New Quality Criteria for the Manufacture of MPC Wood Trusses
(posted 4/30/02, revised 4/08/03)

With the completion and approval of the new ANSI/TPI 1-2002 "National Design Standard for Metal Plate Connected Wood Truss Construction" (revision of ANSI/TPI 1-1995) comes the introduction of a new Quality Standard for truss manufacturers (i.e., TPI 1-2002 Chapter 3). The new Quality Standard is a revision of the quality criteria in Chapter 4 of ANSI/TPI 1-1995, and consists of a blend of familiar criteria (i.e., per ANSI/TPI 1-1995) with some not-so-familiar concepts that are new to the ANSI/TPI 1-2002 standard.

Something New, Something Old

For the most part, lumber, assembly, and plating requirements remain the same in the 2002 standard, as well as the provisions for repressing and replating. What's different is a new procedure for joint inspection to determine acceptance of a joint, as it relates to plate placement (i.e., maximum allowable plate translation, rotation, and a 20% limitation for lumber characteristics within each plate contact area) and embedment (i.e., maximum allowable plate embedment gap along the plate perimeter). This new method, called the Plate Placement Method (PPM), provides a quick, step-by-step approach to determining acceptance of the joint.

For those who are familiar with ANSI/TPI 1-1995, you will not find anything in the new PPM relating to "total number of effective teeth." However, effective teeth requirements, similar to those in ANSI/TPI 1-1995 (Ch. 4), do appear in an alternate joint inspection method presented in an Annex to Chapter 3. This alternate method is appropriately called the Tooth Count Method (TCM) and includes the requirement to determine the number of effective teeth at each plate contact area, accounting for any ineffective teeth (e.g., flattened teeth, teeth in knots, etc.) or teeth with reduced effectiveness due to gaps between the plate and wood.

The wood member-to-member gap checks are the same regardless of the joint QC method employed (PPM vs. TCM). What has changed with wood gaps, though, is where the gaps are now measured (at the edge of the connector plate rather than at the cut), and a uniform maximum gap tolerance for a greater variety of joints replaces the differing maximum and average gap criteria for different joints as currently found in ANSI/TPI 1-1995.

A Look at the New Quality Standard

To learn more about the new Quality Standard, Chapter 3 of the new ANSI/TPI 1-2002 standard can be viewed in its entirety here. (For a review of the quality criteria found in Chapter 4 of ANSI/TPI 1-1995, click here.) However, the best understanding of the new Quality Standard will come from the corresponding Commentary to Chapter 3, and other informational articles that will be developed in the months to come.

 

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