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Reviewing Metal-Plate-Connected Wood Truss Submittal Packages

By William Bolduc, P.E., S.E.
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Figure 3. Pitch pocket under plate area. Pitch is the tree sap (liquid or dried) that can fill voids between the growth rings in wood. Photo courtesy of William Bolduc.
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Hey Truss Manufacturer, What’s Your Fabrication Tolerance?

If the big change is related to in-plant quality control, it may seem irrelevant to the truss submittal process for plan examination and inspection.  In fact, the fabrication tolerance affects the plate sizes and one must state it on the truss design drawings submitted for plan examination and inspection.  There is a simple method to discover if the truss manufacturer is following the new building codes.  Ask the question, “What is the fabrication tolerance?”  It should be stated on every truss design drawing that is submitted.

It may be clearly stated on the truss design drawing as the fabrication tolerance or indirectly stated as the quality control factor, Cq.  ANSI introduced the quality control factor, Cq, in ANSI/TPI 1-2002.  The meaning of the quality control factor was changed in ANSI/TPI 1-2007.  In both cases, there is a direct correspondence between the fabrication tolerance and the quality control factor, Cq (see Table 2 below).  ANSI/TPI 1 states that either the fabrication tolerance or the quality control factor, Cq, must be stated on each truss design drawing.  For better clarity, the newer version (2007) of the standard requires that the fabrication tolerance be stated on each truss design drawing.

Fabrication Tolerance

Quality Control Factor, Cq

Per ANSI/TPI 1-2002

Per ANSI/TPI 1-2007

0%

1.25

1.00

10%

1.125

0.90

11%

1.11

0.89

20%

1.00

0.80

30%

0.875

0.70

Table 2.  Equivalent Quality Control Factor for Various Fabrication Tolerances

If you do not see the fabrication tolerance or the quality control factor, Cq, on the truss design drawings, that may indicate that the truss manufacturer is not following the current codes and standards and/or the third-party agency that is required to monitor the in-plant quality procedures is not following the current standards.   In either case, your question of “What’s your fabrication tolerance?” will likely encourage the truss manufacturer to improve quality and follow the building codes and standards.

Wane under plate contact area
Figure 4. Wane under plate contact area of bottom chord. Wane refers to the lack of wood or bark on the edge or corner of lumber. Photo courtesy of William Bolduc.

What is the Correct Value for the Fabrication Tolerance?

There is no single correct value. 
It depends on the quality control procedures used by the truss manufacturer.  The fabrication tolerance may be any value from zero to 30 percent (or even higher).  What is important is that the fabrication tolerance shown on the truss design drawings match (or exceed) the fabrication tolerance used by the truss manufacturer for their quality control procedures.  The third-party agency that audits the quality control procedures used in the truss plant must also agree that the quality procedure matches the fabrication tolerances shown on the truss design drawings.

Typically, truss manufacturers will use a higher tolerance for roof trusses (plates embedded into the wide face of the lumber) than for floor trusses (with plates embedded into the narrow edge of the lumber).  This is due to the greater geometric complexity of roof trusses and the manufacturing process that may require placement of a truss plate on the underside of the connection for roof trusses.

If the fabrication tolerance is less than 20 percent for roof trusses (plates into wide face) or 11 percent for floor trusses (plates into narrow edge), the ANSI/TPI 1-2002 (Section 3.2.4.2) requires that the truss manufacturer provide to the approved inspection agency, or through other means, “justification” for the lower fabrication tolerance.  ANSI/TPI 1-2007 does not have this requirement (see Table 2 for conversion of the fabrication tolerance to the quality control factor, Cq, value).

Conclusion

The new versions of the building codes and standards have significantly improved and clarified the requirements for Metal-Plate-Connected Wood Truss Construction.  Truss designers and manufacturers, third-party quality agencies, plan examiners, and building inspectors need to adopt and follow these new changes.

William Bolduc, P.E., S.E. is a structural engineer with the A.C. Houston Lumber Company. He is an expert on several aspects of wood-frame construction.  He is a Professional Engineer in 27 states and a Structural Engineer in Nevada, Utah and Texas.  Bolduc has a bachelor’s degree in Civil Engineering from the Illinois Institute of Technology and a master’s degree in Civil Engineering from the University of Nevada Las Vegas.

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