MECHANICAL MODEL FOR WITHDRAWAL FAILURE OF SELF-TAPPING SCREWS IN GLULAM

With the development of mechanical fasteners in timber structures, self-tapping screws or threaded rods with sufficient length and optimized thread provide a new technical solution for realizing the connections with high mechanical performance. Considering that the withdrawal property of self-tapping screws in wood has an important influence on the mechanical performance of connections with self-tapping screws as fasteners, the withdrawal failure of self-tapping screws in wood should be avoided to take the full advantage of the fasteners in axial load transfer. Different from existing calculation methods based on the classic Volkersen theory, a type of “assembly unit”, which contains threads is first introduced in a new model, is utilized to research the withdrawal failure mechanism of self-tapping screws in glulam and to predict the anchorage length of self-tapping screws in glulam. The model redefines the withdrawal failure surface according to the typical withdrawal failure phenomenon, considers the effect of the thread on the wood failure region, and reflects the local stress of the wood and the discontinuous transfer of shear stress/shear force on the withdrawal failure surface in the aspects of model simplification, of formula derivation and, of algorithm design. An acceptable agreement is achieved between theoretical and experimental results, and a nearly linear relationship is predicted by the model, between withdrawal failure load and embedment length before self-tapping screw or threaded rod fracture.