Experiments in Proof Loading Structural End-Jointed Lumber

The nondestructive test methods for lumber presently available fail to provide an evaluation or quality control device for end joints in structural lumber. One proposed test method for end joint quality is proof loading, which would fracture all joints incapable of sustaining a minimum level of stress. An initial experiment showed that no incipient compression damage was caused by a bending proof load of finger joints in Douglas-fir to 60, 70, 80, or 90 Percent of the ultimate strength of matched control specimens. In a second experiment conducted to explore the feasibility of a bending proof load for tension members, the bending strength of finger joints was found to be approximately half the tensile strength, and it was concluded that a bending proof load should be about four times the allowable design stress for tension members. A tensile proof load thus appeared more feasible for tension members than a bending proof load. A tension proof load had the additional advantage of not causing incipient compression damage, as a bending proof load at high stress might. Specimen size had a definite effect on the tensile strength of end joints; in hemlock, tests of small, clear, finger-jointed specimens averaged 10,270 psi, while full-size 2 by 4’s averaged only 5,220 psi.

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