your evaluation. For a discussion on selecting an auditor, see my article on Quality Assurance, Safety, and the Benefits of a Non-Biased Audit.

Determining the proper fabrication tolerances to use in your plant may not be a simple calculation, but this guidance should help you make informed decisions.

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.