Investigating the effect of rigidity of the diaphragm and connections constraints on the overall behavior of steel braced frames under fire load

The resistance of steel structures under accidental loading conditions such as fire is affected by the inherent characteristics of the structure such as redundancy, ductility and joint constraints. On the other hand, due to the action of the structural components exposed to fire, the rigidity of the diaphragm and the performance of the connections are subjected to damage. This paper examines effects of the rigidity of the diaphragm and the shear connections constraints as part of the fire-resistant properties of steel braced frames. Two modes of operation of the floors in the form of rigid diaphragm and soft diaphragm, as well as two modes of operation of shear connections in the form of hinged and rolled joints, have been evaluated under a fire scenario. The effect of gravity and fire loads on a 10-story steel braced frame is simulated in these cases using numerical analysis techniques using finite element method. Analysis show that fire resistance time of braced frames and structural collapse mechanisms are severely affected by the presence or absence of diaphragms as well as tensile constraints on shear joints. As in the case of the existence rigid diaphragm of the structure, it undergoes a total collapse in a short time, and also in the absence of tensile constraints of the joints, the collapse mechanism locally changes position. Analysis results can also help engineers understand local and general responses to steel braced frames under fire loading.