Proof Loading to Assure Lumber Strength

The control of within-mill variations in the strength distribution of a given structural grade of lumber is of growing concern to the wood industry. Recent studies have proposed the use of on-line proof loading procedures to maintain quality through the elimination of weak pieces.An experiment was conducted on one thousand two hundred pieces of 2 x 4 No. 2 Dense KD Southern pine to determine the effect of both single and reverse bending proof loads on the strength of lumber tested in both bending and tension parallel-to-grain. The specimens were separated into six groups, each containing two hundred pieces. A pair of control groups, single proof load groups, and reverse proof load groups were tested. One group in each pair was failed in bending, the other was in tension. During the testing, no damage due to the proof loading was detected.Probabilistic analysis of the data indicates that the bending strength of the single proof loaded specimens could be reduced by 12% and still remain as reliable as the control. Reverse proof loaded specimens could tolerate a reduction of 33%. The tensile strength for each single proof loaded member could tolerate a reduction of 13%, while reverse proof loaded specimens could face a decrease of 18% and maintain a level of safety equal to the related control. When using a bending proof load to assure lumber strength, it is necessary to use a higher proof load to assure tensile strength than would be needed just to assure bending strength