Machine Stress Rated Lumber: Challenge to Design

Machine stress rating (MSR) principles of nondestructive testing improve the design potential of lumber through segregation by modulus of elasticity as well as by identification of allowable fiber stress levels through combined MOE and visual criteria. Although significantly more accurate in grading for elasticity than conventional visual grading, use of MSR lumber in conventional housing has been influenced by the paradoxes of stiffness criteria that exist in present wood design. In more engineered use such as trusses, MSR lumber has been employed extensively because of its ability to identify lumber capable of use at high fiber stress levels. In laminated beam design, where both stiffness and strength are important, acceptance hinges on development of design specifications to accomodate the potential that has been demonstrated in exploratory full-sized beam tests. Progress with advanced lumber grading systems is dependent upon enlightened progress in structural design with lumber.