Reverse proof loading as a means of quality control in lumber manufacturing

Abstract

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 proof loading procedure, applied in bending, appears to result in a more uniform product through elimination of the weak pieces resulting from inconsistencies within mill grading procedures, different levels of grader skill and variation in log quality.

Experiments on 400 pieces of 2 x 4 No. 2 Dense KD Southern Pine involved loading 200 pieces to failure in bending and then reverse proof loading in bending the remaining 200 pieces. The reverse proof loaded lumber (RPL lumber) was then destroyed in bending. The probability of failure was calculated for the first 200 pieces (the control lumber) and compared to the probability of failure of the RPL lumber. A reduction of strength procedure was then performed on the RPL lumber data. These results indicated that the reverse proof loading procedure would account for a 30.5% drift in the quality of manufactured lumber.