Behavior of Composite Slim Floor Structures in Fire

Abstract

In recent years, increasing interest has been shown throughout Europe in developing and designing slim floor systems in steel-framed buildings. This paper presents the fire resistance behavior of the composite asymmetric slim floor beam as an isolated member and as a part of the frame using numerical analysis methods. Three schemes were investigated, including isolated beams, a plane subframe with semirigid beam-to-column connections, and a 3D slim floor frame system. The first scheme aimed to explore the fire resistance of the beams according to standard fire-testing methodology. The objective of the second scheme was to reveal the effect of frame continuity on the fire resistance of the slim floor beam and the mechanical interaction between the frame elements. The third scheme was to preliminarily identify the influence of the composite slab on the beam behavior in fire. The investigations show that the isolated slim floor beam has a 60-min standard fire resistance without any additional fire protection, if the load ratio is <0.5. As a part of the frame, the beam still keeps its stability even when the temperature of the bottom steel flange of the beam reaches up to 900°C (90-min ISO fire exposure). The analyzed results indicate that the axial restraints provided by the surrounding parts cause a larger deformation of the beam in the earlier ISO heating phase and a more stable behavior thereafter. The rotational restraints essentially cause the change in the applied load ratio in fire, which can be quantified using the “modified load ratio” proposed in this paper.