From worst-case scenarios to less obvious examples, do you have general guidelines that will help you make informed decisions? For example, when faced with lumber defects on the table, do you know how the builders in your plant will proceed? More importantly, are they being supplied with the information they will need to make informed decisions? Take, for example, the question of culling lumber.
Most fabricators would not even consider using the bottom chord show in the first photo. The real question is: do your builders know how to cull lumber? Do they know when to bump up the plate size and when to replace the truss member? There is no set rule to the situation; however, since TPI regulations allow some defects, the question is how much defect is allowable and when is a defect unacceptable. The technical response is that “it depends on the plate tolerances” and the other factors determining the plate size. The plate size via the truss design software calculates the necessary tooth holding, orientation of the connector plate. The larger the fabrication tolerance, the more flexibility the builder has for locating the connector on the joint. Still other factors must be considered in order to achieve a suitable truss.
Considering the joint in the photo, in this case the wane area does not contribute tooth holding and should not be counted as acceptable tooth holding. This particular plate fails this application because adequate tooth count is not available in the chord. In this instance, it would appear that using a larger plate would have resulted in an acceptable joint.
So how does the truss builder out in the shop determine what they should do? Consider these guidelines:
- If there is a defect of up to 25% of the face of the member in the plated area, then it a good idea to upsize the plate just to be covered. Keep in mind that, if plates are designed oversized, this rule may not be necessary.
- If there is a defect of 25% to 50% of the face of the member in the plated area, then it is imperative that the plate be evaluated and a larger plate should be used. This can be done by upsizing the plate based on the percentage of defect area in the member affected. In the example above, a single plate size each way covers the 1 x 4 inch area. Here a 3 x 4 is substituted with a 4 x 5.
- If there is a defect of 50% or larger of the face of the member in the plated area, then the member should be culled and replaced with a suitable replacement.
Of course these decisions need to be made by the builder at the table in a quick manner. Some situations, such as critical plates, may not allow any defects. That’s why it’s important to give the table enough information. Most plate software providers have shop output that shows minimum tooth count for every joint of the truss. Its good information to have if your lumber has a great deal of defects at the plated area. A little paper saves throwing away an expensive piece of lumber already cut and at the table ready to be used. Providing accurate information at the table in addition to thoughtful guidelines will help ensure that the best results are achieved in practice.