SECTION CONTENTS
What is MSR Lumber?
Quality Control
Specifying MSR Lumber

This information is available in the publications Western Lumber Product Use Manual (A) and Machine Stress Rated Lumber (TG-4) which can be downloaded in Adobe Acrobat (PDF) format.

NOTE: Western Lumber design value information is presented in the Design Values section. However, everything you need for MSR lumber is included in this section. 

Standard Adjustment Factors for MSR lumber are the same as those for Dimension Lumber (as provided in the Design Values section); they are repeated in this section for your convenience. A Checklist for MSR Adjustments is provided.

Refer to the Literature section or the WWPA Online Literature Store for additional titles and design tools.

What is MSR Lumber?

Machine Stress-Rated Lumber (MSR) is dimension lumber that has been evaluated by mechanical stress-rating equipment. The stress-rating equipment measures the stiffness of the material and sorts it into various modulus of elasticity (E) classes.

Research has shown that a direct relationship exists between the bending stiffness of a piece of lumber, its bending strength or modulus of rupture (MOR), and its ultimate tensile strength (UTS).

Since the only way to determine strength values is to actually break the piece, the next best thing is to measure the stiffness, compute the modulus of elasticity (E), and then predict the strength values.

MSR lumber is distinguished from visually stress-graded lumber in that each piece is nondestructively evaluated for bending stiffness and sorted into modulus of elasticity classes.

Following this "E" sorting, each piece must also meet certain visual requirements and daily quality control test procedures for both bending (F b) and modulus of elasticity (E).

Voluntary Procedures

Because there is a direct relationship between specific gravity values and MSR lumber grades (with higher-strength grades having higher specific gravity values), some MSR lumber producers provide additional daily quality control for specific gravity (SG) and/or tension (F t), in addition to the mandatory Fb and E testing.

When these additional levels of quality control are provided, the producer may include the appropriate F t, SG, and specific gravity-related compression perpendicular-to-grain (Fc | ) and horizontal shear (Fv) values on the grade stamp, in addition to Fb and E. MSR producers providing one or more of these additional levels of quality control may choose to limit the number of grades which are subject to Ft and SG testing.

Quality Control

In addition to requiring conformance to the Western Lumber Grading Rules for machine rated lumber, the Association and the producers assume the following additional areas of responsibility for quality control:

WWPA

• Certification of machines
• Calibration of test equipment
• Plant use regulations
• Quality inspections
• Product appearance considerations

Producer

• Adherence to WWPA procedures
• Strength level checks every shift
• Constant visual quality checks
•"E" level checks every shift
• Maintenance of detailed test records
• Specific gravity and/or tension level checks every shift, when additional procedures are implemented

Code Acceptability

MSR lumber produced under an approved grading agency’s certification and quality control procedures is accepted by regulatory agencies and all major building codes.

Specifying MSR Lumber

Specifying machine stress-rated lumber is simple because it is generally marketed by strength and stiffness values, F b and E.

When ordering, specify lumber gradestamped MSR or Machine Rated and list the F b and corresponding E values.

The only time it is necessary to specify a particular species is when design criteria require a horizontal shear (F v), specific gravity (SG), or compression perpendicular-to-grain (Fc | ) value associated with a specific species, or when design criteria call for higher specific gravity (and related Fc | and Fv values) resulting from additional specific gravity quality control.

Grade Stamp Facsimiles

All MSR lumber must be gradestamped by an American Lumber Standard-approved agency. The grade stamp must include a product designation such as "MSR" or "MACHINE RATED," the registered trademark of the grading agency, the mill number or name, moisture content designation, species designation, and the F b and E rating. Ft, SG, Fc | and Fv values may only appear when appropriate additional voluntary quality control processes are implemented. The following grade stamp facsimiles are examples only, the exact format may vary as a result of stamping equipment requirements.

About MSR Design Values

Design values are in pounds per square inch. When designing with MSR lumber, the appropriate adjustments must be applied to the Base Value numbers. Adjustment tables are provided in this section for your convenience. They are the same Adjustment Factor tables as provided with Dimension Lumber Design Values in the Design Values section.

F b: For any given value of Fb, the average modulus of elasticity (E), may vary depending on species, timber source and other variables. The E value included in the Fb-E grade designations are those usually associated with each Fb level. Grade stamps may show higher or lower E values (in increments of 100,000 psi), if machine rating indicates the assignment is appropriate. When an E value varies from the designated Fb level in the table, the tabulated Fb, Ft and Fc values associated with the designated Fb value are applicable.

Flatwise Use: The tabulated F b values are applicable to lumber loaded on edge. When loaded flatwise, refer to Flat Use Adjustment table.

F c | and Fv: Design values for compression perpendicular-to-grain (Fc | ), and horizontal shear (Fv), are the same as assigned to visually graded lumber of the appropriate species.

Refer to the Design Values section for design value information on visually-graded lumber.

Information Tables (Note: you need Adobe Acrobat to read the tables, which appear in a new browser window.)

Design Values for Western Machine Stress-Rated Lumber - Table 3(msr.pdf)
    Deriving Compression Perpendicular-to-Grain Values
    Deriving Horizontal Shear Adjustment
    Checklist - Adjustments for MSR Lumber

Adjustment Factors for Base Values - Tables A-C (adjust.pdf)
    Size Adjustment Factors
    Repetitive Member Adjustment Factors
    Duration of Load Adjustment
    Horizontal Shear Design Values

Additional Adjustment Factors for Base Values - Tables D-G (adjust2.pdf)
    Flat Use Factors
    Adjustments for Compression Perpendicular to Grain
    Wet Use Factors
    Incising Factors

While every effort has been made to ensure the accuracy of the Western lumber information included in this Online Technical Guide, WWPA accepts no responsibility for errors or omissions in the information presented herein, or for errors which may occur in downloading and printing any of the files, nor any liability resulting from the use of this information for design and construction applications.

The WWPA Online Technical Guide is copyrighted by Western Wood Products Association in Portland, Oregon. Ó 1997 WWPA