Experimental and numerical study on fire-induced collapse of unprotected steel framed structures

To investigate the fire -induced collapse, a fire test on a two -storey unprotected steel framed structure was performed. This test lasted until the complete collapse, providing valuable insights into the entire collapse process. Detailed temperature distribution and structural responses were measured and presented. The experimental results revealed that the frame collapsed after 82 -min heating with an average component temperature around 900 degrees C. This duration was 215.4% longer than the fire resistance of single frame column. The eventual collapse was due to damaged slab -beam connections and extensive rotation in beam -column connections. Notable observations included a sink collapse mode, pronounced membrane action in slabs, and extensive horizontal displacement. Column buckling occurred at approximately 40 min, yet it did not break the structural integrity. Based on the experimental results, a 3D numerical method employing explicit dynamic analysis was proposed and validated, with the displacement deviations within 20 mm. Considering various fire scenarios, the numerical analysis identifies three potential collapse modes for steel framed structures: the sink collapse mode from edge bay fire and central bay fire, along with flip collapse mode from edge bay fire. The sink collapse mode is more likely to occur compared to the flip collapse mode, which necessitates more specific conditions. When extrapolated to multi-storey steel framed structures, the collapse mechanism mirrors that of the tested frame, including three distinct stages: thermal expansion, column buckling, and collapse stage. The buckling of columns and the formation of plastic hinges are the critical indicators for the column buckling and collapse stages, respectively.