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This article investigates the fire performance of Light Steel Frame (LSF) walls commonly used in buildings. Six full-scale LSF tests with different layouts are validated through numerical simulations using uncoupled thermal and mechanical analyses. The hybrid numerical model incorporates experimental data to accurately predict the LSF wall temperature, solving the non-linear transient thermal analysis. Three mechanical simulations are developed: elastic buckling analysis for instability mode, Geometric and Material Non-Linear Imperfection Analysis (GMNIA) for load-bearing capacity at room temperature, and thermo-mechanical analysis considering temperature effects under constant load. Model validation compares six experimental tests under room temperature and fire conditions. The Root Mean Square Error is used for each comparison. Results show that the fire resistance (R) of LSF walls decreases with the load level. The impact of the cavity insulation is examined, revealing potential improvements in fire resistance for cavity-insulated LSF hollow stud walls compared to non-insulated ones. Notably, hollow section studs generally exhibit higher fire resistance than corresponding lipped section studs when void cavities are used. The investigation proposes a new approach to determine the fire resistance based on the relationship between the critical temperature of steel studs (Hot flange) and load levels. This relationship allows us to predict the fire resistance time through a preliminary thermal analysis of LSF walls.