Abstract
In this paper, the results of load-bearing fire test are introduced that targets a composite flooring system composed of a flat RC slab and an unprotected beam. When the unprotected steel beam applied as a secondary beam is exposed to fire, it loses strength at an early stage of fire. On the other hand, for flat RC slabs supported in 2 ways, when deflection increases, membrane stress occurs within the cross section of the slab, and as a result the load bearing capacity of the slab is improved due to this membrane action. In RC slabs in which two-tier reinforcements are arranged, when subjected to fire heating, the temperature at the upper-side reinforcement becomes lower than that at the lower-side reinforcement, and thus the tensile strength at the upper-side reinforcement is kept to a high level for a long time. As a result, it is forecast that the retention of strength by the slab due to the membrane action during fire will become more conspicuous. In order to understand these behaviors, a load-bearing fire test was carried out by means of standard fire heating in accordance with ISO 834, targeting a composite flooring system composed of a two-tier reinforcement-arranged flat RC slab and an unprotected steel beam. The following will summarize the results of the tests.
• The RC slab to which the bending collapse load based on the yield-line theory was given sustained load-bearing capacity even after being subjected to 3.5-hours of standard fire heating while involving deflection surpassing 1/20 that of the short-side span.
• In the RC slab in which two-tier reinforcements were arranged, even when the temperature of the lower-side reinforcement reached 700 °C, the temperature of the upper-side reinforcement was about 400 °C, thereby allowing for the slab to retain its load-bearing capacity. The reason for this successful retention was considered to be that the tension resistance of the upper-side reinforcement worked effectively due to the membrane action of the RC slab.
• In the composite flooring system used in this test, the effect of the application of an unprotected steel beam on fire resistance was small.
• The effect of the headed studs arranged in the four peripheral sections of the RC slab on the deflection behavior of the RC slab during fire was small. Due to the small effect, even when the rotational and horizontal displacement at the end of the slab was not restricted, the membrane action of the RC slab worked effectively, and it was thus shown that the load-bearing capacity of the composite flooring system was retained during fire.
• It was shown from these test results that the application of an unprotected secondary beam in the composite flooring system is made possible by means of the membrane action of the RC slab.
