Numerical Study of the Fire Behavior of Encased and Slim-Floor Composite Beams

This paper reports comparative studies of the fire behavior of partially encased composite beams (PECBs), slim-floor composite beams (SFCBs), and fully encased composite beams (FECBs) using validated numerical models. The effects of parameters such as load ratio, concrete slab width, concrete and steel strength, and section dimension of the steel beam were considered. The findings indicate that the mechanical behavior of PECBs, SFCBs, and FECBs undergoes four distinct phases when exposed to elevated temperature, namely an elastic phase, an elastic-plastic phase, a plastic yielding phase, and a plastic instability phase. The failure criterion proposed by ISO 834 is more appropriate for predicting the failure of PECBs, SFCBs, and FECBs than those proposed by BS 476-20 and ASTM E119-18. The failures of SFCBs and FECBs under fire were both demonstrated to be the crushing of concrete and yielding of steel beam, whereas when PECBs fail, the steel beam yields, but whether the concrete of PECBs is crushed is determined by the load ratio. At elevated temperatures, the web of PECBs and SFCBs yields earlier than the lower flange. Additionally, the lower flange of PECBs and SFCBs has the potential to convert from a state of tension to compression. Increasing the web height and thickness or decreasing the load ratio can prolong the duration of plastic yielding phases and plastic instability phases, thus improving the fire resistance of PECBs, SFCBs, and FECBs. In addition, the fire resistance of PECBs, SFCBs, and FECBs are barely affected by the concrete slab width, steel strength, and shear connection degree. Although PECBs and SFCBs both have good fire resistance, additional measures such as welding the reinforcement to the web or applying a fire protection layer to the lower flange are required to meet 2-h fire resistance at a load ratio of 0.4. Moreover, FECBs can achieve 2-h fire resistance even if the load ratio is up to 0.8.