Mass Timber Panels (MTP) are a new generation of engineered wood panels that are available in large plane dimensions to facilitate fast floor construction with the obvious environmental benefit of being from a renewable material. In floor construction, concrete slab or topping is often applied over the MTP panels to improve various performance attributes, including structural, acoustic and vibration serviceability. Mass Timber Panel-Concrete (MTPC) composite floor system often consists of a Mass Timber Panel (MTP) connected to the concrete layer with mechanical connectors such as Self-Tapping Screw (STS) and a sound insulation layer in between the MTP and concrete. Lack of design standards and guidelines are the most important barrier limiting wide spread use of this MTPC composite floor system.
The capacity of this type of composite system mostly depends on the strength of the interlayer connection. Also, the allowable floor span is often governed by serviceability performance requirements, such as deflection and vibration, which are directly dependent on the stiffness of the interlayer connection. Usually, connection tests are performed to characterize connection strength and stiffness required for structural design. In this research, three types of MTPs with normal weight concrete, three insulation thicknesses, two screw embedment lengths and two screw angles were tested to characterize connection strength and stiffness. Test results showed that connections with screws at an insertion angle of 30-degree had a larger strength and stiffness than connections with screws inserted at a 45-degree angle. Stiffness appears to be more sensitive to the presence of an insulation layer compared to strength. Overall, 5-15% and 22-34% reduction of strength and 35-50% and 55-65% reduction of serviceability stiffness were noticed for an insulation thickness of 5 mm and 15 mm, respectively. In lieu of testing, analytical models can be developed to directly calculate connection strength and stiffness based on component properties. To that end, two analytical models each were developed for solid and layered timber, for directly predicting the stiffness and strength of a connection with inclined screws and an insulation layer. Usually, connection properties 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. Therefore, in the developed models, both the bearing and withdrawal actions of the screw are considered. The connection stiffness and strength model were validated with the connection tests with a wide range of parameters. It was found that the strength models are capable of predicting the mode of failure of a connection and the load-carrying capacity within 10% of the experimental value, while, the stiffness models are capable of predicting the stiffness of connection to within 18% of the experimental value.
The commonly used Gamma method to design a timber-concrete composite floor has limitations and cannot predict the load-carrying capacity, bending stiffness and failure modes of the composite floor system when there are widely spaced discrete connectors. Therefore, an analytical model has been developed considering the interlayer connector behaviour under the elastic-plastic range along with an acoustic layer between timber and concrete, to predict the capacity, bending stiffness, failure modes and the load-deflection response of MTPC composite floor system. One-way acting composite floor panels were also tested under four-point bending with different configurations to investigate the influence of different parameters and to validate the developed system prediction model. It was found that the model is capable of predicting the capacity of the MTPC composite system within the range of -6% to +26%, bending stiffness within the range of -15% to +10% of the bending test values and the associated failure mode. The Gamma method cannot predict the system capacity, and it tended to over-estimate the bending stiffness on average by 43% and was found not appropriate for MTPC composite system with discrete shear connectors and MTP. This developed connection and system models for MTPC composite floors will facilitate the use of such a system in mass timber construction.