Numerical modelling of externally insulated LSF floor–ceiling systems under fire conditions

Light Gauge Steel Framed (LSF) floor–ceiling systems composed of cold-formed steel joists are increasingly used due to their high strength-to-weight ratio and cost efficiency. However, research into their fire performance beyond standard plasterboard ceiling configurations remains limited. Conducting fire tests for every floor configuration is expensive, time-consuming, and practically restrictive, making validated numerical modelling a beneficial alternative. This study aims to develop suitable numerical models using Ansys Fluent and Abaqus to accurately simulate the thermal and structural behaviour of externally insulated LSF floor–ceiling systems under fire conditions, based on the authors' previous experimental studies involving three short-span floor fire tests. Although Abaqus is widely used for heat-transfer analyses of LSF systems, its standard thermal modelling framework does not explicitly represent buoyancy-driven natural convection within enclosed cavities, which can influence joist cold flange temperatures. To address this limitation, Ansys Fluent was utilised, incorporating cavity natural convection and surface-to-surface radiation, to accurately predict the temperatures of sheathing and joists. The thermal predictions obtained from Ansys Fluent were subsequently applied in sequentially coupled structural modelling using Abaqus, effectively replicating the experimental failure modes and failure times observed in the fire tests. Finally, a parametric study of full-scale joist floors was performed for six commonly used industry joist types to quantify the effects of joist depth (100–300 mm), joist thickness (1–3 mm) and load ratio (0.2–0.7), and to develop simplified predictive relationships for failure time and average joist failure temperature suitable for design-oriented assessment including fire resistance level tables.