Minimum insulation concrete upper flange thickness values of beam-and-block floor systems under 60 minutes in standard fire exposure

This document deals with insulation criterion I for beam-and-block floor systems (with non-resisting blocks or semi-resisting blocks) including a distribution cast-in-situ concrete slab over the whole floor surface and forming a continuous compression slab.
Several fire design methods are permitted including tabulated data. Depending on the fire duration required for the floor, the latter provide minimum concrete upper flange thickness values for the section of the structural floor.
In the NF P19-205 standard, the practice of the DTU Feu Béton has been selected at 30 min. Beyond that, the current more penalizing practice of the NF EN 1992-1-2 standard has been selected. At 60 min, the thickness of the compressed upper flange changes from 60 mm to 80 mm. A numerical study has showed that this minimum thickness of the compression slab (80 mm) can be optimized.
With a conservative thermal hypothesis, it is shown that for spaces between beams up to 75 cm, the insulation criterion I60 is satisfied with the following conditions:
• For beam-and-block floors systems with PSE blocks 120 mm, 150 mm or 200 mm high, the 80 mm concrete thickness criterion is justified. In the particular case of blocks where nibs thickness is less than or equal to 30 mm and which allowing the lengthening of the effective contour of the interface between prestressed concrete beams and cast-in-situ concrete, the minimum compression thickness slab can be reduced to 70 mm (for distances between beams up to 70 cm);
• For floor systems with concrete blocks 120 mm, 160 mm or 200 mm high, the 80 mm thickness criterion is changed to 60 mm.
Those thickness values can be refined according to the type of studied assembly (twin beams, distances between beam axis, hypothesis for thermal analysis, …).

Keywords: beam-and-block floor systems, compression slab, fire exposure, flange thickness