Timber-concrete composite (TCC) floors utilize the best mechanical properties of timber and concrete. It also combines the sustainability, high strength-to-weight ratio of timber with the strength and durability of concrete, resulting in an efficient and eco-friendly solution for long-span structures. While most research has focused on simply supported TCC floor systems under positive bending, structural engineers increasingly favor continuous spans for their ability to distribute the loads, higher overall stiffness compared to simply-supported slab, reduced deflections, and higher redundancy in the system that increases the margin of collapse for gravity load resisting systems. However, continuous spans require the development of negative bending moments at interior supports (i.e., above girders or column supports), reversing the stress distribution and placing concrete in tension and timber in compression. In order to develop resistance to negative bending moments at the supports, the concrete should be reinforced by rebars, and the tensile force in the rebars should be transferred to the cross-laminated timber (CLT) section by means of shear connections that provides both stiffness and strength for the composite action between the concrete and CLT sections. Currently, there is no validation of the current design models (γ-method and Elastoplastic method) to investigate the behavior of TCC floor systems under negative bending moment. This study investigates and validates the behavior of TCC floors under negative bending moment through a full-scale experimental program. It examines the composite action between CLT and concrete, performance of shear connectors, failure modes, strength and ductility capacity, and overall flexural stiffness. The findings of this study revealed that the commonly used Gamma method overestimates the initial stiffness of TCC systems under negative bending, leading to potentially unconservative serviceability predictions. In contrast, the nonlinear Elastoplastic method showed strong agreement with experimental results in predicting both ultimate capacity and failure progression, making it a more reliable analytical tool for design of TCC floor systems. Experimental testing also demonstrated significant ductility and structural integrity, with the floor systems exhibiting load-carrying capacity even after reaching the peak capacity, which helps re-distribution of loads to other members and increase the margin of collapse for gravity load resisting systems with TCC floor slabs. These outcomes highlight the robustness of TCC systems in negative bending regions and support their safe application in continuous-span configurations. The integrated experimental and analytical results advance modeling accuracy, inform design methodologies, and support future code development for TCC floors, facilitating their adoption in hybrid structural systems such as mid- to high-rise buildings.
Monther Mohammad Nayfeh (Wed,) studied this question.