Experimental research on fire performance of U-shaped steel-concrete composite beam under uniformly distributed loading

Four U-shaped steel-concrete composite beams with channel connectors (CUSCBs) were designed and tested under the uniformly distributed vertical load and ISO-834 standard fire condition. The sectional temperature distribution, mid-span deformation development, fire resistance, and failure modes of each CUSCB under fire were investigated. The test results reveal the following facts: (1) All specimens fail flexurally; (2) The interface between the concrete flange and the U-shaped steel web remain intact and no local buckling occurs in U-shaped steel; (3) The concrete temperature decreases from the fired surface to the non-fired surface gradually; and (4) The temperature of the concrete at the corner between the lower flange and the web of U-shaped steel is higher than the concrete within the beam. Moreover, the fire resistance increases with the increase of fire protection layer thickness; and the bottom reinforcement in U-shaped steel can improve the fire resistance. The fire behavior of CUSCBs was also analyzed using the finite element program ABAQUS and the simulated temperature distributions and fire resistances were found to be in good agreement with the test results. Furthermore, the parametric analysis results show that the thickness of fire protection layer, the diameter of bottom reinforcement, the load ratio, and the thickness of U-shaped steel have significant impact on the fire resistance, while the section size and material strength have little impact. Lastly, design suggestions for the fire resistance design of CUSCBs are proposed.