Stiffness prediction of Mass Timber Panel-Concrete (MTPC) composite connection with inclined screws and a gap

Abstract

An increasingly popular wood composite floor system consists of a Mass Timber Panel (MTP) connected to a concrete slab or topping with mechanical connectors such as Self-Tapping Screw (STS) with a sound insulation layer in between the MTP and concrete. Allowable floor span for this type of MTP-concrete composite system is often governed by serviceability performance requirements, such as deflection and vibration, which are directly dependent on the stiffness of the interlayer connection. Often tests are performed to characterize connection stiffness required for structural design. In lieu of testing, analytical models can be developed to calculate connection stiffness based on component properties. To that end, two analytical models were developed for solid and layered timber, for directly predicting the stiffness of a connection with inclined screws and an insulation layer. Usually, stiffness of laterally loaded connection is controlled by the dowel bearing effect of the fastener in timber, but inclined screw connection has a more complex behaviour due to the combined bearing and withdrawal action of the screw. In the developed models, both the bearing and withdrawal actions of the screw are considered along with the bending stiffness of the screw by applying a theoretically derived correction factor for the embedment stiffness modulus based on the beam on elastic foundation and friction between the concrete and MTP. Both models were experimentally validated with a wide range of material properties. It was found that the models are capable of predicting the stiffness of the MTP-concrete connection to within 18% of the experimental value. It was also noted that the model for solid timber panel is simpler in form and can be adopted for layered timber panel such as CLT with a small difference in solutions. Friction was found to be notable when there was no insulation gap in between timber and concrete.