Embedment and withdrawal stiffness predictions of self-tapping screws in timber

Abstract

• Empirical equations for embedment and withdrawal stiffness of self-tapping screw in timber. • Non-linear regression analysis on embedment and withdrawal test data. • Embedment stiffness of screw increases with increasing insertion angle; decreases with increasing diameter. • Withdrawal stiffness of screw increases with increasing penetration length; decreases with increasing insertion angle and diameter. In the construction of modern multi-storey mass timber structures, self-tapping screws are the preferred fasteners. In order the maximize the strength and stiffness of self-tapping screw connections, the screws are often inserted into timber members at an angle other than 90 degrees to the face of the member. With this fabrication preference, both withdrawal and embedment stiffness relative to the grain angle of wood are important fastener properties that influence the performance of self-tapping screw connections. In certain timber systems that incorporate self-tapping screws, such as composite floor systems, their designs are often dictated by serviceability performance, which is dependent on connection stiffness. Models to predict the stiffness of self-tapping screw connection with inclined fasteners exists. These models require inputs of embedment stiffness and withdrawal stiffness when the screws load wood at different grain angles. To facilitate the application of these connection stiffness models, in this paper empirical equations are presented to directly predict the embedment and withdrawal stiffness of self-tapping screws. The models were derived by analyzing embedment and withdrawal test data obtained from various testing programs using non-linear regression analysis. The use of the proposed empirical equations will facilitate the use of connection stiffness models in design and reduce the need to conduct connection tests for a large number of combinations of fastener size, wood density and insertion angle.