Research On Effective Flange Width Of Steel-Concrete Composite Beam Under Service Stage

In a steel-concrete composite structure, because of shear lag effect, the normal stress in the parts of the slab near the grid webs bigger than that remote from the webs, resulting in an incorrect calculation of the vertical deflection and the stress of a composite structure when using the elementary theory of beam. Domestic and foreign codes introduce a concept of the effective flange width as a simplified practical method for design a steel-concrete composite structure, which influences the mechanical properties of the beam directly. However, at home and abroad, the research on the effective flange width of the concrete slab in the hogging moment region is not enough. What’s more, the method, when acquiring the longitudinal reinforcement stress in the hogging moment area through Chinese specification, is to only take the steel beams and reinforced into consideration. They think the concrete slab quits working when it’s in tension, ignoring the role of the concrete slab completely. But, this case only applies the structure to achieve the ultimate limit state in the hogging moment region, and it will lead to the calculated result of longitudinal steel stress is too large, leading to further concrete crack width. It’s apparently inconsistent with the actual situation.Therefore, this paper is designed to give a more rational method to calculate the effective flange width and longitudinal reinforcement stress of composite beams in hogging bending zone during service stage. Composite beam under sagging moment during service stage is elasticity. The concrete of Composite beam under hogging moment during the normal using stage may crack. Based on practical experience, this paper selects the stage when the longitudinal stress of the middle section of reinforced concrete slab in the 30%-50% of the yield stress as the basis for determining the stage at the hogging moment during service stage. This paper adopt the theoretical formula to calculate under sagging moment because of its efficiency, and when the composite beam in hogging moment, the concrete may crack. The theoretical formula no longer applies this case, so selecting the finite element method to analyze under hogging moment.First of all, this paper adopted nonlinear finite element software of ABAQUS to simulate the whole loading process of two composite beams respectively at the sagging and hogging bending moment provided by researchers. It can be acquired clearly that the result is accurate enough, proving the accuracy and applicability of the finite element model. Then, this paper also deduces a theoretical model under the action of pressing and bending simultaneously, and proves the correctness of this theoretical model by comparing with the result of the finite element model. Then, using theoretical formula to analyze the effective flange width of composite beams under sagging moment in elastic stage. Based on previous studies, this paper analyzes the effective flange width under different ratios of width to span、load properties, and proposes appropriate simplified formula to compare with the codes all over the world. Then, adopting the finite element method and a new formula to make a detailed analysis of the effective flange width at the hogging bending moment during service stage. Some parameters are analyzed on the influence of the effective flange width, such as the sectional position along the span direction, the width-span radio of the composite beams, the concrete slab thickness, the longitudinal reinforcement radio and the form of the loads. The main factors are extracted to fit a simplified formula which is used to compare with the result at the sagging bending moment. Finally, comparing the longitudinal reinforcement stress and concrete crack width calculation results calculated through ignoring the role of the concrete completely with the result from this paper.The result shows that, at the hogging bending moment during service stage, the concrete slab cracking results in the reduction of the effective flange width. But, the longitudinal reinforcement stress and concrete crack width calculation results calculated through ignoring the role of the concrete completely is too large.