Fatigue behaviour of timber–concrete composite connections and floor beams

Abstract

In recent years, timber–concrete composite systems have become more widely used as a new construction technique for buildings and bridges. The main advantage is that the compressive strength of concrete is exploited through the use of composite action while timber beams are able to resist the tensile stresses. The level of composite action, which can be achieved by the system, is dependent on the type of shear connector used. There is a lack of knowledge, however, on the performance of these types of connections when subjected to cyclic loading, which is typical for bridges. Testing was performed in the Structures Laboratory of the University of Canterbury to analyse the fatigue behaviour of two types of timber–concrete connections via push-out specimens, and two beam specimens representing strips of composite floor with the same connection types. The two types of connection investigated were: (i) a rectangular notch connection reinforced with a coach screw (also known as lag screw); and (ii) a connection with toothed metal plates punched into laminated veneer lumber (LVL). The stiffness of the connection was monitored throughout the cyclic loading along with the total amount of slip occurring between the concrete and timber. After the application of 2million cycles, the push-out and beam specimens were loaded to failure in order to quantify their maximum strength. The strength of the rectangular notched connection after cyclic loading was 0.95 times of the one without cyclic loading, while for the metal plate connection was 0.60 times. For the metal plate connection, a continuous increase in slip was observed with increased cycles possibly due to accumulated damage from repeated loading. The rectangular notch connection displayed more resistance to changes in slip, strength and stiffness than the metal plate connection. No obvious loss of stiffness was observed in the rectangular notch connected floor beams after 2million cycles, and when tested to failure the stiffness was very similar to the same floor beam that had not been cyclically loaded. The floor beam with metal plate connections did not perform well and failed after 350,000cycles. The loss of strength, stiffness and composite action in this floor beam compared to the one without cyclic loading was significant. In this respect, the rectangular notch connection system is recommended for use in bridge design as opposed to metal plate connections.