Last month’s fine article by Glenn Traylor, “How Do I Choose Which Plates for a Critical Plate Inspection?”, reminded me of investigative experiences I’ve had throughout my career. The work of truss builders is extremely important, as evidenced by instances of truss failures.
Along with permanent bracing deficiencies, the most common cases of plating error that I’ve seen have been W-trusses clear spanning about 50-ft. As Glenn discussed, a common thread for the peak joint misplacement issue occurs when the top chords are 2x6 (and greater) with 2x4 webs. Apparently, truss builders may (improperly) shift the plates upward in an attempt to “cover” more of the larger top chords.
Truss Fabrication Quality—A Life-Safety Issue
In one case involving a fatality during truss installation, a 2x6 36-ft Fink truss (2-ft. on-center) failed at the peak joint when an installer was standing on the center panel of the bottom chord. The truss design drawing required 5x8 plates to be centered on the intersection of the four members at the peak joint.
As shown in the photo [see PDF or View in Full Issue], the 5x8 plates were improperly shifted upward onto the 2x6 chords about 1-1/4 inches. In addition to the plate misplacement issue, the teeth in contact with the webs area were not fully embedded, exacerbating the misplacement error. For the truss cases I’ve investigated, this case was the easiest to diagnose because photographic evidence taken before the collapse showed that temporary truss bracing (both lateral and diagonal per industry recommendations) had been installed.
What “Looks Right” May Not Be “Safe”
Though it may be unknown to many truss builders, the design axial compressive force in truss joints was permitted to be reduced by 50% in 1995 with the release of ANSI/TPI 1–1995. Known as the “1/2 Rule,” the tooth count requirement/contact area for joints loaded to the same level in “compression” was permitted to be ½ as many as needed for two members loaded in “tension.” The net effects of the truss engineering rules are plating designs for larger top chords in compression (2x6 to 2x12) having less plate coverage over the butted members; in contrast, the same size chord in tension without the 50% tension force reduction might have almost total coverage of the butted member ends.
It can be noted that the historic “1/2 Rule” provisions were revised in the 2007 and 2014 editions of ANSI/TPI 1. The current version of ANSI/TPI 1 permits a compressive force reduction factor ranging from 0–100% depending upon numerous complicated factors. Details can be found in ANSI/TPI 1–2014, Section 8.3.3.3 (p.72–73) and Commentary to 8.3.3.3 (p.72–74). Visit https://www.tpinst.org/new-tpi1-2014page for a free read-only version of the Standard.
The Take-Away
I invite CMs to consider using this example case in training and education of truss builders. Some of the points that could be made are:
- The job of truss builders is very important, especially when you consider that construction workers walk on trusses during the installation process.
- During truss installations, truss chords are just as important as wood-based “scaffold planks” which have a safety factor about twice the safety factor of framing lumber, such as joists and rafters.
- Regardless of truss span, accurate plating of all truss joints is structurally important to ensure both worker safety during truss installation and reliability in-service when exposed to the code-required design loads.
- Due to the complexity of modern wood truss designs, it is critically important for the fabrication team to carefully review the Truss Design Drawings (TTDs) during the plating process to ensure compliance with the TDDs.
In addition to concerns for quality and in-service performance, the importance of proper training and education should not be understated when considering matters of safety.
Frank Woeste, P.E., Ph.D., is Professor Emeritus, Virginia Tech, and frequently consults with the public, design professionals, contractors, and building code officials on various aspects of engineered wood construction and residential construction, including decks and balconies. Frank, along with his colleagues, continues to offer continuing education programs at Virginia Tech annually.