Mechanical Response Mechanism of Steel-Concrete Composite Continuous Beams Under Fire

This paper employed the ABAQUS finite element （FE） software to conduct a three-dimensional shell-solid FE analysis on the fire resistance of steel-concrete composite continuous beams. Based on validation of the FE model， the effects of load ratio， load position ratio， shear connection ratio， and fire protection layer thickness of steel beams on the fire resistance of continuous beams were subsequently investigated. Additionally， the relations between the variation of internal forces and the deformation stages of composite continuous beams under fire were elucidated. It further revealed the mechanical response mechanisms， including the interface slip behavior， the formation of plastic hinges， and the failure modes. Based on these findings， a differentiated fire protection design strategy of “reinforcing the side spans while simplifying the mid-span” was proposed. The analysis results revealed that： 1） The mid-span of the three-span continuous beam underwent four deformation stages： elastic， elastoplastic， plastic， and catenary action. The catenary effect reduced the required thickness of the fire protection layer. As the restraint stiffness decreased， particularly in the edge span of the continuous beam， the failure mode of the beam shifted from overall lateral instability to failure due to insufficient bearing capacity. In such cases， the fire resistance was equivalent to that of a simply-supported beam due to the absence of the catenary effect. 2） Due to the thermal expansion of the composite beam being constrained by the intermediate supports， a large negative bending moment was generated. Consequently， the plastic hinge at the supports formed earlier than that at the mid-span. Additionally， the positive bending moment at the mid-span of the composite continuous beam decreased or even reversed to negative in the early stages of the fire. Throughout the fire exposure， the continuous beam underwent a significant redistribution of internal forces. 3） The fire resistance of the composite continuous beam was almost unaffected by the shear connection degree η. However， when the steel beam had no fire protection， the end slip decreased significantly with increases in η. As the thickness of the fire protection layer increased， the influence of η on the beam-end displacement weakened， and the end slip was markedly reduced. 4） For composite beams with a load ratio of 0.4， a differentiated fire protection strategy was adopted， which was characterized as “reinforcing the side spans while simplifying the mid-span”. Specifically， the fire protection thickness for the side spans was determined in accordance with the calculation method for simply-supported beams stipulated in the GB 51249—2017 Code for Fire Safety of Steel Structures in Buildings. For the mid-span， fire protection could be omitted entirely， or a minimal layer could be applied to mitigate early-stage deflection during a fire.