Block shear model for axially-loaded groups of screws

Primary axially loaded groups of screws, applied perpendicular or under an angle to the grain of structural timber elements lead locally to shear, rolling shear and tensile perpendicular-to-the-grain stresses. Under certain conditions, these stresses lead to brittle and undesirable joint failure mechanisms, such as tension perpendicular-to-the-grain, row or block shear. The focus of this contribution is on the block shear failure mechanism, which is characterized by a crack formation along the volume enclosed by the usually cubic group design. Block shear was observed by testing groups of screws in solid timber, glulam (GLT) and cross laminated timber (CLT) made of Norway spruce (Picea abies) at (i) joints with outer steel plates taking off when loaded, (ii) tight group designs, i.e. small spacings parallel (a1) and especially perpendicular-to-the-grain (a2 < 5 d), even complying common regulations in diverse ETAs of wellknown screw producers. The current version of Eurocode 5 demands to consider block shear failure also in the design of axially-loaded groups of screws, however, without providing any design rules. Therefore, model is proposed to describe the block failure mechanisms. Thereby, five potential, lateral failure planes surrounding the group are defined and load sharing between them by a parallel acting system assumed. The elastic material properties, geometric and stress distribution parameters provide corresponding stiffnesses. A complement set for those parameters is presented and additionally, the mechanical model is extended to a probabilistic-mechanical model, which enhances the resistance prediction and allows the formulation of the block shear model on the characteristic level. Limits to other joint failure mechanisms and a comparison with other available model proposal is worked out. The proposed block shear model in its comprehensive version is rather complex for use in practice. With focus on ease-of-use, a simplified version is presented in addition, which is seen as envisaged contribution to the extension of the Eurocode 5 and beyond.