Research on Global Stability Design Method of Steel-Concrete Composite Beams of Negative Moment Region

Steel-concrete composite beams are widely used in medium and small span bridges because of their reasonable acceptance, light lifting weight and convenient in construction, but they have lateral instability in the negative moment region. For the global stability design in the negative moment region, each code gives the calculation method according to the elastic foundation beam theory or inverted U-frame theory, but the results of different codes are different. How to select a method with good accuracy and relatively simple to assist the design that has become an urgent problem to be solved.
Using MATLAB numerical calculation method, this paper analyzed the parameter sensitivity of span, web height thickness ratio and reinforcement ratio of concrete slab of the global stability calculation method of composite beams in Eurocode 4, Code for Design of Steel and Concrete Composite Bridges(GB 50917-2013) and Standard for Design of Steel Structures(GB 50017-2017), and analyzed the influence of parameter changes of the stability reduction coefficient of composite beams. Based on ABAQUS nonlinear finite element analysis method, the arc-length method was used to simulate the global stability of composite beams. The differences were compared between the elastic-plastic critical buckling moment calculated by the three code and the finite element analysis under the changes of span and web height thickness ratio.
Through a large number of data analysis, it is found that:increasing the reinforcement ratio of concrete slab will slightly reduce the stability reduction coefficient, on the one hand, the increase in longitudinal reinforcement ratio will improve the stiffness of steel beams, on the other hand, it will lead to the upward movement towards neutral axis and increase of compression region. In both cases, the adverse effect of upward movement towards neutral axis is slightly greater than the beneficial effect of increasing section stiffness. On the whole, improving the reinforcement ratio of concrete slabs has no obvious effect on the stability of negative moment region; the height thickness ratio has a great influence on the stability of composite beams. With the increase of height thickness ratio, the stability reduction coefficient decreases obviously; the upper flange of the composite beam is restrained and the lateral displacement is limited, when the span increases to a certain extent, the instability mode changes from one half wave to two halves wave. Therefore, when the span is small, the stability in the negative moment region decreases with the increase of the span. When the span increases to a certain value, the increase in the span has little impact on the stability in the negative moment region. The results show that the elastic-plastic critical moments calculated by the three design methods are less than the simulated values, and there is a certain safety margin; the calculation results of the Eurocode 4 is closer and more conservative than the finite element simulation results, Code for Design of Steel and Concrete Composite Bridges is simple and can also ensure a certain degree of safety, Standard for Design of Steel Structures is closer to the finite element simulation results, and the safety reserve is relatively low than the other two code.