Timber-concrete composite (TCC) floors are a structural system where a concrete slab is laid over a mass timber panel or a series of wood beams, attached by means of shear connectors, to contribute to the structural resistance of the floor. The composite action of wood and concrete components is considered in the structural analysis of a TCC floor for the serviceability and ultimate limit states by means of analytical models that determine the distribution of internal forces using the strength and stiffness properties of the components. It is known that the TCC components (concrete, wood, and shear connections) exhibit time-dependent behaviour leading to the increased deflection of the composite floor over time (creep). Therefore, the design of TCC floors is often governed by the long-term deflections. While the short-term properties of the materials are defined in relevant material standards, the long-term properties are not readily available for all components. Furthermore, the causes, rates and magnitudes of the time-dependent behaviour differ between the TCC components. Therefore, evaluation of the long-term behaviour of a TCC floor accounting for the redistribution of internal forces due to rheological effects, such as duration of load (DOL), creep and relaxation, is not a trivial task.
Although it is critical to develop and validate models predicting the long-term resistance and deflection of TCC floors, not many studies have been undertaken to date, because the experimental tests are generally expensive (i.e., material variability requiring multiple samples to quantify effects, or connections that need to be tested full-scale and in groups) and take a long time (i.e., lack of an accelerated test method). Several analytical and numerical methods have been developed to estimate the long-term performance of TCC floors by extrapolating the existing methods for concrete and wood structures. This report presents an overview of the state-of-the-art on the long-term performance of TCC floors with special attention to analytical procedures validated by experimental tests and serves as a background for the design procedure proposed by the authors for adoption in the 2024 edition of CSA O86. A design example is provided at the end of this document.
