The most common problem typically encountered is excessive defects in the plated area of the truss. While some defects are acceptable in the plated area, the degree of defects depends on the actual joint situation and forces at the joint.
Connector sizes are controlled by design, handling, and fabrication tolerance. They can also be limited by minimum tooth bite set by the fabricator. For example, some fabricators can decide to increase their minimum tooth count per member in response to issues that may have been encountered as the trusses leave the plant and travel to the jobsite and the structure. Plating to design limits does not include handling and transportation considerations, but fabricators may choose to take these factors into account.
The best variable to make adjustments for wane in plated area is the “fabrication tolerance.” It can be adjusted within the software. Adjusting this number higher allows for more defects and will also allow for more mis-placement of connectors.
Despite setting a high “fabrication tolerance,” some defects are never acceptable. The following photos show defects impacting a roof truss and a floor truss. Each is not acceptable because defects at the joint are well outside the acceptable limits.
The general rules of thumb when addressing wane are the following. These rules need to be adjusted as tolerances are reduced.
- If the plated area is 1/3 of the face of the member, plating with the connector as designed will generally be acceptable; however, it still has much to do with the location of the connector and the Joint Stress Index of the connector. Lowering the plate fabrication tolerance to zero may eliminate the opportunities to have defects in the plated area.
- If the plated area is 1/3 to 1/2 the face of the member, plating should be increased to provide the required tooth count. Low fabrication tolerances eliminate any excessive wane situation.
- If the waned area is more than 1/2 the face of the member, the member should be replaced.
In the first photo (floor truss), there is no plate upgrade that will result in a satisfactory joint connection. The floor truss web should be replaced. In the second photo (roof truss), upsizing the plate would have made the joint acceptable utilizing the existing lumber. This defect is less than 1/2 of the face of the member and the defect is in the plated area. Installing a larger plate can transfer load, assuming the tooth count requirements are met.
Once again, to build a quality product, it’s important to remember some simple rules of thumb as well as your common sense. Then you’re even more likely to avoid problems created by defects.
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.