Failure and analysis of composite timber beams with box section

This paper presents the results from a study on a particular type of long-spanning composite timber beams with box section, which can potentially used for floor joists. In this study, four-point bend tests of four composite timber beams of 2400×200×105 mm3 were performed on a fixture with inner span of 700 and outer, 2100 mm. The beams consist of two wood flanges connected via two plywood webs, forming a rectangular hollow section. The wood flanges are made of MGP12 with thickness of 90 mm and depth of 45 mm (AS1720.1), and plywood webs, ID Code 7-24-3 with total thickness of three plies, 7.2 mm (AS2269.0-2008). The experiments demonstrated that with very strong rigid bonds joining the flanges and webs, the beams invariably failed from the buckling and then shearing of web. For the beam with weaker flange-web bonds, the beam will prematurely fail because of debonding. These observations are analysed in terms of stress distributions in the beams and the strength behaviours of individual components in the beams.