A QUANTITATIVE RELATIONSHIP BETWEEN THE CROSS SECTIONAL SHAPE OF COMPOSITE FLOOR AND FIRE RESISTANCE TIME

Abstract

The dependence of the thermal responses of composite floors on the cross sectional shapes is calculated by a model of heat and mass transfer. The model takes into account of the heat and mass transfer, the desorption of physically adsorbed water, and the thermal decomposition of water of crystallization. While the average thickness is kept constant (115mm), the rib sizes are changed. The fire resistance time is determined for each shape based on the ISO 834 criteria for temperature rise of unexposed surface. In case of small ribs, the fire resistance time is determined by the average temperature rise of the unexposed surface. On the contrary, as the rib becomes large, the fire resistance time is determined by the maximum temperature rise. Between these rib sizes, the two criteria are exceeded at the same time. We call these shapes as 'thermally optimum' in the sense that there is no redundancy of concrete. It is shown that the thermally optimum shapes are the imit of rib size in order to assure the insulation function of composite floor. It is also shown that the rib volume (rib height times rib width) is approximately constant in thermally optimum shapes.