The ANSI/TPI 1 standard is a critical document that governs the design and quality criteria for metal plate-connected wood trusses. It serves as a guideline for truss manufacturers and engineers alike, providing the technical framework to design and construct reliable and code-compliant trusses used in residential, commercial, and industrial buildings.
As an ANSI-accredited developer, the Truss Plate Institute (TPI) follows a rigorous consensus-based process to ensure that ANSI/TPI 1 remains current and relevant to industry needs. The ANSI process mandates that the standard be opened for review and update at least every five years, allowing for advancements in engineering, materials, and construction practices to be incorporated. In keeping with this process, the standard was most recently updated in 2022, and the latest edition is now referenced in the 2024 ICC model codes.
The latest update to ANSI/TPI 1 included more than thirty changes, though most were clarifications that generally won’t impact truss designs. However, there were several changes that may have a significant impact on typical truss designs. Some of the most critical changes are discussed below and include:
- Person Loading
- Multi-ply Girder Load Distribution
- Web Reinforcement Design
- Extended-Chord Bearing Trusses
Person Load (ANSI/TPI 1-2022 Section 6.2.2.5)
This new provision essentially requires trusses spanning more than 18 feet to be designed to support a 300-pound concentrated load applied at the midpoint of each bottom chord panel. This check is to be done without environmental loads (such as snow or wind) and allows for a 1.6 duration of load factor. The provision was introduced to enhance safety by ensuring that the bottom chord can support a worker during temporary conditions, such as access for maintenance or installation activities.
Multi-ply Girder Load Distribution (ANSI/TPI 1-2022 Section 7.5.2.4)
Previously, multi-ply girder trusses were designed under the assumption that loads applied to one face were evenly distributed across all plies. However, extensive testing has shown that the ply closest to the applied load consistently carries a greater share of the load, regardless of the connection method used. To address this, a new provision in the standard now specifies the minimum percentage of load each ply must be designed to support. This change may lead to an increase in connector plate sizes for some girders and, in some cases, require higher-grade lumber. However, other updates to the standard, such as adjustments to repetitive member factors, may help offset the impact on lumber requirements.
Web Reinforcement Design (ANSI/TPI 1-2022 Section 7.3.2.1)
A new design method has been added to the standard to provide an engineered approach for analyzing reinforced web members (e.g., T-braced and I-braced webs). Previously, web reinforcement was addressed prescriptively with a table in the BCSI, which imposed geometric limitations and often required higher-grade lumber than necessary. The new provisions allow for more flexible and economical design solutions, as they can now be incorporated into truss design software, offering greater efficiency when web reinforcement is used in place of continuous lateral restraint (CLRs).
Extended-Chord Bearing Trusses (ANSI/TPI 1 Section 7.3.13)
Another design methodology that was added to the standard is for extended-chord bearing trusses, also known as top-chord-bearing trusses. Previously, these trusses relied on a prescriptive table in ANSI/TPI 1 to limit the reactions for specific top-chord-bearing conditions unless other values were determined through testing. The updated standard introduces new provisions that allow designers to analyze any top-chord-bearing condition analytically, rather than relying solely on the prescriptive limits.
Since many plate suppliers have already incorporated in their software proprietary analytical methods for designing top-chord-bearing trusses, significant changes in truss designs may not be noticeable. However, the new provisions offer designers more flexibility in evaluating specific conditions.
Additionally, because of this new thorough analytical method, a new requirement has been added regarding lumber wane in the top-chord-bearing area. If wane is not managed through a quality control process during manufacturing, a new factor (Cwane) will be applied, reducing the reaction limit. Manufacturing plants that effectively limit wane in these areas can apply a Cwane factor of 1.00, meaning no reduction in bearing capacity.
The updates to ANSI/TPI 1 reflect TPI’s ongoing commitment to keeping the standard relevant and aligned with current engineering practices and technology. By incorporating new design methodologies and refining existing provisions, these changes improve the accuracy, efficiency, and safety of truss designs. Keeping ANSI/TPI 1 up to date is essential in maintaining the metal plate connected wood truss industry on a sound engineering basis. The 2022 edition of the standard is available for purchase from TPI at www.tpinst.org.