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Don’t Forget! You Saw it in the
April 2023 #15285 Page #84
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Truss Design Value Comparison Tables David Conner, Vice President, Timber Products Inspection
T
imber Products Inspection has compiled design value comparison tables for several popular species and species groups as a helpful tool for truss manufacturers to determine if a lumber grade and/or lumber species substitution satisfies the original truss design. The data used comes from the 2018 National Design Specification® (NDS®) for Wood Construction Supplement and is normalized to a 2x4 application. These tables can be used to compare the design values for visual grades of 2x6–2x12 as well, since the size adjustment factors are consistent regardless of grade and/or species. These 2x4 table values are valid for comparing visual grades of 2x4 to 2x4, just as they would be valid for comparing visual grades of 2x6 to 2x6, 2x8 to 2x8, 2x10 to 2x10, and 2x12 to 2x12. Note 1 – These tables were not intended to be used for size substitution purposes, since size substitutions can cause other practical issues beyond just design value application. Note 2 – MSR grades can be compared to each other using these tables, regardless of size. MSR grades can be compared to visual grades using these tables for 2x4 only, though; any other size would require the size adjustment factors to be applied to the visual grade’s design values. Example of table use – the truss design calls for 2x6 SPF NO.1, and the substitution considered is a 2x6 SYP NO.2 (different species and different grade). The tables provide the following 2x4 values: ITEM
Fb
Ft
Fv
Fc-perp
Fc-par
MOE
G
2x4 SPF NO.1
1300
675
135
425
1300
1.4
0.42
2x4 SYP NO.2
1100
675
175
565
1450
1.4
0.55
The SYP NO.2 meets or exceeds SPF NO.1 for the Ft, Fv, Fc-perp, Fc-par, MOE, and G. But since the Fb is lower for SYP NO.2, it cannot be substituted for SPF NO.1. The next higher visual grade of SYP NO.1 can be compared for possible substitution. The tables provide the following 2x4 values: ITEM 2x4 SPF NO.1 2x4 SYP NO.1
Fb 1300 1500
Ft 675 1000
Fv 135 175
Fc-perp 425 565
Fc-par 1300 1650
MOE 1.4 1.6
G 0.42 0.55
The SYP NO.1 exceeds SPF NO.1 for the Fb, Ft, Fv, Fc-perp, Fc-par, MOE, and G. So the 2x6 SYP NO.1 can be substituted for 2x6 SPF NO.1. If you have questions or would like more information, please call us at 770-922-8000 or visit http://www.tpinspection.com/. Continued next page
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