Potential of strength grading of timber with combined measurement techniques: Report of the Combigrade-project - phase 1

About 100 pieces of both spruce (Picea abies) logs and pine (Pinus sylvestris) logs were sampled and sawn timber produced. Non-destructive measurements of logs and sawn timber were made by 15 organisations producing 40 different measured quantities for each test piece. Finally, test material was loaded to failure in bending, and grade determining properties (modulus of elasticity, bending strength and density) were measured. Degrees of determination were calculated between non-destructively measured indicators and grade determining properties. Also strength grading procedure determined by prEN 14081 was simulated numerically, and effectiveness of some potential grading methods was analysed. It was concluded that the coefficient of determination between strength and most non-destructive indicators was remarkably higher with pine than spruce. Especially knot size and density are potential grading parameters for pine but not for spruce. This does not, however, indicate that yield to high grades would be in general better with pine than spruce, but it indicates that pine has larger variability of knot sizes and density, and consequently higher variability of strength. Log scanning can also produce strength indicators which are on the level of some existing grading methods. Simulation of European machine grading procedure indicated that there is a random factor which affects the yield to different grades. It can be counteracted by increasing the sample size when determining the settings for grading.