Performance of Ultra Shallow Floor Beams (USFB) exposed to standard and natural fires

This paper investigates computationally the fire performance of a plug steel-concrete composite flooring system, the partially encased ultra-shallow floor beams (USFB). The investigation of the behaviour of USFBs exposed to standard and natural fires is crucial in determining their fire resistance and evaluating their overall performance in contemporary construction. Although the product providers usually indicate the fire resistance of USFBs based on ΕΝ1994-1-2 procedures, the response to elevated temperature effects remains yet neither well documented nor clearly understood. This analysis involves two different beams of 5 m and 8 m span. Results show that the unprotected beams experience severe temperature gradients while exposed to standard fire, as the lower flange still remains unprotected in contrast to the upper steel parts of the cross-section which are encased in concrete. Their fire resistance rating is found approximately at 40 min. Moreover, different thermal gradients are developed when the USFBs are exposed to natural fires (slow and fast burning). When the lower flange is protected with intumescent coatings, the USFBs have shown increased fire resistance and they can survive a full duration of a natural fire under realistic utilization ratios. From the parametric analyses, the optimized thicknesses for the required intumescent coating were obtained to achieve 60, 90, and 120 min of fire resistance and for surviving of natural fires exposures.