Using a 6 x 6 plate instead of a 3 x 6 plate can make the Plate Placement Method fail the polygon rule. So what is happening? And how can this be resolved?
What’s Happening
In the image, the yellow area represents the connector optimal placement for the designed size of a 3 x 6 connector. The yellow and green area represents the builders’ substitution of a 6 x 6 plate. When completing a critical plate analysis because of this substitution, the plate center is no longer within the polygon area shaded in purple as indicated by the orange dot. Per the plate placement method, this plate would fail. Because the plate size is increased, the center of the plate also shifts.
How It’s Resolved
Once it’s determined that the center fails the polygon requirement, what else can the inspector do to determine if the plate fails or is acceptable? The most logical next step would be to use the tooth count method. The tooth count method does not require the plate center in a specific area. In this case, because the plate is oversized, there are plenty of teeth per member. Another option to avoid counting teeth is that the truss can be rerun with the new size. The plate placement method would generate a new polygon, as illustrated in the image with the light blue. With this redesigned plate, selection of the polygon area will change, indicating the location of the plate can be shifted.
The Bottom Line
The rule is: If the Plate Placement Method fails, then count teeth.
- If the tooth count passes, the plate passes.
- If the tooth count fails, the plate placement method will fail.
And what if you’re unwilling to count teeth? If the plate used is different than the size indicated on the output paperwork, then regeneration of the paperwork will be necessary to confirm that the upsized plate is acceptable.