Chapter 3 of ANSI/TPI 1-2022, the National Design Standard for Metal Plate Connected Wood Trusses, provides the component manufacturer with the Quality Criteria they must follow to ensure the metal plate connected wood trusses (MPCWT) they build meet the design assumptions. Section 3.4.2 Lumber Substitutions provides the requirements when you need to substitute a different material than what is called for by the Truss Design Drawing (TDD):
Section 3.4.2 Lumber Substitutions. Truss lumber of a different grade shall be permitted if the substitute grade meets or exceeds the specified grade for each of the following engineering design properties:
a) Bending (FB)
b) Tension (FT)
c) Compression Parallel to Grain (FC)
d) Compression Perpendicular to Grain (FCꓕ)
e) Shear (FV)
f) Specific Gravity (G)
g) Modulus of Elasticity (E)
h) Modulus of Elasticity for Stability Calculations (Emin).
Note that this section starts with “Truss lumber of a different grade….” Per this section, you can substitute only members of the same size. If a size change is needed, the truss design will have to be reanalyzed within your design software. If any of the design criteria change, the truss design should be reanalyzed in your design software.
When designing with wood, these are the eight design properties that are used. These design properties can be found in the American Wood Council’s (AWC) National Design Specification® (NDS®) for Wood Construction Supplement. The NDS Supplement is essentially a clearinghouse of the values for structural sawn lumber and other wood products provided by the lumber grading agencies or product associations. You can view this standard for free on the AWC website.
NDS Supplement Table 4A covers all species of visually graded sawn lumber except Southern Pine (SP). We will use the three following species from Table 4A: Douglas Fir-Larch (DFL), Hem-Fir (HF), and Spruce-Pine-Fir (SPF). For SP, we go to Table 4B in the NDS Supplement. When comparing a member from one of these three species to SP, an additional step is needed: accounting for the Size Factor CF. This is because the Southern Pine Inspection Bureau (SPIB) submits its design values with the CF already taken into account.
Now that we know where to obtain the lumber design values we need to compare, here are some steps we recommend you follow. [For all images, See PDF or View in Full Issue.]
First, let’s look at item f from the list, the Specific Gravity (G) value. See Table 1 for the G value of the four common species of lumber used.
No matter the grade of the new material, you cannot substitute material with a lower G value for a member with a higher G value. Your connector plate sizes are based on the G value of the lumber at the joint. So, if the only available material to substitute has a lower G value, the truss design should be reanalyzed in your design software.
Now let’s look at some specific situations.
Condition 1: Same Species and Material Grading Method
Other than different G values, this is the easiest of the situations you may find yourself in. You can always substitute a higher grade for a lower grade. For example, a #1 for #2, or 1800f-1.5E for 1650f-1.5E MSR, are acceptable.
If the only material you have available to substitute is a lower grade than that called out in the TDD, the truss design will need to be reanalyzed in your design software.
Condition 2: Different Species but Same Material Grading Method
Your TDD calls out for a #2 Spruce-Pine-Fir, and you have #2 Southern Pine available. When comparing the design values for this case, the CF factor should be included. See Table 2, for the CF values for the #2 SPF.
See Table 3 for the comparison of #2 SPF to #2 SP. As you see, the Fb, or bending value of #2 SP, falls below that of #2 SPF. So, you could not substitute #2 SP for #2 SPF. With only one design value falling short, it may be worth reanalyzing the design, making the substitution, and seeing what happens. Otherwise, a #1 grade member would be required, if available.
Condition 3: Different Species and Material Grading Method
Your TDD calls out for a #2 Spruce-Pine-Fir, and you have 1650f-1.5E SPF MSR available. See Table 4 for the comparison of #2 SPF to 1650f-1.5E SPF. All the values for 1650f-1.5E are higher than the #2 SPF, so this substitution can be made.
To maintain product quality and safety, it is very important to follow the lumber substitution guidelines in ANSI/TPI 1 and the design properties in the NDS Supplement.
For additional information, or if you have questions, please contact the MiTek Engineering department.