Research On Degradation Of Stiffness Of Main Beam Caused By Concrete Cracking In Negative Moment Zone Of Composite Continuous Beam

With the gradual popularization of large and medium-span steel-concrete composite continuous beams,the increase of negative bending moment of mid-support leads to more and more cracking of concrete slabs,and the degradation of stiffness of main beams caused by crack propagation becomes more and more prominent.Therefore,it is necessary to study the degradation of stiffness of main beams caused by crack propagation of concrete slabs in negative bending moment region.From the point of view of crack generation and propagation,this paper studies the main law of cracking of concrete face slab in negative moment area by means of literature review,theoretical analysis and finite element simulation,puts forward the crack model of cracking of concrete face slab in negative moment area,analyses the degradation law of stiffness of main beam in cracking process of concrete face slab,and studies the stress of composite beam after cracking of concrete face slab.Influencing factors of performance and measures to improve stiffness and bearing capacity of composite beams in negative moment zone are proposed.The main conclusions are as follows:(1)The general law of crack propagation of concrete face slabs in negative moment zone of composite beams is obtained.Cracks generally occur at the top transverse reinforcement position with the largest bending moment at the middle support,and then expand to both sides.Cracks are located at the top transverse reinforcement position,and the spacing is the top transverse reinforcement spacing.The length of cracks is the width of the concrete face plate,the depth of cracks is the thickness of the concrete face plate,and the crack shape penetrates the main cracks.(2)A finite element model analysis method is proposed to reflect the degradation of stiffness of the main beam in the process of crack generation and propagation in the negative moment zone of composite beams.(3)The main regularity of stiffness degradation of the main beam during the cracking process of the concrete face slab in the negative moment zone of composite beams is obtained.The stiffness of composite beams decreases with the crack propagation,and the stiffness changes abruptly from the initial uncracked section stiffness to the initial cracked section stiffness.The further expansion of the crack leads to the reduction of the stiffness of composite beams to the cracked section stiffness,and finally the gradual yield of longitudinal reinforcement and steel beam leads to the further reduction of the stiffness of the main beam.(4)It is pointed out that when calculating the deflection of composite beams in the negative moment zone,the contribution of the tension stiffness effect of concrete slabs to the stiffness of the section should be fully considered,and the calculation method without considering the effect of concrete slabs in the negative moment zone should be conservative according to relevant codes.(5)The effects of various factors on the stiffness and ultimate bearing capacity of the main beam after cracking of the concrete face slab in the negative moment zone of composite beams are obtained.Reinforcement ratio,steel beam roof thickness,steel beam floor thickness and steel beam web thickness have greater influence on the stiffness and ultimate bearing capacity of composite beams after cracking of concrete face slab,while the thickness of concrete face slab and the strength grade of concrete face slab have less influence on the stiffness and bearing capacity of main beams after cracking of concrete face slab.(6)A method to improve the stiffness and bearing capacity of composite beams in negative moment region is proposed.The feasibility of this method is verified by finite element analysis.Then,the corresponding rules are discussed by parameter analysis.In the negative moment zone of composite beams,Z-shaped oblique bars are set at the transverse reinforcement position of the top layer of concrete face slab,which improves the local reinforcement ratio,strengthens the connection between cracks and improves the local mechanical properties of concrete face slabs,thus improving the stiffness and bearing capacity of composite beams.