Design Considerations and Structural Performance of Timber–Concrete Composite Floor Systems Subjected to Negative Bending Moments

Abstract

Timber–concrete composite (TCC) floor systems offer an efficient hybrid solution that leverages the high stiffness and strength capacity of concrete with the lightweight, renewable characteristics of mass timber. Although extensive research has been conducted on simply supported TCC members subjected to positive bending, the structural response of continuous TCC floors, particularly under negative bending moments at interior supports and cantilever slabs remains largely unvalidated. Negative bending moment reverses the stress distribution, places the concrete layer in tension, and requires adequate reinforcement in concrete and reliable shear transfer mechanisms among concrete, shear connectors, and timber elements to maintain composite action. Existing analytical approaches, including the gamma method and the elastoplastic (EP) method, have not been fully studied and experimentally verified for such conditions, which limits their applicability for multispan TCC design. This study addresses this critical gap through a comprehensive full-scale experimental program evaluating the stiffness, moment capacity, slip behavior, strain development, ductility, and failure mechanisms of cross-laminated timber (CLT) concrete composite floors subjected to negative bending moment. Results show that the gamma method overestimates effective stiffness by approximately 8%–14%, leading to unconservative deflection predictions. However, the EP method, with the modifications and refinements proposed in this study, accurately predicts the ultimate strength and observed failure progression. Experimental results demonstrate that the ultimate capacity of TCC floor systems under negative bending is governed by compression-controlled failure of the outermost CLT laminations, rather than tensile rupture, which contrasts with conventional assumptions used in current design methods. The effective bending stiffness of TCC floors subjected to negative bending is approximately 2.4 times that of CLT-only floors, while the ultimate moment capacity is nearly three times greater. Moreover, the TCC floor systems demonstrated substantial postyield load-carrying capacity and a stable postpeak response, exhibiting ductile behavior and reliable internal force redistribution, which confirms the structural integrity and reliability of TCC systems subjected to negative bending at the supports of cantilever and multispan floor systems. These findings provide a basis for refining current design methods for TCC floor systems and support broader adoption of TCC floors in multispan applications, advancing their use in hybrid timber–concrete structural systems, and contributing to the development of US design guide for TCC floor systems.