Minimum Geometric and Execution Requirements for Axially Loaded Groups of Screws in Hardwood

Societal demand on sustainable buildings motivate timber as construction material. Actually, deciduous tree species (hardwoods) play a minor role despite their considerably increasing harvest stock in Europe. Rules for design and execution of joints established for softwood must be adapted or are need for hardwood products. With focus on mainly axially-loaded screws, applications in hardwood usually coincide with distinctively higher insertion moments but also higher capacities and stiffnesses. Joint type, dimensions of the members, the group arrangement parameters and angels are key characteristics which determine the stress distribution and consequently the resistance and failure behaviour of the joint. To enable a comprehensive and structured analysis it is proposed to differentiate between (i) failure modes sufficiently determined by single fastener properties, e.g. withdrawal of steel tension failure, and (ii) joint failure modes describable only in conjunction with timber properties, e.g. block, plug or row shear. The experimental investigations done, cover the possible range of joint types, from end-grain joints to hangers loaded perpendicular-to-grain with, as well as joints with inclined screws. A check of current regulations according to Eurocode 5 and to diverse ETAs is subject of this contribution. The results with glulam of beech (Fagus s.) and birch (Betula pendula) show for the former, that they are sufficient to reach failure modes (i). For requirements close to that of diverse ETAs all failure are observed. Results of LVL of beech indicate the need for larger penetration length and wider geometric requirements. The recommendations for minimum geometric and executions requirements for axially loaded groups of screws are seen as worthwhile contribution for regulating such joints in hardwood.