Preemptive Design

“Understanding the plate design leads to a greater appreciation for why QC is so important,” says Scott Ward, principal of Southern Components, Inc. in Shreveport, Louisiana and chair of SBCA’s QC Committee. “It helps avoid potential truss repairs in the field, which can be a real headache.”

Ward wants all of his employees to make the connection between good design and quality control topics like lumber size and grade selection, plate placement, and plate-joint matching (the process of ensuring the plate size used on a joint matches what is shown on the truss design drawing).

To someone starting out in the industry, the concept of manufacturing a truss with the proper connector plate lateral resistance can seem fairly straightforward: at each joint of a truss, the metal connector plate must have sufficient plate-to-wood contact area to resist withdrawal of the plate teeth from each wood member in the joint. ANSI/TPI 1-2014 requires metal connector plate manufacturers to conduct testing to determine metal connector plates’ lateral resistance (that is, the holding power of the plate teeth). The lateral resistance design values derived from testing are used in truss design software so the program can correctly size and place a plate for each joint, even taking issues like imperfect lumber and the occasional bent tooth into account.

If the software always picked the perfect plate, QC inspections would be simple; but that’s not quite all there is to it.

For practical reasons, testing can’t fully account for the entire range of joint conditions and plate sizes commonly used in truss fabrication. The challenges of plate testing result in design values that must be adjusted to account for the considerations specific to an individual joint. Some of these adjustments are left to the judgement of the plate manufacturer’s engineers.

One adjustment applied to a lateral resistance design value, and one which greatly affects QC, is the aptly-named quality control factor (C_q). Based on the fabrication tolerance built into truss design software, the quality control factor typically ranges from 1.0 to 0.8 (sometimes it can be even less), and it reduces the lateral resistance of the truss plate in design calculations to account for the possibility of strength-reducing characteristics (such as pitch pockets, wane, loose knots or flattened teeth) in the plate contact area of the lumber.

Component manufacturers (CMs) can work with their plate suppliers to adjust this value based on their in-plant QC inspection results. Reducing the C_q factor (i.e., increasing the fabrication tolerance) increases the allowable defects for a given joint (i.e., the strength-reducing characteristics of the lumber in the plate area), but also requires a more-perfect plate placement (as the size of the tolerance polygon is reduced) or a larger plate.

The designer’s goal is to strike a balance between providing a sufficient reduction in lateral resistance to account for joint strength reducing characteristics without either shrinking the plate placement tolerance beyond what is practical for the company’s normal production quality control procedures or increasing the plate size beyond what is readily available and cost effective. Optimizing QC means pitting an ideal design that minimizes plate size against the reality of manufacturing with an organic raw material like lumber and an imperfect manufacturing environment.

“While focusing on the right height and span makes the building fit together and look right,” Ward explained, “the real engineering performance exists at the truss plate design and placement level.” The value of a manufactured component comes from the testing, structural analysis, joint design, and fabrication precision that goes into creating a truss. It’s what makes CMs important to their customers and what makes suppliers important to CMs.

“We are really only as good as our lumber, plate and software suppliers,” said Ward. Design, QC and the connection between them hold everything together. “Our QC program allows us to take greater advantage of their products and technology,” Ward explained. When truss designers understand the assumptions built into the design software and use SBCA QC inspections and the management data generated from those inspections to identify opportunities to adjust software settings, CMs can take full advantage of lumber quality and plate performance to generate manufacturing efficiencies.