Generally speaking, ANSI/TPI 1 makes common sense and is very logical. However, there are several items that can be a little confusing because they seem to defy logic.
When we upgrade lumber per ANSI/TPI 1 Section 3.4.2, we know that lumber strength needs to be maintained in order to meet the design requirements. In my February article, “What is the Risk of Using Incorrect Lumber?”, I discuss how 8 different lumber properties must be equal to or greater than the specified grade. Seems reasonable!
When plates are upgraded, either to accommodate wane in the plated area or maybe because there is not sufficient inventory of the needed connector, the truss builders can upsize the connector using a “stronger” connector. A very specific passage in ANSI/TPI 1 addresses this situation:
3.6.3 Substitutions.
A Metal Connector Plate with larger dimensions in one or both directions, but of the same type and gauge specified on the Truss Design Drawing, shall be an acceptable substitution provided the requirements of Section 3.6.5 are met.
This section is specific, but explanation might be needed for “of the same type and gauge.” You can use a larger connector, but it has to be the same type or gauge. This is because design requires values such as tooth holding capacity, which is influenced by lumber properties and the number of teeth transferring the load. The strength of the connector depends on gauge and teeth configuration. For example, examining an ordinary Alpine 20 gauge connector versus a high-strength 20 gauge connector reveals the latter has 25% fewer teeth to increase the section—cross section—determining the connector’s ability to transfer loads, assuming adequate tooth capacity. Therefore, these plates are not interchangeable. Some connector manufacturers can vary as much as 50% tooth count.
Importantly, even if they sound like they will be the same or better, connector plates will vary across different manufacturers as well as within individual manufacturer product lines. Just consider the number of choices available: 20 gauge connectors, 20 gauge high strength connectors, 18 gauge connectors, 16 gauge connectors, super strength connectors… I think you get the picture!
Show here are three different MiTek connectors (for images, See PDF or View in Full Issue). In order to replace these connectors, a new design is required. Chances are that “higher” gauge connectors may work, but this will not be known until the design is run with the new plates.
Remember to Follow the Rules
If you are attempting to upsize a connector without specific engineering, then these rules must be followed:
- The plate must be of the same manufacturer as specified by the design.
- The size must be equal to or greater than the original design in both dimensions.
- The plate must be of the same style.
- The plate must be positioned over the entire original plated area.
- The plate cannot change the functionality of the truss.
- The plate cannot extend beyond the profile of the truss.
- If the plate is a replacement of a removed connector, then an increase in size may be required to cover the entire used area.
Yes, a change can be made while manufacturing the component—but only if you follow the rules and keep in mind that additional engineering may be necessary. When in doubt, remember that you can always take the time to rerun the truss with the new design. That’s easier, and cheaper, than having to replace a truss after it’s been built.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. 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.