Machine stress rated (MSR) lumber is softwood dimension lumber that has had its strength predicted by mechanical means rather than by relying on visual indicators. MSR lumber has traditionally been used for producing engineered wood products such as roof trusses and is now also commonly used in producing glue-laminated (glulam) beams, chords for wood I-beams and webs in stressed-skin panels.
The grading system for MSR lumber is based on the established relationship between the stiffness of a piece of lumber and its bending strength. An MSR machine non-destructively tests each piece of MSR lumber to determine its stiffness so that it can be assigned a permitted design stress. Grades of MSR lumber are assigned “f-E” values (e.g., 1950f-1.7E). The “f” value designates the predicted strength in pounds per square inch (psi) and the “E” value designates the average stiffness measured in millions of pounds per square inch (106 psi).
Products like roof trusses will use different grades of MSR lumber within their structure, depending on the particular stresses experienced by the chords composing each roof truss. Most MSR lumber is 2x4; 2x6 and 2x8. MSR lumber is also produced in smaller quantities.
Engineers prefer MSR lumber, since the strength characteristics are known within a narrow range. MSR lumber is also used in furniture manufacture, for scaffold planks, in assembling ladders, and as electric utility pole cross-arms. As building codes become more rigorous and region-specific, the market for lumber with more defined strength characteristics (i.e., MSR lumber) is expected to grow. For example, in regions that are redesigning building codes as a result of increased risk of hurricane damage, MSR lumber is gaining market share in residential construction for use as wall studs.
MSR machines are located predominantly in the interior of British Columbia and in the southern U.S., to take advantage of the strength inherent in lodgepole pine and southern yellow pine.
In 2011, Canadian and U.S. production of MSR was 1.3 million cubic metres, a substantial decline from 2005, when production peaked at 3 million cubic metres.
However, overall production is expected to increase as a result of increasing house construction in the U.S. and the revised lower strength ratings for non-MSR lumber made from certain species in building codes and standards. Production is expected to decrease in British Columbia (because of the mountain pine beetle epidemic) and to increase in the southern U.S. to fill this gap.
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