Reverse Proofloading of Lumber

The determination of the strength distribution of a given structural grade of lumber from any chosen mill is of growing concern to the lumber industry. Lumber in the lower range of strength values is of prime interest. A reverse proofloading procedure, applied in bending, results in a more predictable product through elimination of the weak pieces resulting from imperfect performance of the visual stress rating model, variation in grader interpretation, and fluctuation in log quality. Experiments on 400 pieces of 2 by 4 No. 2 Dense KD Southern Pine involved loading 200 pieces to failure in bending and reverse proofloading in bending the other 200 pieces. The pieces that survived the reverse proofloading were then tested to failure in bending. The data was analyzed using a load and resistance reliability model. Results indicated that the reverse proofloading procedure could maintain the claimed allowable design value of 1,800 psi in the face of a 27 percent downward shift in strength distribution within the grade.

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