The Predictive Model for Stiffness of Inclined Screws as Shear Connection in Timber-Concrete Composite Floor

Interest in timber-concrete composite (TCC) floors has increased over the last 30 years. TCC technology relies on timber and concrete members acting compositely together. Both timber and concrete exhibit a brittle behaviour in bending/tension and compression respectively whilst the shear connection is identified as the only contributor of ductile behaviour. Therefore, the strength, stiffness and arrangement of the shear connection play a crucial role in the design parameters of TCC system including deflection and stiffness of floor. Hence, calculation of stiffness is of interest to study the structural performance of TCC floor. Material properties of timber, fastener and concrete influence the overall load-displacement response of shear connection.

There are only few investigations on analytical closed-form equation to predict the stiffness and strength of TCC joints as input values to design a partially composite floor. For example, Eurocode 5 recommends the empirical equations for the slip modulus of dowels and screws which are limited to vertically inserted fasteners only. Eurocode 5 only recommends that the strength and stiffness of unconventional joints should be determined by push-out tests. Previous investigations reported that the inclination of a fastener significantly increase the initial stiffness and ultimate strength of the TCC joints and consequently composite floor.

This paper presents a model for prediction of the stiffness of TCC joint using crossed inclined proprietary screws (SFS Intec). The model assumes the behaviour of inclined screw as a beam on a two-dimensional elastic foundation, and considers the timber as the elastic foundation consisting of orthogonal springs with differing stiffness in the parallel and transverse to the grain directions. The experimental aspect of the research consists of embedding and push-out tests aiming to verify the stiffness model of TCC joints with inclined screws. The model is reasonably accurate in predicting the characteristic stiffness. This research suggests the model to facilitate the design of inclined screw shear connections for TCC construction.