Lumber Grades and Design Values
- Lumber grades take into account natural characteristics, including knots, splits, checks, shake, and wane.
- Lumber design values are assigned to lumber grades to provide a means for using this material in structural applications.
- Values are published in Design Values for Wood Construction, a supplement to the National Design Specification® for Wood Construction.
- The six design values are bending (Fb), shear parallel to grain (Fv), compression perpendicular to grain (Fc⊥), compression parallel to grain (Fc), tension parallel to grain (Ft), and modulus of elasticity (E and Emin).
Visually Graded and Machine Graded Lumber
- The current changes in design values affect visually graded lumber only.
- Specifically, the changes being made to southern pine lumber in 2012 are the result of laboratory testing of the strength properties of visually graded lumber.
- Because a machine is assessing strength properties during production, Machine Evaluated Lumber (MEL) and Machine Stress Rated Lumber (MSR) grading automatically adjusts to changes in the internal quality of lumber.
- Changes to visual grades have no impact on the design values of MSR and MEL because of the way these products are manufactured and tested.
Assurance from Machine Graded Lumber
- Produced since the 1960s, MSR lumber has documented and tested structural design values.
- In contrast to visual grades, machine rated lumber is produced by measuring physical properties of each piece of lumber in the production line.
- Manufacturers are required to test samples during each shift and the entire quality assurance process is audited by independent third parties to confirm conformance with published design values.
- MSR and MEL lumber consistently meet design value requirements even as timber growing practices and environmental conditions change.
Choosing MSR and MEL
- You can be confident in the strength and performance of MSR and MEL lumber.
- Designers can rely on its properties, use full-design strength, and avoid overbuilding.
- MSR and MEL lumber have proven to be cost-effective, quality lumber products for component manufacturers and builders.
Common Grades of MSR Lumber
The machine grading process sorts dimension lumber by strength and stiffness to improve consistency. The table below shows four common MSR grades and design values. For a comprehensive list, see Table 4C in Design Values for Wood Construction, a supplement to the National Design Specification® (NDS®) for Wood Construction.
| Grade Designation | Bending - Fb | Tension Parallel to grain - Ft | Compression parallel to grain - Fc | Modulus of Elasticity - E |
|---|---|---|---|---|
| 1650f-1.5E | 1650 | 1020 | 1700 | 1,500,000 |
| 1800f-1.6E | 1800 | 1175 | 1750 | 1,600,000 |
| 2100f-1.8E | 2100 | 1575 | 1875 | 1,800,000 |
| 2400f-2.0E | 2400 | 1925 | 1975 | 2,000,000 |
Design values in pounds per square inch (psi)
Design Values By Species
MSR lumber simplifies many design considerations because a grade such as 1650f-1.5E maintains the same Fb, Ft, Fc, and E values regardless of the species or size considered. Other properties, however, change by species as they relate to the specific gravity (density) of wood. A few examples are listed below; see Table 4C in the NDS supplement for more detailed information or refer to information produced by grade rules writing agencies.
| Grade Designation | Species | Specific Gravity - 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 |
Design values in pounds per square inch (psi)
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 Table 2.3.1 in the NDS supplement. 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.
Quality Control of Machine Graded Lumber
Machine graded lumber certification and quality control procedures are based on approved American or Canadian lumber standards (ALS or CLS), and 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, and 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.