ANSI/TPI 1 is a practical guide to manufacturing metal plate connected wood truss. It considers rational expectations and reasonable manufacturing capabilities that help ensure a sound product. Due to the nature of wood, cutting equipment and processes gaps do occur. These gaps can cause deflection. This can occur over time. Due to labor cost and waste involved, eliminating all gaps can be expensive and not cost effective.
So, which gaps are permissible, or not? Shown is an excerpt from the ANSI/TPI 1 standard, Figure 3.7-4 Wood Member to Wood Member Gaps. [For all images, See PDF or View in Full Issue.]
In this diagram, the areas denoted in red illustrate allowable gaps.
Assumptions
Determining if a gap is acceptable relies on a few assumptions:
- The design software assumes that joints are in contact with each other.
- Connectors are not designed to prevent buckling when presented with a compressive force.
- Forces in tension are not subject to buckling but the forces can reverse due to variations in load cases. An example of force reversal would be wind uplift on a normal gravity load.
- When gaps exist, even allowable gaps, eventually over time, the truss will settle, reducing the gap by compressing existing contact area.
- When building floor trusses, consider using a tension design rather than a compressive design (see configuration in the diagram).
- Floating chases help eliminate gaps in floors (see my August 2022 article, What is the Value of a Floating Chase?).
Reality
What if I have a gap? What can I do about it?
If during fabrication a gap larger than allowed per Section 3.7.6.3 occurs, then a metal shim can be installed to fill the gap. This galvanized shim must be installed with a suitable and approved fastener to prevent accidental removal. Coated metal banding makes a great shim, or truss manufacturers can cut their own shims from galvanized materials.
Importantly, wood cannot be used to fill gaps because the material compacts and has very little compression strength. The photo shows an example of wood shims used to fill gaps – this is not allowed per the standard.
Mind Your Gaps
Gaps can occur, but careful manufacturing procedures can almost eliminate the problem. When they do occur, you don’t have to take the truss apart. Follow the standard and install a metal shim. It’s an approved option that will preserve the strength and integrity of your trusses.
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.