Fire experiment and fire design method of U-shaped steel-concrete composite beams

Four U-shaped steel-concrete composite beams (USCB) were designed and tested under the ISO-834 standard fire curve and distributed vertical load. The research parameters were the load ratio and shear connector form. The temperature distribution, fire resistance, and failure modes of the USCB specimen were investigated. The fire tests showed that the failure mode of the specimen is flexural damage and the connection interface between the flange and web of the composite beam remains intact. The temperature of the fire-resistance reinforcements is below 500°C when the specimens reached the fire resistance, which is in the early stage of material degradation and has high residual strength. The fire resistance increased (improve by 51%) with the decrease of load ratio (0.7 to 0.4) and was not affected by the shear connector form. Then a numerical simulation was applied to establish the temperature field model and fire resistance model of USCB, and the simulation results of the temperature field and fire resistance were in good agreement with the test results. The parameter analysis indicated that the load ratio, the diameter of fire-resistance reinforcement and fire protection layer have an obvious impact on the fire performance. Based on the heat transfer mechanism analysis, the temperature calculation methods for U-shaped steel, fire-resistant reinforcement and concrete were proposed. Moreover, a flexural bearing capacity calculation method for the USCB under the standard temperature rise curve is proposed, which can be applied to calculate fire resistance.