Fire design rules to predict the moment capacities of thin-walled floor joists subject to non-uniform temperature distributions

• Investigated the behaviour of LSF floor joists subject to non-uniform temperature distributions in fire.
• Fire test and numerical analysis results of three LSF floor configurations were used.
• Developed simple fire design rules to predict the moment capacities of lipped channel floor joists and fire ratings.
• Neutral axis shifts due to effective section and elastic modulus variation were considered.
• Load ratio versus time curves for varying LSF floor configurations were predicted.

Thin-walled floor joists in LSF floor panels are protected by gypsum plasterboards and are subjected to a non-uniform temperature distribution across their cross-section in fires. Fire tests have shown that the failure of floor panels was governed by the section moment capacity of joists governed by local buckling effects. This research was aimed at developing simple design rules to predict the failure times (fire resistance) of LSF floors made of lipped channel section joists exposed to standard fires. The use of ambient temperature section modulus led to over-estimation of the moment capacity as the mechanical properties of steel varied across the joist cross-section while deteriorating with increasing joist temperatures in fires. New simplified fire design rules were proposed to determine the section moment capacities of joists at elevated temperatures based on the current cold-formed steel design specifications. Their accuracy was verified using finite element analysis and fire test results.