Reliability of proof-loaded Norway spruce

Strength grading of structural timber is a prerequisite for the use of timber in load-bearing constructions. The grading methods used in production today are mainly based on estimation of the modulus of elasticity (MOE) determined from measurements of either flatwise bending stiffness or resonant vibrations. By using proof-loading techniques, the yield can be increased and the reliability of the graded material can be improved. Unfortunately, this technique can cause damage to some of the graded material. This paper deals with the quantification and severity of damage caused by proof-loading of Norway spruce (Picea abies) timber. A total of 410 specimens were loaded in cyclic bending with increasing load level in each cycle until failure. The maximum loads during the tests were compared to the failure loads of a reference sample. If the failure load was lower than the maximum load, the specimen was assumed to have been damaged in the previous load-cycle and was defined as a rogue. The technique used is similar to the technique used for example in Australia. The results obtained fit well with the results obtained from Australian species. Examination of the MOE indicates no damage in the specimens at stress levels below 75% of the strength. Damage is in this case defined as a decrease in the MOE. Regarding Norway spruce grown in Sweden it can be concluded that the problem of damage due to proof-loading is small for timber graded to strength classes C24 or C30. For both classes the number of rogues can be expected to be less than 5%. For timber graded to higher strength classes the problem of damage due to proof-loading will increase.