Lumber Substitution
Question:
If the truss design drawing specifies using 2x4 1650F SPF, can 2x4 No. 2 southern pine be used instead? What are some things that need to be checked?
Answer:
Section 3.4 of the Truss Plate Institute's ANSI/TPI 1 permits direct substitution of a higher-stress grade of the same size and species combination, because the allowable properties of the substitute lumber will be equal to or greater than the material it is replacing. If the lumber substitution includes a change in grade, species combination (both true for this case) or size, additional analysis is required to ensure the substitution is acceptable.
Unless you re-run the design in your computer, you will need to compare the allowable design properties of the substitute lumber with those of the lumber you want to replace. If the allowable properties of the substitute lumber are equal to or greater than the specified lumber, then substitution should be ok.
In this example, the allowable design values are:
|
No. 2 southern pine 2x4 (effective Jun 1, 2013) |
1650f-1.5E SPF 2x4 |
|
Fb = 1100 psi |
Fb = 1650 psi |
|
Ft = 675 psi |
Ft = 1020 psi |
|
Fv = 175 psi |
Fv = 135 psi |
|
Fcperp = 565 psi |
Fcperp = 425 psi |
|
Fcpar = 14500 psi |
Fcpar = 1700 psi |
|
MOE = 1.4 million psi |
MOE = 1.5 million psi |
The southern pine values are significantly lower than the SPF values, so a direct substitution is not going to work for this case based strictly on a comparison between allowable properties.
A second factor that is important to remember is that plate design values are lower for SPF than they are for southern pine. This is due to the density differences between southern pine and SPF. In this case, substituting southern pine for SPF would be fine if lateral resistance (gripping) strength determines the plating requirements. SYP typically has the best truss plate design values, so substituting SYP for other species is easiest; it gets more complex going the other way.
The quickest, most precise and probably best way to check the adequacy of this proposed substitution is to re-run the design and see what effects the substitution has on the original design. Depending upon where the substituted lumber is to be used, it is possible that it may work fine, even if some of the allowable values are less than those for the SPF.