Per ANSI/TPI 1–2014, National Design Standard for Metal Plate Connected Wood Truss Construction, plates should be installed within a 10 degree tolerance. So what happens when the plate is rotated and exceeds this requirement?
The simple answer is: the configuration and design of the connector plate provides different values depending on the orientation of the teeth. For example, the plate in the photo which has elongated teeth will have a higher cross section value, and therefore strength, in one direction than in the other direction. Teeth have a stronger resistance to withdrawal based on their configuration. This results in differing values based on the angle.
As the plate is rotated, this value will change. The 10% rule is a limit to serve as a guideline for using the values generated on the shop drawing. The tolerances can be adjusted, but that requires specific design changes. Using a plate monitor, a designer can rotate a plate to any angle and analyze the impact. But remember, this manipulation would need to be communicated to the shop so they would know the new angle orientation and build accordingly.
Things to know:
- If during visual inspection the truss plates are identified as outside the 10% tolerance, then the plates should be marked as failed. Corrections can be made by removing the plate and reinstalling a larger plate with the necessary tooth count. Remember, there is a 50% reduction in the areas where the plate is removed and then reinstalled.
- The plate can be reevaluated by the design software. This design would need to pass engineering, and a record of this should be retained for potential questions.
- Odd rotations and adjustments make it difficult in the shop. It’s easier to hold parallel to a chord or perpendicular to a chord so a quick inspection will be possible rather than having to refer to a specific angle.
- In critical plate placement method analysis, if the plate is out of angle tolerance, despite having an adequate tooth count, the plate fails.
- Often angulations problems manifest during back plating. Frequent inspections during production should be done to ensure proper orientation.
- Mirrors installed just after the finish roller under the truss can be helpful to identify missing or mis-placed plates.
- Vertical racking improves the visual inspection by the truss stacker. If trusses are flat stacked, some method should be employed to insure proper placement.
- Convenient gauges are available to quickly identify angulations issues at: https://www.tpinst.org/tpi-store/in-plant-truss-tolerance-guide.
Keeping your “plates straight” is more than a cosmetic concern. But sure to check that your angles are within the tolerance of the design to ensure the manufacture of a quality product that will be built to last.
Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. While he is a TPI 3rd Party In-Plant Quality Assurance Authorized Agent covering the Southeastern United States and performs 3rd party safety auditor services, these articles represent his personal views, knowledge, and experience. 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.