The simple answer to the title question, can designs be changed during manufacturing, is yes. But, the situation is rarely straightforward and simple in reality. The better question is: What considerations are necessary when making modifications to the design while manufacturing a truss?
To answer the broader question, the answers stack up as follows. First, designs can be modified if there is engineering to back up the change and it is documented. Second, as we build product there are adjustments that we can make to accommodate for the defects of the materials we are using. Third, the lumber should be to grade and all the rules in ANSI/TPI 1 should be followed. Wane and other defects that impact tooth holding can force adjustments to the design. If these defects are within acceptable parameters during fabrication, they can be made and still meet the code requirements. A simple example might be upsizing a connector for excessive wane in the plated area. Larger plates also may be used due to limits of inventory, upsizing for handling, or simplification of plate picking. To understand the big picture though, it can be helpful to consider specific situations.
The first picture [See PDF or View in Full Issue] brings into focus the question of what can we do when we have a broken web or chord? Why can’t we simply put a plate over the damage like this photo illustrates? If we had a specific sealed engineering design, we could make this repair. The problem is that such a repair typically is not worth the time, cost, and effort required to document the damage, initiate the repair, deliver the repair directions to the shop, and then track the documentation connecting it with the project. For this reason, repairs of broken webs and chords are not allowed and should never be done unless there is that specific design.
The next photo [See PDF or View in Full Issue] illustrates a similar repair that was made incorrectly but could have had a significant impact if the truss had made it out of the shop and onto a project. The repair was made using a (random) connector and, because of the location of the break, the plate was positioned in a location that was incapable of transferring the load. This could have resulted in a catastrophic situation.
The same rules apply for mill cuts and other cuts that are random and undesirable because they create stress concentrations in the component that could lead to a failure in the member. The third photo [See PDF or View in Full Issue] is an example of an incorrectly “repaired” mill cut. The design originally required a 3 x 6 plate centered on the joint. It was replaced with a 6 x 12 connector. In this situation, the chord should have been culled.
The last photo [See PDF or View in Full Issue] is of a truss that broke in the final press. Workers were preparing to add a connector to the break and feed it back through the finish roller. This would have resulted in a seriously defective truss hidden by the connector until the joint failed. Repairs like this create a ticking time bomb. Not a great prospect for the end user, builder, or fabricator.
In every situation, the truth is that modifications can be made – but the better question is should they be made.
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.