Sustainable Steel-Timber Hybrid Structures in Fire

Steel-timber hybrid structures are emerging as low-carbon alternatives to conventional steel-concrete construction, typically consisting of steel framing supporting cross-laminated timber (CLT) floor systems. However, their structural behaviour in fire remains poorly understood, particularly regarding lateral-torsional buckling (LTB) and composite action. Current fire design practice typically neglects composite action, while the lateral restraint provided by mechanical connectors in steel-timber assemblies remains insufficiently quantified, creating uncertainty in predicting structural stability and fire performance.

To address these gaps, this thesis investigates the structural fire performance of representative steel-CLT downstand configurations using Eurocode-based analytical formulations and advanced finite element simulations. LTB and composite action are examined independently under different boundary conditions to isolate their respective mechanics. The analyses account for variations in heating scenarios, material degradation, connector performance, and restrained thermal expansion.

The results demonstrate that partial lateral restraint enhances stability compared with unrestrained beams, whereas full lateral restraint is not representative of the behaviour of steel-timber hybrid members connected with self‑tapping screws in fire. Axial restraint induces compressive forces that accelerate the onset of LTB, highlighting limitations in conventional design assumptions derived from steel-concrete composite systems.

Conversely, composite action increases failure temperatures compared with bare steel beams; however, composite systems may exhibit greater early-stage deflections due to restrained thermal expansion of the steel beam against the cooler CLT slab. As heating progresses, degradation of the bottom CLT layer and the steel-timber interface reduces longitudinal shear transfer, potentially leading to a progressive loss of composite action and a transition towards non-composite behaviour.

Overall, the findings demonstrate that current design approaches do not adequately capture the complex thermo‑mechanical response of steel-timber hybrid systems in fire. This study advances understanding of LTB and composite behaviour at elevated temperatures, providing a foundation for more robust performance-based fire design and future experimental investigation.