Fire resistance performance and design method of steel-concretecomposite continuous curved box girders

As a strategy to improve the fire resistant performance of steel-concrete composite continuous curved box girders, a three-span steel-concrete composite continuous curved box girder was selected as a research object to establish a two-stage three-dimensional nonlinear analytical model under fire by employing the commonly used finite element software ANSYS. Based on the existing thermal-structural coupled analytical method, the developed model considered the radiation heat transfer in the cavity of steel box girder and the contact boundary conditions at the interface between the top flange of steel box girder and the concrete slab. The prediction results obtained by the model were compared with the experimental data to verify the model's reliability. The established model was used to conduct a parameter sensitivity of mid-span deflection of the steel-concrete composite continuous curved box girder under different longitudinal fire exposure positions, fire intensities, and load levels. The decay laws of ultimate bearing capacity and stiffness of the steel-concrete composite continuous curved box girder was studied. With the mid-span deflection under fire used as the evaluation indicator, a fire resistant design method of steel-concrete composite continuous curved box girders was proposed. Research results show that the deflection of the outer edge of steel-concrete composite continuous curved box girder is greater than that of the inner edge under the symmetrical fire and structural load, and this effect is more significant with greater loads and more severe fire. The stiffness decreases faster than the ultimate bearing capacity under a large burned area such as that resulting from a fuel tanker fire. Compared with the ultimate bearing capacity and stiffness of steel-concrete composite continuous curved box girder under normal temperature, the ultimate bearing capacity and stiffness reduce to 29% and 14%, respectively, when the side span is exposed to fire for 16 min, and they further reduce to 31% and 22%, respectively, when the middle-span is exposed to fire for 28 min. In the fire resistant design of steel-concrete composite continuous curved box girders, improving the stiffness of outer steel box girder under fire is necessary. Increasing and widening the longitudinal stiffeners of the bottom plate of outer steel box girder can reduce the mid-span deflection difference between the inner and outer steel box girders by 23% and 30%, respectively, when the side span is exposed to fire for 20 min, and by 22% and 27%, respectively, when the middle-span is exposed to fire for 32 min. 1 tab, 15 figs, 31 refs.