Finite element analysis on the behaviour of multi-storey steel frame with concrete slabs against progressive collapse under fire conditions.
Nene, Ntethelelo Syabonga.
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This research was conducted to study the behaviour of a multi-storey steel frame with concrete slabs against progressive collapse under fire conditions. An implicit coupled thermomechanical analysis was executed using a commercial finite element modelling, ANSYS while considering the computational effort and the simulation time required to solve the models. A three-dimensional steel frame model was modelled using line bodies while concrete slabs were modelled with shell elements. The research studies the collapse modes and load redistribution action of the steel frame exposed to elevated temperatures. Two fire scenarios were investigated namely, (i) heating individual ground floor columns at various location i.e., corner, short edge, long edge and interior column (ii) four columns being heated at the same time within the same compartment (also referred as compartment fires). The findings illustrate that when individual columns are heated, the steel frame does not show any signs of progressive collapse; however, the premature local buckling was observed on the heated columns. This was due to material softening or an increase in compression on the heated column. The interior column was more likely to cause progressive collapse. This was due to column load-ratio i.e., the interior columns carry twice the loads in comparison with the perimeter columns. For compartments, no progressive collapse was observed for all the scenarios; however, premature local failure was observed for all the heated columns and for some non-heated columns. For corner and interior compartment fire, non-heated perimeter columns showed premature local failure due to the lateral movement caused by the sagging of slabs above the upper floor of the ground floor column under tension forces. The global progressive collapse was restrained by the load redistribution action which transferred the loads which were previously carried by the buckled column to the surrounding structural elements. In addition, more loads were transferred along the short span because the stronger axis (x-axis) was orientated along the short span. Hence, loads are more likely to be transferred to the axis with the greater stiffness (stronger axis). In conclusion, the collapse modes were influenced by the fire location and the load redistribution action.