Investigations Concerning the Force Distribution along Axially Loaded Self-tapping Screws

Self-tapping screws, as simple fasteners with a high load carrying potential if stressed axially, are frequently applied in timber engineering as tensile joints in wide span GLT truss systems or as reinforcements against stresses perpendicular to grain. In fact, force distribution along axially loaded screws has a very important influence on the joint behaviour. Some models based on Volkersen’s theory combined with fundamentals of linear elastic fracture mechanics already exist for glued-in rods or lag screws.

This paper provides a measuring technique estimating the force distribution based on the determined elongation of the threaded part over the inserted length by several strain gauges while the composite “timber-screw” is stressed axially. Therefore, 16 withdrawal “push-pull” tests were carried out in solid timber varying the slenderness

λ

, given as the ratio

l

ef

/

d

, from 5 to 20 and with angles of screw axis to grain direction

α

of 0°, 45° and 90°. The results are used to verify existing models of comparable configurations which are further adapted to self-tapping screws.

Beside the fundamental knowledge of the withdrawal behaviour, also structural analysis of tensile joints with self-tapping screws can be improved considering the location of the stress centre and its impact on eccentricities due to the non-linear distributions, and in regard to recommendations concerning load introduction perpendicular to grain.