BENDING MOMENT RESISTANCE OF COMPOSITE BEAMS PROTECTED ONLY AT THE BEAM ENDS WITH CONTINUOUS FLOOR SLAB IN FIRE

Study on fire resistance of composite beams with pin joint at the ends Part2

Abstract

 In the case of steel-concrete composite beams pinned with steel girders, and the beam ends are rotationally restrained by the girders, reinforcing bars in the concrete slab work effectively so that the hogging moment resistance at the pin joints can be exerted considerably. Considering this effect, there is a possibility that fire protections of the steel beams can be omitted or reduced when the load level is low. However, the hogging moment resistance at pin joints considering rebars and the influence of the rebar specification on it have not been clarified.

 This paper presents the results of load-bearing fire tests of composite beams protected only at the beam end with continuous floor slab conducted to clarify the hogging moment resistance at pin joints considering rebars and the influence of the rebar specification on it. 3 specimens (Pure steel beam, Composite beam using composite slab, Composite beam using RC slab) were tested, and the main test parameter was the specifications of the rebars in the concrete slab (the amount of main rebar, the type of rebar, the fixing method). The load level was 40% of the long-term allowable moment, and the heating was in accordance with ISO 834 standard curve. In this study, the hogging moment resistance of the joints and steel temperature of composite beam in fire were discussed.

 The main conclusions from the test results were as follows:

 1) In the case of the cross section of the steel beam used in this tests, it was confirmed that the temperature of the joints during heating for 1 hour remained at about 600 ℃ by protecting at the joints using the fire resistive materials for 1 hour.

 2) The pin joints have sufficient rotational ability at elevated temperature without fracture of the bolts even in bending collapse.

 3) The pure steel beam with protected pin joints had the same ultimate strength as that of unprotected fixed end.

 4) In the case of composite beam using RC slab, the calculated value by the evaluation model of the hogging moment resistance at pin joints considering the rebars shown in Fig. 17(d) corresponded with the experimental values well.

 5) In the case of composite beam using composite slab, the rebars did not contribute to the hogging moment resistance at pin joints because the rebars were fractured at the initial stage of heating. It is necessary to evaluate the hogging moment resistance at the beam ends as simple steel beam without the effect of rebar when the rebars are fractured prematurely.