Design Values of Machine Stress Rated Lumber

Machine Stress Rated lumber (MSR Lumber) assures the performance and reliability of your engineered components and structures. The machine grading process sorts dimension lumber by strength and stiffness to improve consistency. Daily quality control testing for strength and stiffness ensures that products meet structural requirements. Machine graded lumber certification and quality control procedures are based on approved American or Canadian lumber standards (ALS or CLS).

Design values in pounds per square inch (psi)

Notes:
(1) Other grades of Machine Stress Rated Lumber: The grades listed above are meant as examples of commonly produced Machine Stress Rated (MSR) grades. This is not in-tended to be a complete list. The Supplement to the National Design Specification (NDS) for Wood Construction (AF& PA, 1997) provides a good summary of the established grades in Machine Stress Rated (MSR) and Machine Evaluated Lumber (MEL) in Table 4C on page 35.

(2) Quality Control of Machine Stress Rated Lumber: QC testing takes place on a daily basis for all machine graded lumber products. Depending on the grade requirements, testing takes place for one or more of the following properties: E, Fb, Ft. This testing process is established to verify that production meets design requirements for all product shipped to customers as MSR, MEL, or E-rated laminating grades.

(3) Common Species used in Machine Stress Rated Lumber: The Machine Stress Rated Lumber Production Survey (MSR Lumber Producers Council, 1998) identifies Spruce-Pine-Fir (SPF) as the species in which the majority of Machine Stress Rated Lumber (MSR Lumber) is produced (69% of total Machine Stress Rated in 1998). Douglas Fir-Larch (DFL), Hem-Fir (HF), and Southern Pine (SYP), share significant volumes as well (5% to 14% of total Machine Stress Rated Lumber (MSR Lumber) in 1998). Consult the MSR Lumber Production Survey for more information.

(4) Species-specific design values for Machine Stress Rated Lumber: Machine Stress Rated lumber (MSR lumber) simplifies many design considerations since a grade like 1650f-1.5E maintains the same Fb, Ft, Fc//, and E values no matter what species or size is considered. Other properties, however, change by species as they relate to the specific gravity (density) of wood. A couple of examples are listed below. Please refer to NDS Supplement footnotes #2 for Table 4C for more detailed information. Grade rules writing agencies are another good source of up-to-date information on this topic (see NDS Supplement Section 1.1 on page 2).

Grade Designation	Species	SG	Shear parallel to grain Fv	Compression perpendicular to grain Fc
1650f-1.5E	SPF	0.42	135	425
1650f-1.5E	HF	0.43	145	405
2400f-2.0E	SPF	0.50	170	615
2400f-2.0E	SYP	0.57	190	805

(5) Adjustment Factors: Adjustment factors must be applied to the allowable design values presented here. Adjustments to values are taken for duration of load, repetitive member situations, beam and column stability, and other factors as summarized in NDS Table 2.3.1 on page 9. Machine graded lumber enjoys specific advantages in beam stability, column stability, and buckling stiffness factors as a result of the consistency of E compared to visually graded lumber.

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