Experimental, numerical and analytical analysis of a single-span steel portal frame exposed to fire

The thermo-mechanical behaviour of single-span steel portal frames in fire conditions illustrates a sequential transition from an ambient temperature state through to eventual collapse, encompassing axial forces, moments and deformations. To investigate the thermo-mechanical behaviour, a fire test on an 8 m x 6 m steel portal frame assembly is conducted. Detailed thermal and structural responses of the frame are measured and presented. The experimental results reveal that the frame collapses post 31-min heating with a peak temperature approximating 800 degrees C. A typical inward collapse mode is observed. 2D and 3D finite element models are established and validated against experimental results, respectively. The numerical model is divided into three heating regions to mimic the behaviour exhibited during the fire scenario. Furthermore, a thermo-mechanical theory is proposed to analyse the fire behaviour of a single-span steel portal frame. Based on the fire response of a restrained steel beam, the thermo-mechanical behaviour can be segmented into four stages: the ambient temperature stage, thermal expansion stage, elasto-plastic stage and catenary stage. A comprehensive series of calculation formulas determine the displacement, internal forces and moments at different stages. The analytical computations exhibit a close consistence with the numerical results, within the deviation of 10% for the majority. This analytical method can be effortlessly employed for both the fire-resistant design and assessment of the current state of steel portal frames in fire conditions.