Sometimes, a designed and engineered connection will appear to look like a member to member gap. Because of that, we need to ask the question: should the truss designer always try to eliminate a member to member gap in a truss? Generally speaking, yes, it’s always worthwhile, but there are occasions when a gap occurs and it’s not a design error or fabrication error. [For all images, See PDF or View in Full Issue.]
The photo shows a circumstance where a significant gap was created between members which appears to be a fabrication error. In this case, the joint was not fabricated wrong — the builders followed the design.
The significant gap was created because the lines the web was cut against were not defined and the software attempted to pick the best possible situation.
Web 2 was cut against the angle cut of Chord 2. Also, Web 1 was single cut against the Chord 1 piece. This configuration creates a gap, as shown with the orange arrows.
Because it was done per design, the fabrication was not an error.
With this being the case, should the design have been designed differently?
My contention is yes.
The design creates a very small area contact of Web 2 against Web 1. That means additional design considerations need to be addressed to ensure proper performance and safety.
Because of the small area of contact between the webs, the plate needs to be designed stiff enough to prevent collapsing this joint under design load and, more importantly, under installation loading. Loading over time will eventually collapse this gap until a sufficient amount of material is crushed into Web 1.
How much this joint will collapse is a matter of discussion. Many designers argue that it is very minimal. Others feel any effort to prevent this circumstance is worthwhile. I agree it should be prevented, but there are several other reasons besides long-term loading.
With most major plate manufacturers providing easy-to-use tools to recut joints that don’t automatically heal this condition, it’s just a click or two to make the necessary adjustments. There are some software manufacturers that don’t provide a quick adjustment, but with a few more clicks it’s easy to resolve.
You may be asking if there are reasons other than joint collapse to avoid this situation — the answer is yes. In this joint, double cutting both Web 1 and Web 2 would make the joint easier to assemble. A very specific location would be defined by using this method. Thus, the preferred approach would be that both webs are double cut.
When considering cutting, also keep this in mind. On a component saw, a double cut web length is more accurate than a single cut. This is due to crooks or bows that naturally exist in lumber. Cutting with a linear saw helps eliminate this problem, because most linear saws clamp or hold the lumber closer to the cut end, thereby limiting the variable that bows or twist cause. Another reason to eliminate this circumstance is because it looks better and does not look like a mistake. I would argue that there is a value for this too.
The Bottom Line
Even when a truss is sealable by an engineer, it may not be a design you should build. The software may let you design it a certain way, but you should still consider questions about its long-term performance. Some designs have inherent flaws that will manifest themselves into problems years later.
An ANSI/TPI 1 3rd Party Quality Assurance Authorized Agent covering the Southeastern United States, Glenn Traylor is an independent consultant with over 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, expert witness and ANSI/TPI 1 compliance assessments. Glenn provides new plant and retrofit designs, equipment evaluations, ROI, capacity analysis, and CPM analysis.