A research program aimed at examining some issues arising from use of fingerjoined lumber for long-span metal plate connected trusses was carried out in two phases. The first phase examined the effect of truss plate over-embedding and fingerjoint offset on plate capacity in small wood specimens. The results indicated that the presence of joint offset had a significant effect on plate capacity, and that over-embedding the plate into the joint bonded with polyvinyl acetate had significant effects on joint strength.
In the second phase, two follow-up studies were conducted to evaluate: i) the effect of truss plate over-embedding on strength of fingerjoints bonded with phenol-resorcinol formaldehyde, and ii) the effect of joint offset on plate capacity in tension perpendicular to grain and in tension parallel to grain in large wood members.
The present over-embedding study corroborated the results obtained in the first phase, i.e., over-embedding had a significant effect on fingerjoint strength. In bending, the severity of effect depended upon plate location. Where plates were applied on edge face, over-pressed plates gave 27% lower bending strength (MOR) compared to normally pressed plates and 35% lower than control (not plated), and normally pressed plates showed 11% lower MOR compared to control. Where plates were applied on flat face, over-pressed plates yielded 20% lower and normally pressed plates 14% lower MOR compared to control, but there was no significant difference between over-pressed and normally pressed plates. Plate location had no significant effect on MOR of normally pressed plates and that of control, but did have a significant effect on over-pressed plates. In the latter, plating on flat face gave 23% greater MOR compared to that on edge face. In tension, the severity of over-embedding effect also depended upon plate location. Over-pressed plates gave 29% and 16% lower ultimate tensile strength (UTS) compared to normally pressed plates when plates were applied on edge and flat face, respectively. On both faces, over-pressed plates yielded 18% lower UTS compared to control, but there was no significant difference between normally pressed plates and control.
In the fingerjoint offset study, the presence of offset did not have a significant effect on truss plate capacity in tension perpendicular to grain. However, the joint control without offset yielded significantly greater (about 6%) ultimate tensile load (UTL) compared to solid horizontal control. Plate location also had a significant effect on plate capacity. Plating on flat face gave 13% greater UTL compared to that on edge face. This was similar to that observed in the plate over-embedding study. Also, the presence of joint offset did not have a significant effect on plate capacity in tension parallel to grain in 2 x 4 wood members. The present results showed similarities to and differences from those obtained in the first phase depending upon plate grip location.
One negative implication is that over-embedding truss plates over fingerjoints should be avoided. This should be inspected as part of truss fabrication quality inspection. On the positive side, generally the NLGA 1/16-inch maximum fingerjoint offset should not impact truss plate capacity in tension perpendicular or parallel to grain.