The influence of the grading method on the finger joint bending strength of beech

During the last meeting in Karlsruhe the authors presented a paper [1] providing background information about the determination of the characteristic bending strength of beech glulam. A design proposal was derived to calculate the characteristic bending strength depending on the characteristic tensile strength of the lamellae and the characteristic finger joint bending strength. It was experimentally and numerically proved that visual strength grading of beech provide for strength class GL36 and mechanical grading for GL48. The current paper gives now more detailed information about the influence of the strength grading method on the characteristic finger joint bending strength with regard to beech glulam requirements. Therefore 108 bending tests on finger joints manufactured from visually graded beech boards were performed. A further 319 tests on finger joints manufactured from mechanically graded beech boards were carried out. All the bending tests were conducted flatways according to EN 408 with a span of 15 times the height. The test results confirm a characteristic finger joint bending strength of 56 N/mm2 in case of visual and 70 N/mm2 in case of mechanical grading.