Influence of cavity fires on cold formed steel modular construction systems

Abstract

Fire safety challenges associated with continuous intermodular cavities have raised concerns about fire/heat and smoke spread in modular buildings. However, their interaction with steel modular buildings remains unexplored, contributing to uncertainty in fire safety design. Therefore, this research investigates the influence of cavity fire on fire safety and structural response of cold-formed steel (CFS) columns in modular buildings through experimental testing and validated numerical simulations. Results highlighted that even narrow, non-combustible cavity widths of 25–50 mm compromise upper-level safety, enabling vertical flame to spread up to 9 times and smoke spread up to twice that of non-cavity fires. Structural analysis demonstrated that such cavity fire exposure causes complex column behaviour, including restrained thermal expansion, thermal bowing, and buckling, contributing to reducing the axial load capacity by 2.4% to 35%. The observed failure modalities included global-local buckling interaction and local buckling. The quantified axial load increment due to restrained thermal expansion further revealed the requirement to consider additional service loads of 52% at intermodular connections and the column. Cavity widths of 25–50 mm were identified as the most vulnerable for fire resistance, with unprotected columns often failing to meet the 60 – 120 minute fire resistance requirements. Fire resistance improvement strategies were evaluated in terms of both performance and spatial efficiency. For cavity fire intensities up to 30 kW, reducing cavity width was found to be effective, while intensities between 30 – 60 kW necessitate plasterboard protection. In conclusion, this research demonstrates the critical impact of narrow intermodular cavities on the fire safety of steel modular buildings. The presented insights on critical cavity width, fire resistance levels and critical temperature levels contribute to advancing fire safety standards and practices for steel modular buildings.