Structural behaviour of timber concrete composite connections and floors utilising screw connectors

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The traditional materials such as reinforced concrete and structural steel have been widely used in the construction market. These construction materials produce a large quantity of greenhouse gases as a by-product. An environmentally sustainable solution to decrease the production of greenhouse gases is creation of composites with other materials such as timber to reduce the amount of steel and concrete used in construction. Timber-concrete composites (TCC) structures, extends upon this by combining timber and concrete in order to form a composite structural member that utilises the properties of both materials. Since the 1990s, Timber Concrete Composite (TCC) floors have been gaining wider recognition as being a viable and effective alternative to both reinforced concrete and traditional timber floors. TCCs are a structural form whereby a concrete slab is fixed to a timber joist at the interface using a suitable shear connector which transfers shear forces and impedes slip between concrete and timber. Hence, the strength, stiffness, location and number of shear connectors used at the composite interface are the key factors in determining the composite action, the strength and stiffness of a TCC system. TCC exploits the mechanical properties of each material favourably with the concrete in compression and the timber in tension. TCCs have several advantages over full timber construction, including improved strength (double), stiffness (triple), vibration control, fire performance and thermal and sound insulation. TCCs also have advantages compared to full concrete construction, including a much higher load capacity per unit of self-weight and a lower embodied energy. Mechanical fasteners for example screw and dowel TCC connectors are relatively simple and easy to install, cost effective and structurally efficient connectors with lower labour requirement. With these considerations, mechanical fasteners can be preassembled in prefabrication or cast in situ TCC solutions. Hence, application of mechanical fasteners in TCCs overcomes the drawbacks of alternative connection such as notch type connection and reduces the time required to construct a TCC system. This research investigates the experimental parametric study on the effect of different types of high-performance concrete on mechanical properties of TCC connections and floors using locally available materials in Australia to evaluate their potential for use in the construction market. A parametric study of mechanical fasteners such as crossed SFS VB, crossed SPAX and coach screws connections in different lengths (short and long SFS VB), angles (±30°, ±45° and ±60°) to the connection face and a number of crossed SFS VB and SPAX at 45° series utilising 17mm plywood formwork interlayer and different types of concrete was carried out. Hence, the effects of connector type, inclination angle and length of screw and existence of plywood interlayer on mechanical properties of the TCC connections were investigated. Moreover, two innovative TCC shear connection systems were put forward and assessed for their suitability as a substitute or replacement for existing connection systems using push-out test. The application of high-performance concrete such as light-weight concrete and self-consolidating concrete provides a great deal of benefits in TCC technology to minimize the dead-load on the timber component or increase the concrete workability and accelerate the process of pouring. Such weight reduction and increased workability may be favourable in the renovation of old timber floors. The use of TCC technology is also advantageous in new multi-storey buildings for aspects such as prefabrication and mitigation of excess dead load – leading to saving on foundation and walls and/or column sizes. This research investigates the effect of different types of high-performance concrete on the mechanical properties of TCC connections and floors using locally available materials in Australia to evaluate their potential for use in construction market. Push-out test was used to determine the mechanical properties and failure modes of shear connections and once the mechanical properties of connection type were identified, full-scale TCC modules utilising different types of shear connector and concrete properties were subjected to four-point bending tests. Hence, the predictions of full beam behaviour using the connection properties were validated and the effect of shear connection and concrete type on structural behaviour of an entire floor was investigated. Literature reports a significant lack of information on analytical closed-form equations to predict the strength and stiffness of TCC connections utilising vertical and inclined fasteners to be used in the design of timber composite beams. This study reviewed the methodology of available analytical models for prediction of the strength and serviceability stiffness of vertically inserted single timber to timber and TCC shear connections and validated their accuracy using the experimental push-out test results. Moreover, an analytical strength model based on some adjustment to EYM to predict the strength of TCC connections utilising single and crossed screws inclined to the timber grain was proposed. This research also presented a model for the stiffness of TCC connections using crossed inclined screws. The Winkler theory of beam on elastic foundation proposed was extended to derive the serviceability slip modulus of TCC connections with inclined screws which were loaded in tension and compression. In addition, a 3-D FE model has been put forward to simulate different TCC connections such as single and multiple wood screws and inclined coach screw utilising the commercial FE analysis software ANSYS. The findings of this part demonstrate that by using a simple numerical model, the behaviour of TCC connections can be accurately modelled and can therefore be used for parametric study of changes in end distance, edge distance, member thickness, screw diameter, screw length and number of screws.
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