There is interest in the lumber and truss industry to supply and use fingerjoined lumber for metal plate connected wood trusses. To support this, it is necessary to provide evidence that fingerjoined lumber meeting the requirements of a recognized fingerjoined lumber product standard can be used with the lumber design provision provided in the governing wood engineering design code.
In consultation with the truss and lumber industry, it was agreed that fingerjoined machine graded lumber meeting the requirements of the National Lumber Grades Authority (NLGA) Special Product Standard 4 (SPS 4) would be assessed for truss applications. The assessment would need to show no issues with applying the lumber design provisions in Clause 5.5.13 of CSA O86, the Canadian Engineering Design in Wood Code, to NLGA SPS 4 fingerjoined lumber. This is necessary because Clause 5.5.13 was originally developed for non-fingerjoined lumber and applies specifically to the design of lumber in truss applications.
The tests carried out under this program included bending test specimens with 1 to 4 joints per specimen tested to failure under three different bending moment configurations, and single fingerjoints tested to failure under pure axial tension or compression, and then under eccentrically applied axial tension or compression to induce bending in addition to the axial loading. All test specimens were prepared using a 2100f-1.8E grade spruce-pine-fir lumber and because the test to failure was typically less than 5 minutes, polyvinyl acetate (PVA) adhesive was used to bond the fingerjoints to facilitate joint fabrication.
Additional testing was also carried out to extend the testing protocol developed in 2008-09 for assessing fingerjoint adhesives under sustained tension loads. Samples bonded with a known performing adhesive, phenol resorcinol formaldehyde (PRF), were substituted with samples bonded with PVA, a known poor performer under sustained loads.
In the bending test, test span configuration and characteristic number of joints showed strong effects on the average bending capacity of the fingerjoints. While more joints in the region of maximum bending moment were expected to contribute to lower bending capacities, this was not as evident in this study. This is likely due to the small sample sizes and the tight control over the joint strength (i.e. low strength variability). Instead, having one or more fingerjoints in the maximum moment zone but near the load points appeared to have a stronger effect. The bending strength reductions were on the order of 5 to 10%.
In the combined loading test, loading eccentricity showed a strong effect on the capacity of the fingerjoints in both tension-bending and compression-bending. The tension-bending interaction should be noted for those evaluating online or offline tension test results. Both the tension-bending and compression-bending results are consistent with the assumptions in the CSA O86 design code.