Once in a while, I’m asked—Is there ever an occasion when connectors on both sides of the truss need exact placement? Well, the answer might surprise you. There are, in fact, several situations where the top face connector and the bottom face connector need to be exactly placed.
The first situation where precision is essential is when the Joint Stress Index (JSI) is 100% and the mid point polygon on the plate placement plan or method (PPM) has a very small polygon. This small polygon occurs because the point limits any movement.
A second situation can occur when the outside geometry limits any movement. This happens sometimes at heels where the plate size equals the heel size. Like the limited movement encountered in the polygon example, this situation should be avoided by the designer if at all possible, because the standard is very difficult to achieve, if not impossible.
There is a third situation that can’t be avoided nor can the plates be misaligned. That third situation is when using hinged plates. Hinges plates allow the field to reestablish the profile of the truss beyond equipment and trucking capacity, avoiding additional framing and piggy back over-framing. The most important relationship between the top face plate and bottom face plate is the alignment of the pivot point. This center must align, otherwise undesirable consequences will result as the truss is moved into field position. In the photo [see PDF or View in Full Issue], the centers do not align. As the truss members are folded into position to press, the finish roller will flatten and realign the plate and teeth, resulting in distorted and folded teeth greatly reducing its design capabilities. If the holes do not align when folded and pressed, the swinging members will move out of the truss plane, resulting in bracing and load transfer issues.
Especially in the third situation, careful education and evaluation is necessary to ensure accuracy. Some plants use a training aid to facilitate alignment—it’s just a steel pin welded to a small metal plate that sits under the truss and connectors. After some experience, this device won’t be necessary, but its initial use will be invaluable to help ensure accurate plate placement. We may all be accustomed to less-than-perfect alignments between top and bottom plates, but we should all remain aware of the times when their alignment is essential.
Glenn Traylor is an independent consultant with almost four decades of experience in the structural building components industry. While he is a TPI 3rd Party In-Plant Quality Assurance Authorized Agent covering the Southeastern United States and performs 3rd party safety auditor services, these articles represent his personal views, knowledge, and experience. 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.