Behaviour of stub columns utilising mild-steel plates and VHS tubes under fire

This paper explores the mechanical behaviour of innovative fabricated stub columns, during ambient and elevated temperature conditions, under axial loads. The fabricated columns utilise very high strength (VHS) steel tubes welded to the corners of square and triangular conventional mild steel columns, of a 500 mm length. The nominal dimensions of the very high strength steel tubes used were 31.8 mm and 38.1 mm diameters with 2.3 mm and 1.6 mm thicknesses respectively, whilst the mild steel plates had widths of 120 mm and 400 mm and a 3 mm thickness. Individual mild steel and VHS tubes were also tested under elevated temperatures. Under ambient temperature, the testing consisted of applying an axial force to the columns to determine the ultimate capacities of the fabricated sections. The experimental tests at elevated temperatures consisted of loading the specimens to 70% of the corresponding ultimate capacity, and increasing the furnace temperature in accordance with the ISO 834 curve, until failure was achieved. When the applied axial load is set to be the same at elevated temperatures, the fabricated VHS steel columns significantly outperformed the conventional mild steel columns. However, when degree of utilisation is set to be the same, the VHS specimens demonstrated a small reduction in strength compared to the corresponding mild steel columns. Moreover, numerical analysis was performed to verify the experimental results whilst a good agreement was achieved between the experimental results with those obtained from finite element modelling.