Study On Mechanical Properties Of Composite Beams With Shear Connectors Partially Debonded Under Negative Bending Moment

Steel-concrete composite beam has been widely recognized by practitioners in the field of construction because of its outstanding performance in stability,earthquake resistance,construction convenience and so on.However,the above advantages of steel-concrete composite beams can only be effectively displayed when they are subjected to positive bending moment.Due to the difference of mechanical properties between concrete and steel beam materials,the damage of concrete slab and local instability of steel beam are easy to occur in the steel-concrete composite beam in the negative bending moment area.This kind of phenomenon seriously damages the safety,applicability and durability of the building structure.To solve the cracking problem of concrete slab in the negative bending moment area of steel-concrete composite beam,Nie Jianguo designed a"pull-resistant and shear-resistant"connector from the perspective of releasing tensile stress in the concrete slab in the negative bending moment area.The characteristic of this kind of connector is that it can weaken the bonding force between the connector and the concrete,thus reducing the tensile stress in the concrete slab in the negative bending moment area,and finally improving the cracking of the concrete slab in the negative bending moment area^[1].Based on the degree of shear connection,casting methods,whether against shear connector partial debonding,the types of shear connectors for the parameter design,5 of the steel and concrete composite beams of these four shear connectors of steel and concrete composite beams with partial debonding process,while 1 root for reference with the shear connectors not debonding process of the project group of composite beam specimens.In order to make the designed steel-concrete composite beam bear the internal force similar to the negative bending moment area of the beam-column joint of the composite frame structure,the composite beam is placed and simply supported in the way of"concrete slab facing down,steel beam facing up",and the vertical downward load is applied in the mid-span position of the composite beam.Based on the static test of negative bending moment of steel-concrete composite beams,the test phenomenon and failure characteristics of five steel-concrete composite beams are analyzed,and the elastic limit state,plastic limit state,rotational capacity,characteristic load,crack development and slip distribution of each composite beam are also analyzed.On the basis of the above test results,a finite element analysis model was established for the composite beam with partial debonding of shear connectors,and the correctness of the finite element model was verified.Based on the finite element model of PHBCB-UB,the effects of high strength post-pouring strength,force ratio,high strength bolt preloading force and concrete slab strength on the mechanical properties of steel-concrete composite beams with local debonding of shear connectors under negative bending moment were investigated.The results show that the elastic ultimate bearing capacity of steel-concrete composite beams under negative bending moment is in good agreement with the measured values when only the combined effect of steel beam and reinforcement is considered.However,the stiffness of composite beams in elastic limit state without slippage effect is larger than that measured in the test.Therefore,it is suggested that the adverse effect of slippage effect should be considered in the calculation of the stiffness of steel-concrete composite beams with local debonding of shear joints under the action of negative bending moment.The metal corrugated sleeve on the prefabricated composite beam has a great influence on the crack development and distribution,and the slip of the prefabricated composite beam is greater than that of the cast-in-place composite beam.The finite element analysis results show that the strength of concrete slab and the preloading force of high-strength bolts have little influence on the ultimate bearing capacity and stiffness of steel-concrete composite beams under negative bending moment.The lifting force ratio and the strength of high strength castable can increase the ultimate bearing capacity of steel-concrete composite beams under negative bending moment without changing the stiffness of composite beams in the elastic stage.