The most common use of finger-joined lumber is in exterior shearwalls and load bearing walls as studs. The most important factor for studs is straightness. Finger-joined studs will stay straighter than solid sawn studs when subjected to heat or humidity. This feature results in significant benefits to the builder and homeowner including a superior building, the elimination of nail pops in drywall and other related wall problems. This also makes finger-joined lumber with a "Stud" grademark an ideal candidate for interior non-load bearing partitions.
Finger-joined lumber for VERTICAL STUD USE ONLY can be manufactured from any commercial species or species group specified by a certified grading agency. The most commonly used species group from which finger-joined lumber is produced is Spruce-Pine-Fir [S-P-F].
NLGA in Canada was one of the first certified rules-writing agencies to develop finger-joined lumber standards, SPS 1 and SPS 3, and hence has one of the most recognizable grade markings that includes the product standard designation on it. In December 1999 the SPS 3 standard was revised by NLGA to allow additional grades to be produced under the standard and to include a new grademark designation, "VERTICAL STUD USE ONLY". Lumber with this grademark is typically Stud grade, which is separately marked on the grade stamp. VERTICAL STUD USE ONLY describes how it is most commonly used, however it is not limited to this use. See Table 1 for other permissible uses.
Finger-joined lumber can be used in wall construction taking into account the same design factors as would be considered with solid sawn lumber.
For example, one could substitute S-P-F finger-joined lumber with a "Stud" grademark for regular studs marked "Standard" grade, because the Standard design values are lower. It could also substitute for No.3 grade, because the No.3 regular grade values are the same as for Stud. One could not normally substitute finger-joined lumber marked "Stud" grade for "No.2" grade because No.2 grade design values are higher than for Stud; however, if the application only requires Stud grade lumber, the finger-joined product is fine.
Regarding species groups, substitution also depends on the application. In high-wind or seismic areas, walls may be required to resist horizontal loads as well as vertical loads. Shearwalls are the primary structural elements that provide resistance to applied horizontal loads. In earthquake zones such as California, designs have traditionally been based on Douglas fir, however, other species may also be used.
Let's look at an example where finger-joined S-P-F lumber is considered as an alternative for Douglas fir studs. If the exterior wall acts as a shearwall, then the reduced base shear capacity of the lower density S-P-F needs to be considered. One possible solution is to decrease the edge nail spacing to meet or exceed the required base design shear as shown in Table 2 below.
Another factor to consider in a shearwall is the capacity of the hold-down to transfer tension forces. Direct substitution of a lower density material will result in a lower connection capacity, which may require a redesign of the connection.
However, California structural consulting engineer Norman Sheel describes another alternative for direct field substitution of Douglas fir solid lumber with S-P-F finger-joined lumber in a shearwall application without requiring redesign. He notes that, " If the members where the edge nailing is applied are left as Douglas fir, and the top and sill plates are left as Douglas fir, I feel that the Douglas fir [base shear] design values are valid." This suggests that, because of stress distribution, intermediate framing that does not occur at panel edges can be directly substituted without reducing the shearwalls base shear capacity to resist wind and earthquake loads. With this in mind, over 95% of the wall framing in a typical California single-family residence could be built using S-P-F finger-joined lumber. Only the 4x4 posts at the end of shearwalls and 2x3 members adjoining the panel edges in shearwalls would be required to remain as Douglas fir to preserve shearwall resistance.
|Table 1: Comparison of structural properties of S-P-F finger-joined VERTICAL STUD USE ONLY lumber.|
|Fb||E||Base Shear Capacity*|
|D.Fir-L Solid Sawn||1050 psi||1.4 x 106 psi||392 plf - 6" edge nail spacing|
|S-P-F Finger-joined||1015 psi||1.2 x 106 psi||457 plf - 4" edge nail spacing|
|*Based on 2x4", Stud grade, 16"o/c, 3/8" Type I structural sheathing, SBCCI wind, 8d nails, 12" field nail spacing|
Finger-joined lumber is also commonly used for flange material in wood I-joists. This application of the product requires the wood fibers and the glued joint to resist long-term tension loads when in use. For this reason, finger-joined lumber used for manufacture of I-Joists must comply with the requirements of a standard such as SPS 1. Wood I-joist manufacturers undertake additional quality assessment procedures during production.