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August 2024 #16301 Page #84
Glued Trussed Floor Joists By Tuomo Poutanen, PhD Professor, Patent Attorney
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n this publication, I reviewed glued parallel chord truss joists (or glued-joint open web wood trusses) in the August 2021 issue and glued roof trusses in the April 2023 issue. This article focuses on parallel chord glued trussed floor joists utilizing the new G-technology, https://www.gjoist.com/.
G-Joist has some unique patented features, web fingers extending completely through the chord, as shown in the image. This web-chord interface maximizes the glue area by utilizing the entire overlapping area between the chord and web with longest web fingers exceeding 2”. The three critical failure modes in a glued truss are knot failure in the web finger, splitting failure in the web, and chord splitting along the grain. These failures are eliminated when employing the G-technology. Knot failure in the finger is prevented using large fingers, which accommodates chord knots and small web knots, while large web knots are graded off. Web and chord grains cross each other throughout the joint, reinforcing the grains against web and chord splitting analogous to the reinforcement seen in plywood. With the G-joint, only two types of failures are possible: web finger cut and glue line failure. Finger cut failure is unlikely when the webs are graded to exclude major knots. Glue line failure is also improbable due to the low mean glue line shear stress of 70–100 psi (0.5–0.7 N/mm²). By keeping all webs identical, and with no verticals, all joints are similar, ensuring uniform resistance throughout the joist. The truss can maintain structural integrity even when installed upside down. Joint routing is done using a saw blade at two blade inclinations to create tapering fingers, as shown in the figure. This two-step routing process is somewhat slow, but it offers multiple advantages: the cutting surfaces are ideal for adhesive bonding, maintenance of the saw blades is inexpensive and simple, the saw blade works well even with knotty timber, the cost of the saw blade is low, and when assembled, there are no open routings, water pockets, or cavities. The web-chord interface (WCI) joint behavior is similar to, and as reliable as, the web-flange interface (WFI) commonly employed in the manufacture of wood I-joists. The historical data and low coefficient of variation (COV) of this technology eliminates the need to proof test each component, requiring only intermittent random testing.
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