Preliminary Evaluation of Two Tongue-And-Groove Joints for Five-Ply Structural Sandwich Wood Panels for Floors

Two designs of tongue and groove (T & G) joints were evaluated for 5-ply, 7/8″ thick, structural sandwich wood panels This type of sandwich panel has been suggested as a single layer floor (combination of subfloor and underlayment). The panels consist of 3/8″ thick particleboard core (underlayment quality) and 1/8″ southern yellow pine veneers, D crossbands and B-C faces, bonded together and to the core with an extended phenolic resin. One of the two T & G designs was the standard T & G design recommended by the American Plywood Association for use with 3/4″ plywood. The other was a modified T & G joint especially designed by the senior author for this sandwich panel. This joint allows a thin portion of the crossband veneer to remain on the surface of the tongue to provide structural reinforcement of the tongue and protection against chipping during transportation and installation. In the test was included 3/4″ T & G plywood for comparison. Results of limited tests indicate that T & G joints of 3/4″ plywood and 5-ply, 7/8″ sandwich contribute significantly to the stiffness of jointed panels at the joint by reducing the deflection under the loaded joint by 50 percent as compared to deflection of straight, unsupported edges of corresponding panels. The modified T & G joints under the same loading conditions and load, supported considerably larger maximum loads than the standard T & G joints in the 5-ply sandwich panels. However, no difference in deflection was found under the same load in the standard and modified joints in the 5-ply sandwich panels. Finally, the results indicate that the 5-ply sandwich panels jointed by the modified T & G joint and loaded at the joint under a condition simulating a critical function of single layer floors, are as strong and substantially stiffer at the joint than jointed 3/4″ T & G plywood panels.

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