A
Th e
Component Manufacturing dverti$er
Don’t Forget! You Saw it in the
June 2019 #11239 Page #57
Adverti$er
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: 1. The job of truss builders is very important, especially when you consider that construction workers walk on trusses during the installation process. 2. 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. 3. 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. 4. Due to the complexity of modern wood truss designs, it is critically important for the fabrication team to carefully review the Truss Design Drawings (TDDs) 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.
PHONE: 800-289-5627
Read/Subscribe online at www.componentadvertiser.com
FAX: 800-524-4982