Distorsional buckling is one of the important buckling patterns of the steel-concrete composite beam in the negative moment region. Rotation retention stiffness and lateral retention stiffness make the steel beam bar to the composite steel and concrete bottom plate are key factors in influencing the distortional buckling behavior. In this work a thorough and intensive study is carried out on the rigidity of retention of the rotation and the stiffness of lateral retention that the bar of steel beam to the lower plate of the composite steel-I-concrete beam in the negative moment region. A principle of energy variation is adopted to derive the analytical expressions to calculate the rotation constraint stiffness and lateral retention stiffness. Combined with the theory of thin-walled bars of axial compression in elastic means, the moment of buckling is obtained. Theoretical analysis shows that the rigidity of rotation restriction and the lateral retention stiffness of the steel beam bar seem to have a linear relationship with the external loads and could also be negative. Compared with other methods, the results calculated by the proposed expressions coincide with the numerical results of ANSYS.
The composite steel and concrete beam is composed of I-beam or welded concrete beam and concrete slabs by shear connectors that can withstand external loads together. Because this composite structure combines the tensile strength of the steel with the compressive strength of the concrete, it has the advantages of a higher load capacity, a better plasticity and ductility, building conveniently and at a lower cost, which makes it widely used in long-range bridges and high-rise buildings. In practical engineering, it is not necessary to verify the lateral buckling of composite beams in the region of positive bending moment because of the sufficient bending stiffness and torsional stiffness of the concrete slab. However, with larger variable loads and unfavorable loads, the lower flange of the steel beam in the negative moment region would produce a lateral buckling associated with the transverse deformation of the web. The buckling distortion is then likely to occur.
In recent years, several researchers have used energy variation methods to analyze the overall stability of the composite beam. Some authors calculated the critical buckling load and others compiled the corresponding specifications. These specifications only consider the overall flexural instability of the steel beam network, but do not take into account distortional buckling. In addition, the critical load formulas of these methods are a bit tedious for engineering calculations. Williams and Jemah, Svensson, Goltermann and Svensson and Ronagh study successively the stability of the composite beam under constant axial force, increasing the contribution of the torsional stiffness of the concrete blocks and the area of participation of the steel plate beam Con In order to consider the effect of the variable bending moment gradient compression bar, the axial force is introduced. Diansheng and Xiaomin presented a model to analyze the local buckling property of the composite beam of cold-formed thin-walled concrete. The elastic buckling stresses in the steel beam band and the flange wall are calculated by the energy method. Jiang investigated the local stability in the region of negative moment for the steel beam frame of continuous composite beams in the attempt to establish the simplified model of local stability under various loads and propose the low local critical buckling stress formula a variety of stress states. Ye and Chen improved Svensson's compressive bar model appropriately. Taking into account the effective participation part of the steel beam bar, two formulas were calculated for the critical load of the distorted buckling stability of the variable axial forces as a function of the improved model. Using the finite element, the accuracy of the previous method was analyzed by calculating the distorted buckling load of the composite beam constriction. Zhou et al. used the principle of variation of the energy to deduce the method of calculation of the rigidity of restriction of the rotation and the stiffness of lateral restriction.