Strength grading of construction timber with large cross sections

A non-destructive method of grading construction timber beams with cross sections between 10x12 cm2 and 12x24 cm2 and lengths from 2.3 m to 7.5 m was developed. By means of beam geometry, the propagation time of an ultrasound impulse along the beam, longitudinal vibrations and the beam's mass a prediction of its bending strength MOR and its modulus of elasticity MOE was calculated. The relationships were derived by multivariate regression analysis of data sets of a sample of 450 spruce beams. Since the minimum beam length for bending tests according to EN 408 is given by 19 h, all beams were shortened. During these trials, a dependency of impulse velocity of the beam's length was observed. Limiting values for strength classes were determined by comparing the predicted MOR and MOE to the measured values.