Where Did 1/32” Plate Embedment Tolerance Originate?
In 1992 the WTCA Board of Directors was reviewing TPI-85 Appendix P: Quality Standard for Metal Plated Connected Wood Trusses and found a disconcerting term; “firm embedment”, where a certain percentage of the plate had to be in metal to wood contact. This lead to a WTCA project, by Mr. Pat McGuire, to determine if “firm embedment” was possible for truss manufacturers to comply with consistently.
Ultimately, this work by Pat led to the creation of a new quality standard in ANSI/TPI 1-1995. The outcome of this study was that a typical truss plant could manufacture plated joints with a tolerance of 1/32 inch for plate embedment at any joint type.
From that point in time, TPI 1 has permitted a reduction to be taken for teeth that are not fully embedded. This allows teeth in embedment gaps to use a reduced “effectiveness” percentage based on the measured gap between the plate and the face of the lumber. This partial effectiveness gives component manufacturers (CMs) the ability to salvage tooth strength during TPI 1 inspections, where joints can be reevaluated and deemed acceptable. This was the fallback position when the plate placement method would have otherwise indicated that the joint required repair.
|Tooth Embedment Gap, G||Tooth Effectiveness|
|G = 0"||119%|
|0" < G ≤ 1/32" (0.03")||100%|
|1/32" < G ≤ 1/16" (0.06")||60%|
|1/16" < G ≤ 3/32" (0.09")||40%|
|G > 3/32" (0.09")||0%|
|Table 3.7-1 Tooth Effectiveness.|
With SBCA undertaking testing to better understand how moisture and weather affect stored trusses, the Quality Control (QC) committee determined that improving the industry’s understanding of the ANSI/TPI 1 embedment gap table would be an integral step in the development of quality control charts for use in both the QC joint and digital QC process. With the committee’s direction, SBCA staff dug into existing test reports to understand the methods used and data gathered that dictate how our industry determines tooth effectiveness due to embedment gaps. What SBCA staff found is that little test data exists, and even the quality of the original plate embedment gap testing, referenced by TPI 1, leads to more questions than answers.
The goal of the SBCA QC committee then became to provide data behind Table 3.7-1, and determine if using Table 3.7-1 with the tooth-count method is reliable and should be followed by inspectors.
Our intent with upcoming articles is to review the history of the QC process from TPI-85 Appendix P to the present, with the goal of preserving history and learning from it as we develop a far more sophisticated QC process and move our industry into the “age of digital control chart QC management”.