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September 2024 #16302 Page #35
Are You Interpreting Memberto-Member Gaps Correctly? By Glenn Traylor
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n 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-tomember connections.
Why Gaps Matter Gaps in member-tomember connections can result in movement of the joint and potentially ANSI/TPI 1-2014 Figure 3.7-4 Wood Member-to-Wood Member Gaps. 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. Continued next page
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