In a recent discussion with several very well-informed industry folks, it became clear to us that interpretation of member-to-member gaps may not be clear.
In particular, we need to take a closer look at Chapter 3 Quality Criteria for the Manufacture of Metal-Plate-Connected Wood Trusses of ANSI/TPI 1-2014 (National Design Standard for Metal Plate Connected Wood Truss Construction) and especially Figure 3.7-4 Wood Member-to-Wood Member Gaps.
The figure seems to be turning the focus on scarf cuts outside the plate zone, but unfortunately that focus neglects the real issue of member-to-member connections. [For all figures and images, See PDF or View in Full Issue.]
Why Gaps Matter
Gaps in member-to-member connections can result in movement of the joint and potentially related deflections not desired or intended in the design. If the joint is in tension for the design load case, this can change as the load case changes. An example would be when a truss is designed for snow, but its design also needs to consider wind where loading can occur as uplift. When a truss is designed, the software assumes that, where compression forces exist, the member to the adjoining member transfers the load and does not depend on the strength of the connector to resist collapsing the connector until the load settles to a point where it’s pushing the resisting member. If a gap is allowed to exist and the plate is not capable of resisting the compression forces, then the plate will buckle until the force is resolved by the pressure of pushing against the adjoining member. This is why member-to-member gaps are limited to 1/8”. It’s a compromise to perfect joints, which would depend on very accurate cutting and controls in the assembly of the truss, resulting in a potentially very costly truss.
What Makes a Gap a Gap
Situation One: In the first figure, a connection is exaggerated in a way to illustrate this point. Member 1 cut against Chord 1 is not cut correctly, which results in an excessive gap and therefore non-compliance. In this illustration, the maximum 1/8” gap is exceeded, as indicated by the orange arrows. There is no significance to the fact the member touches on the left side within the zone of the connector – the connection related to this member is non-compliant.
Situation Two: In the same figure, the Member 2 member-to-member gap exceeds the allowable gap on the left side of the member, as indicated by the orange arrows, while the right side of the member is within the tolerance, as indicated by the red arrows. This member is also non-compliant.
Situation Three: In the next figure, Member 3 is cut incorrectly and does not come into contact with Chord 2, even though it is touching on the left in the plate zone. Regardless of the plate touching on the left, this member is non-compliant.
Situation Four: Also in the second figure, Member 4 is not pushed against Chord 2, resulting in a non-compliance due to excessive member-to-member gap, also indicated by the orange arrows.
Situation Five: In the final scenario in this figure, Member 5 is within the required gap against Chord 2, as indicated by the red arrows, but of course the entire joint will need to be reworked to correct the connection.
The Bottom Line
It is very important that ANSI/TPI 1 Figure 3.7-4 Wood Member-to-Wood Member Gaps addresses gaps greater than the 1/8” gap limit outside the plated area. But also important is the need to address gaps that may be effectively hidden under a plate. Whether readily visible or not, all member-to-member gaps should be taken seriously to avoid significant unintended consequences.
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.