Forward and Reverse shear transfer in beech LVL-concrete composites with singly inclined coach screw connectors

Double-shear tests are reported on beech LVL-concrete composite connections based on coach screw connectors inclined at either 45° or 90°. On different specimens with the same screw orientation the longitudinal shear force was applied either in forward or reverse, because in practice concrete shrinkage, moisture-induced timber expansion and oscillatory (e.g. seismic) or moving loads can induce reversal of the force on the connection. The test data show that the connection shear strength increased over three-fold between the 45o compression screw and 90o screw connections, and again increased but by only a further 14% in proceeding to the 45o tension screw connection. By contrast, the slip moduli of the 45° compression screw and 90o screw connections were similar to each other, but were only 20% that of the 45° tension screw connection. The 45° compression screw and 45° tension screw connections were by far the most and least ductile respectively. The four specimens tested within each group showed good consistency of shear strength, but high variability of slip modulus. Within each such group failure occurred consistently by fracture of the screws slightly into the timber, except for the 45o tension screw connections which failed in three distinct modes (despite the consistent failure load). Given these results, it is strongly recommended that both forward and reverse shear testing becomes a protocol for singly inclined coach screw-based timber-concrete connections.